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CX11x, CX31x, CX710 (Earlier Than V6.03), and CX91x Series Switch Modules V100R001C10 Configuration Guide 13

The documents describe the configuration of various services supported by the CX11x&CX31x&CX91x series switch modules The description covers configuration examples and function configurations.

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Principles

Principles

IS-IS Basic Concepts

IS-IS Topology Structure

Overall IS-IS Topology

IS-IS uses a two-level hierarchy (backbone area and non-backbone area) to support large-scale routing networks. Generally, Level-1 routers are deployed in non-backbone areas, whereas Level-2 and Level-1-2 routers are deployed in backbone areas. Each non-backbone area connects to the backbone area through a Level-1-2 router.

Figure 7-66 shows a network that runs IS-IS. The network is similar to an OSPF network typology with multiple areas. The backbone area contains all the routers in Area 1 and Level-1-2 routers in other areas.

Figure 7-66 IS-IS topology I

Figure 7-67 shows another type of IS-IS topology. In this topology, Level-2 routers belong to different areas. All the physically contiguous Level-1-2 and Level-2 routers form the backbone area of IS-IS.

Figure 7-67 IS-IS topology II

The two types of topologies show the differences between IS-IS and OSPF:
  • In IS-IS, each router belongs to only one area. In OSPF, different interfaces of a router may belong to different areas.
  • In IS-IS, no area is defined as the backbone area. In OSPF, Area 0 is defined as the backbone area.
  • In IS-IS, Level-1 and Level-2 routes are calculated using the SPF algorithm to generate the shortest path tree (SPT). In OSPF, the SPF algorithm is used only in the same area, and inter-area routes are forwarded by the backbone area.
IS-IS Router Types
  • Level-1 router

    A Level-1 router manages intra-area routing. It establishes neighbor relationships with only the Level-1 and Level-1-2 routers in the same area and maintains a Level-1 link state database (LSDB). The LSDB contains intra-area routing information. A packet to a destination outside this area is forwarded to the nearest Level-1-2 router.

  • Level-2 router

    A Level-2 router manages inter-area routing. It can establish neighbor relationships with Level-2 or Level-1-2 routers in different areas and maintains a Level-2 LSDB. The LSDB contains inter-area routing information.

    All Level-2 routers form the backbone network of the routing domain. They establish Level-2 neighbor relationships and are responsible for inter-area communication. Level-2 routers in the routing domain must be physically contiguous to ensure the continuity of the backbone network. Only Level-2 routers can exchange data packets or routing information with routers outside the routing domain.

  • Level-1-2 router

    A router that belongs to both a Level-1 area and a Level-2 area is called a Level-1-2 router. It can establish Level-1 neighbor relationships with Level-1 and Level-1-2 routers in the same area. It can also establish Level-2 neighbor relationships with Level-2 and Level-1-2 routers in different areas. A Level-1 router must be connected to other areas through a Level-1-2 router.

    A Level-1-2 router maintains two LSDBs: a Level-1 LSDB and a Level-2 LSDB. The Level-1 LSDB saves for intra-area routing and the Level-2 LSDB saves for inter-area routing.

IS-IS Network Types

IS-IS supports only two types of networks. In terms of physical links, IS-IS networks can be classified into the following link types:

  • Broadcast: such as Ethernet and Token-Ring
  • Point-to-point: such as PPP and HDLC

For a Non-Broadcast Multi-Access (NBMA) network such as the ATM, you should configure its sub-interfaces as P2P interfaces.

IS-IS cannot run on Point to MultiPoint (P2MP) networks.

DIS and Pseudonode

In a broadcast network, IS-IS needs to elect a Designated Intermediate System (DIS) from all the routers. DISs are used to create and update pseudonodes and generate link state protocol data units (LSPs) of pseudonodes to describe available network devices.

The pseudonode is used to simulate the virtual node in the broadcast network and is not an actual router. In IS-IS, a pseudonode is identified by the system ID of the DIS and the 1-byte Circuit ID (its value is not 0).

Figure 7-68 Pseudonode

As shown in Figure 7-68, the use of pseudonodes simplifies the network topology and shortens LSPs. When the network changes, the number of generated LSPs is reduced, and the SPF consumes fewer resources.

Level-1 and Level-2 DISs are elected separately. You can configure different priorities for DISs of different levels. The router with the highest priority is elected as the DIS. If there are multiple routers with the same highest priority on a broadcast network, the one with the highest MAC address is chosen. The DISs of different levels can be the same router or different routers.

DIS election in IS-IS differs from designated router (DR) election in OSPF:
  • On an IS-IS broadcast network, the router with priority 0 also takes part in DIS election. In OSPF, the router with priority 0 does not take part in DR election.

  • In IS-IS, when a new router that meets the requirements of being a DIS connects to a broadcast network, the router is elected as the new DIS, and the previous pseudonode is deleted. This causes a new flooding of LSPs. In OSPF, when a new router connects to a network, it is not immediately elected as the DR even if it has the highest DR priority.

  • On an IS-IS broadcast network, routers (including non-DIS routers) of the same level on a network segment set up adjacencies. In OSPF, routers set up adjacencies with only the DR and backup designated router (BDR).

On an IS-IS broadcast network, although all the routers set up adjacencies with each other, the LSDBs are synchronized by the DISs.

IS-IS Address Structure

The network service access point (NSAP) is an address defined by the OSI to locate resources. Figure 7-69 shows the NSAP address structure. The NSAP is composed of the initial domain part (IDP) and the domain specific part (DSP). The lengths of the IDP and the DSP are variable. The maximum length of the NSAP is 20 bytes and its minimum length is 8 bytes.
  • The IDP is similar to the network ID in an IP address. It is defined by the ISO and consists of the authority and format identifier (AFI) and the initial domain identifier (IDI). The AFI indicates the address allocation authority and address format, and the IDI identifies a domain.
  • The DSP is similar to the subnet ID and host address in an IP address. The DSP consists of the High Order DSP (HODSP), system ID, and NSAP Selector (SEL). The HODSP is used to divide areas, the system ID identifies a host, and the SEL indicates the service type.
Figure 7-69 IS-IS address structure
  • Area Address

    The IDP and the HODSP of the DSP identify a routing domain and the areas in a routing domain. Therefore, the combination of the IDP and HODSP is called an area address, which is similar to an area number in OSPF. The area addresses of routers in the same Level-1 area must be the same, while the area addresses of routers in the Level-2 area can be different.

    In general, a router can be configured with only one area address. The area address of all nodes in an area must be the same. In the implementation of a device, an IS-IS process can be configured with a maximum of three area addresses to support seamless combination, division, and transformation of areas.

  • System ID

    A system ID uniquely identifies a host or a router in an area. In the device, the fixed length of the system ID is 48 bits (6 bytes).

    In actual applications, a router ID corresponds to a system ID. If a router takes the IP address 168.10.1.1 of Loopback 0 as its router ID, its system ID used in IS-IS can be obtained in the following way:
    • Extend each part of IP address 168.10.1.1 to 3 bits and add 0 to the front of any part that is shorter than 3 bits. Then the IP address is extended as 168.010.001.001.

    • Divide the extended address 168.010.001.001 into three parts, each of which consists of four decimal digits. Then system ID 1680.1000.1001 is obtained.

    You can specify a system ID in many ways. You need to ensure that the system ID uniquely identifies a host or a router.

  • SEL

    The role of an SEL is similar to that of the "protocol identifier" of IP. A transport protocol matches an SEL. The SEL is always "00" in IP.

A network entity title (NET) indicates network layer information about an IS. A NET can be regarded as a special NSAP. The NET length is the same as the NSAP length. Its maximum length is 20 bytes and minimum length is 8 bytes. When configuring IS-IS on a router, you only need to configure a NET but not an NSAP.

Assume that there is a NET: ab.cdef.1234.5678.9abc.00. In the NET, the area address is ab.cdef, the system ID is 1234.5678.9abc, and the SEL is 00.

IS-IS PDU Types

IS-IS PDUs include Hello PDUs, link state PDUs (LSPs), and sequence number PDUs (SNPs).

  • Hello PDU

    Hello packets, also called IS-IS Hello PDUs (IIH), are used to set up and maintain neighbor relationships. Among them, Level-1 LAN IIHs apply to the Level-1 routers on broadcast LANs; Level-2 LAN IIHs apply to the Level-2 routers on broadcast LANs; and P2P IIHs apply to non-broadcast networks. Hello packets on different networks have different formats. Compared to a LAN IIH, a P2P IIH does not have the Priority and LAN ID fields, but has a Local Circuit ID field. The Priority field indicates the DIS priority on a broadcast network, the LAN ID field indicates the system ID of the DIS and pseudonode, and the Local Circuit ID indicates the local link ID.

  • LSP

    LSPs are used to exchange link-state information. There are two types of LSPs: Level-1 and Level-2. Level-1 IS-IS transmits Level-1 LSPs; Level-2 IS-IS transmits Level-2 LSPs; and Level-1-2 IS-IS can transmit both Level-1 and Level-2 LSPs.

    The meanings of major fields in an LSP are as follows:

    • ATT field: When a Level-1-2 IS-IS transmits Level-1 LSPs in a Level-1 area, Level-1 IS-IS in the area can communicate with devices in other areas through the Level-1-2 IS-IS if the ATT bit is set in the Level-1 LSPs.

    • OL field: indicates the LSDB overload.

      LSPs with the overload bit are still flooded on the network, but these LSPs are ignored during the calculation of the routes that pass through a router in overload state. After the overload bit is set on a router, other routers ignore the router when performing SPF calculation and consider only the direct routes of the router. For details, see "IS-IS Overload" in Principles.

    • IS Type field: indicates the type of IS-IS that generates the LSP. The value 01 indicates Level-1, and the value 11 indicates Level-2.

  • SNP

    SNPs describe the LSPs in all or some databases to help synchronize and maintain all LSDBs.

    SNPs include complete SNPs (CSNPs) and partial SNPs (PSNPs). They are further classified into Level-1 CSNPs, Level-2 CSNPs, Level-1 PSNPs, and Level-2 PSNPs.

    A CSNP contains the summary of all LSPs in an LSDB. This maintains LSDB synchronization between neighboring routers. On a broadcast network, the DIS periodically sends CSNPs. The default interval for sending CSNPs is 10 seconds. On a point-to-point link, CSNPs are sent only when the neighbor relationship is established for the first time.

    A PSNP lists only the sequence number of recently received LSPs. A PSNP can acknowledge multiple LSPs at one time. If an LSDB is not updated, the PSNP is also used to request a neighbor to send a new LSP.

The variable length fields in an IS-IS PDU are multiple type-length-values (TLVs). Figure 7-70 shows the TLV format. A TLV is also called a code-length-value (CLV).

Figure 7-70 TLV format

TLVs vary according to PDU types, as shown in Table 7-37.

Table 7-37 PDU types and TLV names

TLV Type

Name

Applied PDU Type

1

Area Addresses

IIH, LSP

2

IS Neighbors (LSP)

LSP

4

Partition Designated Level2 IS

L2 LSP

6

IS Neighbors (MAC Address)

LAN IIH

7

IS Neighbors (SNPA Address)

LAN IIH

8

Padding

IIH

9

LSP Entries

SNP

10

Authentication Information

IIH, LSP, SNP

128

IP Internal Reachability Information

LSP

129

Protocols Supported

IIH, LSP

130

IP External Reachability Information

L2 LSP

131

Inter-Domain Routing Protocol Information

L2 LSP

132

IP Interface Address

IIH, LSP

TLVs with the type value ranging from 1 to 10 are defined in ISO 10589, and the other TLVs are defined in RFC 1195.

IS-IS Basic Principles

IS-IS is a link-state routing protocol. Each router generates an LSP that contains link state information about all the IS-IS interfaces on the router. The router can establish IS-IS neighbor relationships with neighboring devices and update its LSDB to synchronize the local LSDB with the LSDBs of all the other devices on the IS-IS network. Based on the local LSDB, the router uses the SPF algorithm to calculate IS-IS routes. If the router finds that an IS-IS route is the optimal route to a destination, the router adds the route to the local IP routing table to guide packet forwarding.

Establishment of IS-IS Neighbor Relationship

Two IS-IS routers need to establish a neighbor relationship before exchanging packets to implement routing. On different networks, the modes for establishing IS-IS neighbors are different.

  • Establishment of a neighbor relationship on a broadcast link

    Figure 7-71 uses Level-2 routers as an example to describe the process of establishing a neighbor relationship on a broadcast link. The process of establishing a neighbor relationship between Level-1 routers is the same as the process of establishing a neighbor relationship between Level-2 routers.

    Figure 7-71 Process of establishing a neighbor relationship on a broadcast link

    1. RouterA broadcasts a Level-2 LAN IS-IS Hello PDU (IIH) with no neighbor ID specified.
    2. RouterB receives this packet and sets the status of the neighbor relationship with RouterA to Initial. RouterB then responds to RouterA with a Level-2 LAN IIH, indicating that RouterA is a neighbor of RouterB.
    3. RouterA receives this packet and sets the status of the neighbor relationship with RouterB to Up. RouterA then sends RouterB a Level-2 LAN IIH indicating that RouterB is a neighbor of RouterA.
    4. RouterB receives this packet and sets the status of the neighbor relationship with RouterA to Up. RouterA and RouterB establish a neighbor relationship successfully.

    The network is a broadcast network, so a DIS needs to be elected. After the neighbor relationship is established, routers wait for two intervals before sending Hello packets to elect the DIS. The IIH packets exchanged by the routers contain the Priority field. The router with the highest priority is elected as the DIS. If the routers have the same priority, the router with the largest interface MAC address is elected as the DIS.

  • Establishment of a neighbor relationship on a P2P link

    Unlike the establishment of a neighbor relationship on a broadcast link, the establishment of a neighbor relationship on a P2P link is classified into two modes: two-way mode and three-way mode.

    • Two-way mode

      Upon receiving a P2P IIH from a neighbor, a router considers the neighbor Up and establishes a neighbor relationship with the neighbor.

    • Three-way mode

      A neighbor relationship is established after P2P IIHs are sent for three times. The establishment of a neighbor relationship on a P2P link is similar to that on a broadcast link.

    Two-way mode has distinct disadvantages. For example, when two or more links exist between two routers, the two routers can still establish a neighbor relationship if one link is Down and the other is Up in the same direction. The parameters of the link in Up state are used in SPF calculation. As a result, the router that does not detect the fault of the link in Down state still tries to forward packets over this link. Three-way mode addresses such problems on unreliable P2P links. In three-way mode, a router considers the neighbor Up only after confirming that the neighbor receives the packet sent by itself, and then establishes a neighbor relationship with the neighbor.

Basic rules for establishing an IS-IS neighbor relationship are as follows:
  • Only neighboring routers of the same level can set up the neighbor relationship with each other.

  • For Level-1 routers, their area IDs must be the same

  • Network types of IS-IS interfaces on both ends of a link must be consistent.

    Ethernet interfaces can be simulated as P2P interfaces to establish a neighbor relationship on a P2P link.

  • IP addresses of IS-IS interfaces on both ends of a link must be on the same network segment.

    IS-IS runs on the data-link layer and was initially designed for CLNP. Therefore, the establishment of an IS-IS neighbor relationship is not related to IP addresses. In the implementation of a device, IS-IS runs only over IP. Therefore, IS-IS needs to check the IP address of its neighbor. If secondary IP addresses are assigned to the interfaces, the routers can still set up the IS-IS neighbor relationship, but only when either the primary IP addresses or secondary IP addresses are on the same network segment.

    When IP addresses of IS-IS interfaces on both ends of a link are on different network segments, a neighbor relationship can still be established on the two interfaces if the interfaces are configured not to check the IP addresses in received Hello packets. You can configure P2P interfaces not to check the IP addresses in received Hello packets. Before configuring Ethernet interfaces not to check the IP addresses, simulate Ethernet interfaces as P2P interfaces.

Process of Exchanging IS-IS LSPs

Causes of LSP generation

All routers in the IS-IS routing domain can generate LSPs. The following events trigger the generation of a new LSP:

  • Neighbor is Up or Down.

  • Related interface goes Up or Down.

  • Imported IP routes change.

  • Inter-area IP routes change.

  • Interface is assigned a new metric value.

  • Periodic updates occur.

Processing of a new LSP received from a neighbor

  1. The router installs the LSP to its LSDB and marks it for flooding.

  2. The router sends the LSP to all interfaces except the interface that initially received the LSP.

  3. The neighbors flood the LSP to their neighbors.

LSP flooding

In LSP flooding, a router sends an LSP to its neighbors and then the neighbors send the received LSP to their respective neighbors except the router that first sends the LSP. In this manner, the LSP is flooded among the routers of the same level. LSP flooding allows each router of the same level to have the same LSP information and synchronize its LSDB with each other.

Each LSP has a 4-byte sequence number. When a router is started, the sequence number of the first LSP sent by the router is 1. When a new LSP is generated, the sequence number of the LSP is equal to the sequence number of the previous LSP plus 1. The greater the sequence number, the newer the LSP.

Process of synchronizing LSDBs between a newly added router and DIS on a broadcast link

Figure 7-72 Process of updating LSDBs on a broadcast link

  1. As shown in Figure 7-72, a new router (RouterC) sends a Hello packet to establish neighbor relationships with the other routers in the broadcast domain.
  2. RouterC establishes neighbor relationships with RouterA and RouterB, waits for the timeout of the LSP refresh timer, and then sends its LSP to a multicast address (01-80-C2-00-00-14 in a Level-1 area and 01-80-C2-00-00-15 in a Level-2 area). All neighbors on the network can receive the LSP.
  3. The DIS on the network segment adds the received LSP to its LSDB. After the CSNP timer expires, the DIS sends CSNPs to synchronize the LSDBs on the network.
  4. RouterC receives the CSNPs from the DIS, checks its LSDB, and sends a PSNP to request the LSPs it does not have.
  5. The DIS receives the PSNP and sends RouterC the required LSPs for LSDB synchronization.
The process of updating the LSDB of the DIS is as follows:
  1. When the DIS receives an LSP, it searches the LSDB to check whether the same LSP exists. If the DIS does not find the same LSP in its LSDB, the DIS adds the LSP to its LSDB and broadcasts the content of the new LSDB.
  2. If the sequence number of the received LSP is greater than that of the corresponding LSP in the LSDB, the DIS replaces the existing LSP with the received LSP and broadcasts the contents of the new LSDB. If the sequence number of the received LSP is smaller than that of the corresponding LSP in the LSDB, the DIS sends its LSP in the LSDB through the inbound interface of the received LSP.
  3. If the sequence number of the received LSP is the same as that of the corresponding LSP in the LSDB, the DIS compares the remaining lifetime of the two LSPs. If the remaining lifetime of the received LSP is smaller than that of the corresponding LSP in the LSDB, the DIS replaces the existing LSP with the received LSP and broadcasts the contents of the new LSDB. If the remaining lifetime of the received LSP is greater than that of the corresponding LSP, the DIS sends its LSP in the LSDB through the inbound interface of the received LSP.
  4. If the sequence number and remaining lifetime of the received LSP are the same as those of the corresponding LSP in the LSDB, the DIS compares the checksum of the two LSPs. If the checksum of the received LSP is greater than that of the corresponding LSP in the LSDB, the DIS replaces the existing LSP with the received LSP and broadcasts the content of the new LSDB. If the checksum of the received LSP is smaller than that of the corresponding LSP, the DIS sends its LSP in the LSDB through the inbound interface of the received LSP.
  5. If the sequence number, remaining lifetime, and checksum of the received LSP are the same as those of the corresponding LSP in the LSDB, the DIS does not forward the received LSP.

Process of synchronizing the LSDB on a P2P link

Figure 7-73 Process of updating LSDBs on a P2P link

  1. RouterA establishes a neighbor relationship with RouterB.

  2. RouterA and RouterB send a CSNP to each other. If the LSDB of the neighbor and the received CSNP are not synchronized, the neighbor sends a PSNP to request the required LSP.

  3. Figure 7-73 assumes that RouterB requests the required LSP from RouterA. RouterA sends the required LSP to RouterB, starts the LSP retransmission timer, and waits for a PSNP from RouterB as an acknowledgement for the received LSP.

  4. If RouterA does not receive a PSNP from RouterB after the LSP retransmission timer expires, RouterA resends the LSP until it receives a PSNP from RouterB.

A PSNP on a P2P link is used as follows:
  • An ACK packet to acknowledge the received LSP.

  • A request packet to acquire LSPs.

The process of updating LSDBs on a P2P link is as follows:
  1. If the sequence number of the received LSP is smaller than that of the corresponding LSP in the LSDB, the router directly sends its LSP to the neighbor and waits for a PSNP from the neighbor. If the sequence number of the received LSP is greater than that of the corresponding LSP in the LSDB, the router adds the received LSP to its LSDB, sends a PSNP to acknowledge the received LSP, and then sends the received LSP to all its neighbors except the neighbor that sends the LSP.
  2. If the sequence number of the received LSP is the same as that of the corresponding LSP in the LSDB, the router compares the remaining lifetime of the two LSPs. If the received LSP has a smaller remaining lifetime than that of the corresponding LSP in the LSDB, the router adds the received LSP to its LSDB, sends a PSNP to acknowledge the received LSP, and then sends the received LSP to all its neighbors except the neighbor that sends the LSP. If the received LSP has a greater remaining lifetime than that of the corresponding LSP in the LSDB, the router directly sends its LSP to the neighbor and waits for a PSNP from the neighbor.
  3. If the sequence number and remaining lifetime of the received LSP are the same as those of the corresponding LSP in the LSDB, the router compares the checksum of the two LSPs. If the received LSP has a greater checksum than that of the corresponding LSP in the LSDB, the router adds the received LSP to its LSDB, sends a PSNP to acknowledge the received LSP, and then sends the received LSP to all its neighbors except the neighbor that sends the LSP. If the received LSP has a smaller checksum than that of the corresponding LSP in the LSDB, the router directly sends its LSP to the neighbor and waits for a PSNP from the neighbor.
  4. If the sequence number, remaining lifetime, and checksum of the received LSP and the corresponding LSP in the LSDB are the same, the router does not forward the received LSP.

IS-IS Authentication

To ensure network security, IS-IS authentication encrypts IS-IS packets by adding the authentication field to packets. When a local router receives IS-IS packets from a remote router, the local router discards the packets if the authentication passwords do not match. This protects the local router.

Authentication Types

Based on the types of packets, the authentication is classified as follows:

  • Interface authentication: authenticates Level-1 and Level-2 Hello packets sent and received on IS-IS interfaces using the specified authentication mode and password.

    You can configure a router to perform interface authentication in the following ways:

    • A router sends authentication packets carrying the authentication TLV and verifies the authentication information about the received packets.

    • A router sends authentication packets carrying the authentication TLV but does not verify the authentication information about the received packets.

  • Area authentication: authenticates Level-1 LSPs and Level-1 SNPs transmitted in an IS-IS area using the specified authentication mode and password.

  • Routing domain authentication: authenticates Level-2 LSPs and Level-2 SNPs transmitted in an IS-IS routing domain using the specified authentication mode and password.

    In area authentication and routing domain authentication, you can configure a router to authenticate LSPs and SNPs separately in the following ways:

    • A router sends LSPs and SNPs carrying the authentication TLV and verifies the authentication information about the received LSPs and SNPs.

    • A router sends LSPs carrying the authentication TLV and verifies the authentication information about the received LSPs. The router sends SNPs carrying the authentication TLV but does not verify the authentication information about the received SNPs.

    • A router sends LSPs carrying the authentication TLV and verifies the authentication information about the received LSPs. The router sends SNPs without the authentication TLV and does not verify the authentication information about the received SNPs.

    • A router sends LSPs and SNPs carrying the authentication TLV but does not verify the authentication information about the received LSPs and SNPs.

Based on the authentication modes of packets, authentication is classified into the following types:

  • Plain text authentication: is a simple authentication mode in which passwords are directly added to packets. This authentication is insecure.

  • MD5 authentication: uses the MD5 algorithm to encrypt passwords before they are added to packets, which improves password security.

  • Keychain authentication: further improves network security with configurable key chain that changes with time.

Mode in Which Authentication Information Is Carried

IS-IS provides a TLV to carry authentication information, with the type of the TLV specified as 10.

  • Type: is defined by the ISO as 10, with a length of 1 byte.

  • Length: indicates the length of the authentication TLV, which is 1 byte.

  • Value: indicates the authentication contents of 1 to 254 bytes, including the authentication type and password.

    The authentication type is 1 byte:

    • Type 0 is reserved.

    • Type 1 indicates plain text authentication.

    • Type 54 indicates MD5 authentication.

    • Type 255 indicates routing domain private authentication methods.

IS-IS Route Leaking

Normally, Level-1 routers manage routes in Level-1 areas. All Level-2 and Level-1-2 routers form a contiguous backbone area. Level-1 areas can only connect to the backbone area, but cannot connect to each other.

A Level-1-2 router encapsulates learned Level-1 routing information into a Level-2 LSP and floods the Level-2 LSP to other Level-2 and Level-1-2 routers. Then Level-1-2 and Level-2 routers know routing information about the entire IS-IS routing domain. To reduce the size of routing tables, a Level-1-2 router, by default, does not advertise the learned routing information of other Level-1 areas and the backbone area to its Level-1 area. In this case, Level-1 routers cannot know routing information outside the local area. As a result, Level-1 routers cannot select the optimal route to the destination outside the local area.

IS-IS route leaking can solve this problem. You can configure access control lists (ACLs) and routing policies and mark routes with tags on Level-1-2 routers to select eligible routes. Then a Level-1-2 router can advertise routing information of other Level-1 areas and backbone area to its Level-1 area.

Figure 7-74 IS-IS route leaking

In Figure 7-74, RouterA sends a packet to RouterF. The selected optimal route should be RouterA->RouterB->RouterD->RouterE->RouterF. This is because the cost of this route is 40, which is smaller than the cost (70) of the other route (RouterA->RouterC->RouterE->RouterF). However, when you check the route on RouterA to view the path of the packets sent to RouterF, the selected route is RouterA->RouterC->RouterE->RouterF but not the optimal route from RouterA to RouterF.

RouterA (Level-1 router) does not know routes outside its area, so it sends packets outside its area through the default route generated by the nearest Level-1-2 router. Therefore, the optimal route is not used to forward the packets.

If route leaking is enabled on Level-1-2 routers (RouterC and RouterD), Level-1 routers in Area 10 can know routes outside Area 10 and passing through the two Level-1-2 routers. After route calculation, the forwarding path becomes RouterA->RouterB->RouterD->RouterE->RouterF, which is the optimal route from RouterA to RouterF.

IS-IS Overload

IS-IS Overload allows a device to use the IS-IS overload bit to identify the overload state. The IS-IS overload bit is the OL field in an IS-IS LSP. After the overload bit is set on a device, other devices ignore this device when performing SPF calculation and consider only the direct routes of the device.

Figure 7-75 IS-IS Overload

As shown in Figure 7-75, RouterB forwards the packets sent from RouterA to network segment 1.1.1.0/24. If the overload bit in the LSP sent from RouterB is set to 1, RouterA considers the LSDB of RouterB incomplete and sends packets to 1.1.1.0/24 through RouterD and RouterE. This process does not affect the packets sent to the directly connected network segment of RouterB.

If a device cannot store new LSPs and fails to synchronize the LSDB, the routes calculated by this device are incorrect. In this situation, the device enters the overload state and does not calculate the routes passing through this device; however, the direct routes of the device are still valid.

A device may enter the overload state because of device abnormalities or is manually configured to enter the overload state. When an IS-IS device on the network needs to be upgraded or maintained, isolate this device from the network temporarily and set the overload bit on the device to prevent other devices from using this device to forward traffic.
  • If the system enters the overload state because of an abnormality, the system deletes all the imported or leaked routes.
  • If the system is configured to enter the overload state, the system determines whether to delete all the imported or leaked routes based on the configuration.

IS-IS Network Convergence

Fast convergence and priority-based convergence can improve IS-IS network convergence. Fast convergence speeds up network convergence by fast calculating routes, while priority-based convergence sets different convergence priorities for routes to improve network convergence.

Fast Convergence

IS-IS fast convergence is an extended feature of IS-IS that is implemented to speed up the convergence of routes. Fast convergence includes the following:

  • Incremental SPF (I-SPF): recalculates only the routes of the changed nodes rather than all the nodes when the network topology changes. This speeds up the calculation of routes.

    In ISO 10589, the SPF algorithm is used to calculate routes. When a node changes on the network, this algorithm is used to recalculate all routes. The calculation takes a long time and consumes too many CPU resources, which affects the convergence speed.

    I-SPF improves this algorithm. Except for the first time, only changed nodes instead of all nodes are involved in calculation. The shortest path tree (SPT) generated is the same as that generated by the previous algorithm. This decreases CPU usage and speeds up network convergence.

  • Partial Route Calculation (PRC): calculates only the changed routes when the routes on the network change.

    Similar to I-SPF, PRC calculates only the changed routes, but it does not calculate the shortest path. It updates routes based on the SPT calculated by I-SPF.

    In route calculation, a leaf represents a route, and a node represents a router. If the SPT changes after I-SPF calculation, PRC processes all the leaves only on the changed node. If the SPT remains unchanged, PRC processes only the changed leaves. For example, if IS-IS is enabled on an interface of a node, the SPT calculated by I-SPF remains unchanged. PRC updates only the routes of this interface, consuming less CPU resources.

    PRC working with I-SPF further improves the convergence performance of the network. It is an improvement of the original SPF algorithm.

  • Intelligent timer: applies to LSP generation and SPF calculation. The first timeout period of the intelligent timer is fixed. Before the intelligent timer expires, if an event that triggers the timer occurs, the next timeout period of the intelligent timer increases.

    Although the route calculation algorithm is improved, the long interval for triggering route calculation affects the convergence speed. Frequent network changes also consume too many CPU resources. The SPF intelligent timer addresses both of these problems. In general, an IS-IS network is stable under normal conditions. The probability of the occurrence of many network changes is very minimal, and IS-IS does not calculate routes frequently. The period for triggering the route calculation is very short (milliseconds). If the topology of the network changes very often, the intelligent timer increases the interval for the calculation times to avoid too much CPU consumption. The original mechanism uses a timer with uniform intervals, which makes fast convergence and low CPU consumption impossible to achieve.

    The LSP generation intelligent timer is similar to the SPF intelligent timer. When the LSP generation intelligent timer expires, the system generates a new LSP based on the current topology. The LSP generation timer is designed as an intelligent timer to respond to emergencies (such as the interface is Up or Down) quickly and speed up the network convergence.

  • LSP fast flooding: speeds up the flooding of LSPs.

    In most cases, when an IS-IS router receives new LSPs from other routers, it updates the LSPs in its LSDB and periodically floods the updated LSPs according to a timer.

    LSP fast flooding speeds up LSDB synchronization because it allows a device to flood fewer LSPs than the specified number before route calculation when the device receives one or more new LSPs. This mechanism also speeds up network convergence.

Priority-based Convergence

Priority-based IS-IS convergence ensures that specific routes are converged first when a great number of routes need to be converged. You can assign a high convergence priority to routes for key services so that these routes are converged quickly. This reduces the impact of route convergence on key services. Different routes can be set with different convergence priorities so that important routes can be converged first. This improves network reliability.

IS-IS Administrative Tag

Administrative tags control the advertisement of IP prefixes in an IS-IS routing domain to simplify route management. You can use administrative tags to control the import of routes of different levels and different areas and control IS-IS multi-instances running on the same router.

Figure 7-76 IS-IS networking

In Figure 7-76, RouterA in Area 4 needs to communicate with RouterB in Area 5, RouterC in Area 3, and RouterD in Area 2. To ensure information security, it is required that other routers in Level-1 areas (Areas 2, 3, and 5) should not receive the packets sent from RouterA. To meet this requirement, configure the same administrative tag for IS-IS interfaces on RouterB, RouterC, and RouterD and configure the Level-1-2 router in Area 4 to leak only the routes matching the configured administrative tag from Level-2 to Level-1 areas. This allows RouterA to communicate with only RouterB, RouterC, and RouterD. Figure 7-77 shows the topology formed on RouterA.

Figure 7-77 IS-IS administrative tag application

The value of an administrative tag is associated with certain attributes. If the cost-style is wide, wide-compatible or compatible, when IS-IS advertises an IP address prefix with these attributes, IS-IS adds the administrative tag to the TLV in the prefix. The tag is flooded along with the prefix throughout the routing domain.

IS-IS Wide Metric

In ISO 10589, the maximum IS-IS interface metric value can only be 63 and the IS-IS cost style is narrow. A small range of metrics cannot meet the requirements on large-scale networks. Therefore, in RFC 3784, the maximum IS-IS interface metric value can reach 16777215, and the maximum IS-IS route metric value can reach 4261412864; in this case, the IS-IS cost style is wide.
  • The following lists the TLVs used in narrow mode:

    • TLV 128 (IP Internal Reachability TLV): carries IS-IS routes in a routing domain.

    • TLV 130 (IP External Reachability TLV): carries IS-IS routes outside a routing domain.

    • TLV 2 (IS Neighbors TLV): carries neighbor information.

  • The following lists the TLVs used in wide mode:

    • TLV 135 (Extended IP Reachability TLV): replaces the earlier IP reachability TLV and carries IS-IS routing information. This TLV expands the route metric and carries sub-TLVs.

    • TLV 22 (IS Extended Neighbors TLV): carries neighbor information.

Table 7-38 lists the cost styles of received and sent IS-IS routing information. The cost styles of received and sent IS-IS routing information vary according to the cost style configured on a device.

Table 7-38 Cost styles of received and sent IS-IS routing information

Cost Style Configured on a Device

Cost Style for Received IS-IS Routing Information

Cost Style for Sent IS-IS Routing Information

narrow

narrow

narrow

narrow-compatible

narrow&wide

narrow

compatible

narrow&wide

narrow&wide

wide-compatible

narrow&wide

wide

wide

wide

wide

When the cost-style is set to compatible, IS-IS sends the information in narrow mode and then in wide mode.

IS-IS in wide mode and IS-IS in narrow mode cannot communicate. If IS-IS in wide mode and IS-IS in narrow mode need to communicate, you must change the mode to enable all routers on the network to receive packets sent by other routers.

IS-IS LSP Fragment Extension

When an IS-IS router needs to advertise the LSPs that contain much information, the IS-IS router generates multiple LSP fragments to carry more IS-IS information.

IS-IS LSP fragments are identified by the LSP Number field in their LSP IDs. This field is of 1 byte. An IS-IS process can generate a maximum of 256 LSP fragments; therefore, only a limited number of routes can be carried.

As defined in RFC 3786, virtual system IDs can be configured and virtual LSPs that carry routing information can be generated for IS-IS.

Concepts

  • Originating system: is a router that runs the IS-IS protocol. A single IS-IS process can function as multiple virtual routers to advertise LSPs, and the originating system refers to the IS-IS process.

  • Normal System-ID: is the system ID of the originating system.

  • Virtual System: is the system identified by the additional system ID to generate extended LSP fragments. These fragments carry additional system IDs in their LSP IDs.

  • Additional System-ID: is assigned by network administrators to identify a virtual system. A maximum of 256 extended LSP fragments can be generated for each additional system ID.
    Like a normal system ID, an additional system ID must be unique in a routing domain.
  • TLV 24 (IS Alias ID TLV): describes the relationship between the originating system and virtual system.

Principles

In IS-IS, each system ID identifies a system, which can generate a maximum of 256 LSP fragments. With more additional system IDs (up to 50 virtual systems can be configured), an IS-IS process can generate a maximum of 13,056 LSP fragments.

After LSP fragment extension is configured, the system prompts you to restart the IS-IS process if information is lost because LSPs overflow. After being restarted, the originating system loads as much routing information to LSPs, adds the overloaded information to the LSPs of the virtual system for transmission, and uses TLV 24 to notify other routers of its relationship with the virtual system.

Operating Modes

An IS-IS router can run the LSP fragment extension feature in two modes.
Figure 7-78 IS-IS LSP fragment extension

Operating Mode Usage Scenario Principles Example Precautions
Mode-1 Some routers on the network do not support LSP fragment extension.

Virtual systems participate in SPF calculation. The originating system advertises LSPs containing information about links to each virtual system. Similarly, each virtual system advertises LSPs containing information about links to the originating system. Virtual systems look like the physical routers that connect to the originating system.

Mode-1 is a transitional mode for the earlier versions that do not support LSP fragment extension. In earlier versions, IS-IS cannot identify the IS Alias ID TLV and processes the received LSP that is advertised by a virtual system as an LSP advertised by an IS-IS process.

In Figure 7-78, RouterB does not support LSP fragment extension, and RouterA is configured to support LSP fragment extension in mode-1. RouterA1 and RouterA2 are virtual systems of RouterA and send LSPs carrying some routing information of RouterA. After receiving LSPs from RouterA, RouterA1, and RouterA2, RouterB considers that there are three individual routers at the remote end and calculates routes. Because the cost of the route from RouterA to RouterA1 and the cost of the route from RouterA to RouterA2 are both 0, the cost of the route from RouterB to RouterA is the same as the cost of the route from RouterB to RouterA1.

The LSP sent by a virtual system contains the same area address and overload bit as those in a common LSP. If the LSPs sent by a virtual system contain TLVs specified in other features, these TLVs must be the same as those in common LSPs.

The virtual system carries neighbor information indicating that the neighbor is the originating system, with the metric equal to the maximum value minus 1. The originating system carries neighbor information indicating that the neighbor is the virtual system, with the metric 0. This ensures that the virtual system is the downstream node of the originating system when other routers calculate routes.

Mode-2 All the routers on the network support LSP fragment extension.

Virtual systems do not participate in SPF calculation. All the routers on the network know that the LSPs generated by virtual systems actually belong to the originating system.

An IS-IS router working in mode-2 can identify the IS Alias ID TLV, which is used as a reference for calculating the SPT and routes.

In Figure 7-78, RouterB supports LSP fragment extension, and RouterA is configured to support LSP fragment extension in mode-2. RouterA1 and RouterA2 are virtual systems of RouterA and send LSPs carrying some routing information of RouterA. When receiving LSPs from RouterA1 and RouterA2, RouterB obtains the IS Alias ID TLV and knows that the originating system of RouterA1 and RouterA2 is RouterA. RouterB then considers that information advertised by RouterA1 and RouterA2 belongs to RouterA. -

When the originating system and virtual system send the LSPs with fragment number 0, the LSPs must carry the IS Alias ID TLV to indicate the originating system regardless of the operation mode (mode-1 or mode-2).

IS-IS Host Name Mapping

The IS-IS host name mapping mechanism maps host names to system IDs for IS-IS devices, including dynamic host name mapping and static host name mapping. Dynamic host name mapping takes precedence over static host name mapping. When both a dynamic host name and a static host name are configured, the dynamic host name takes effect.

On an IS-IS router where host name exchange is disabled, information about IS-IS neighbors and LSDBs shows that each device in an IS-IS routing domain is identified by a system ID with 12-digit hexadecimal number, for example, aaaa.eeee.1234. This device identification method is complex and not easy to use. The host name exchange mechanism facilitates IS-IS network management and maintenance.

The system ID is replaced by a host name in the following situations:
  • When an IS-IS neighbor is displayed, the system ID of the IS-IS neighbor is replaced by its host name. When the neighbor is the DIS, the system ID of the DIS is also replaced by its host name.

  • When an LSP in the IS-IS LSDB is displayed, the system ID in the LSP ID is replaced by the host name of the IS-IS device that advertises the LSP.

  • When details about the IS-IS LSDB are displayed, the Host Name field is added to the LSP generated by the device where dynamic host name exchange is enabled, and the system ID in the Host Name field is replaced by the dynamic host name of the device that generates the LSP.

Dynamic Host Name Mapping

On a device where dynamic host name mapping is enabled, dynamic host name information is advertised as TLV 137 (Dynamic Hostname TLV) in LSPs. When you run IS-IS commands on other devices to view IS-IS information, the system ID of the local device is replaced by the configured host name. The host name is easier to identify and memorize than the system ID.

The Dynamic Hostname TLV is optional and can be inserted anywhere in an LSP. The value of this TLV cannot be empty. A device can determine whether to send LSPs carrying TLV 137, while the device that receives LSPs can determine whether to ignore TLV 137 or whether to obtain TLV 137 for its mapping table.

Static Host Name Mapping

Static host name mapping allows you to configure the mapping between host names and system IDs of other IS-IS devices on a device. Static host name mapping takes effect only on the local device and is not advertised using LSPs.

IS-IS NSR

As networks develop, the demand for data, audio, and video services is growing, which imposes increasing requirements on IP network reliability. If an AMB/SMB switchover is performed on a device due to a maintenance operation or a single point of failure, routes may fail to converge, which may result in traffic loss or even a network breakdown. Non-stop routing (NSR) can address this problem and ensure uninterrupted forwarding of key services.

With Routing Information Protocol next generation (IS-IS) NSR, IS-IS real-time data is synchronized between the AMB and SMB. After an AMB/SMB switchover is performed on a device, the SMB takes over services from the AMB, and neighbors are unaware of the local fault. After the switchover, the new AMB recovers IS-IS immediately based on the synchronized IS-IS real-time data. Therefore, neighbors are unaware of the switchover as well. IS-IS NSR requires synchronization of the following data:

  • All configuration data, such as information about neighbors, timer parameters, and process configurations.

  • Dynamic data, such as the interface parameters and state, and information about neighbors and the link state database (LSDB).

BFD for IS-IS

In IS-IS, the interval for sending Hello packets is 10s, and the holddown time for keeping the neighbor relationship is three times the interval for sending Hello packets. If a router does not receive a Hello packet from its neighbor within the holddown time, the router deletes the corresponding neighbor relationship. This indicates that the router detects neighbor faults in seconds. Second-level fault detection, however, may result in heavy packet loss on high-speed networks.

Bidirectional forwarding detection (BFD) provides light-load and millisecond-level link fault detection to prevent heavy packet loss. BFD is not used to substitute the Hello mechanism of IS-IS but helps IS-IS rapidly detect the faults on neighbors or links and instructs IS-IS to recalculate routes for packet forwarding.

In Figure 7-79, basic IS-IS functions are configured on every router, and BFD for IS-IS is enabled on RouterA and RouterB.

Figure 7-79 BFD for IS-IS

When a fault occurs on the primary link, BFD fast detects the fault and reports it to IS-IS. IS-IS sets the neighbors of the interface on the faulty link to Down, which triggers topology calculation, and updates LSPs so that neighbors such as RouterC can receive the updated LSPs from RouterB. This process implements fast network convergence.

Classification of BFD for IS-IS

BFD for IS-IS includes static BFD for IS-IS and dynamic BFD for IS-IS.

Table 7-39 Two implementation modes for BFD for IS-IS
Implementation Mode Principles Differences
Static BFD for IS-IS BFD session parameters, including local and remote discriminators, are manually configured using commands, and the requests for establishing BFD sessions are manually delivered.
  • Static BFD can be manually controlled and is easy to deploy. To save memory and ensure reliability of key links, deploy BFD on specified links.

  • Establishing and deleting BFD sessions need to be manually triggered and lack flexibility. Configuration errors may occur. For example, if an incorrect local or remote discriminator is configured, a BFD session cannot work properly.

Dynamic BFD for IS-IS BFD sessions are dynamically created but not manually configured. When detecting faults, BFD informs IS-IS of the faults through the routing management (RM) module. IS-IS then turns the neighbors Down, rapidly advertises the changed LSPs, and performs incremental SPF. This implements fast route convergence. Dynamic BFD is more flexible than static BFD. In dynamic BFD, routing protocols trigger the setup of BFD sessions, preventing the configuration errors caused by manual configuration. Dynamic BFD is easy to configure and applies to the scenarios where BFD needs to be configured on the entire network.

BFD uses local and remote discriminators to differentiate multiple BFD sessions between the same pair of systems.

Because IS-IS establishes only single-hop neighbors, BFD for IS-IS detects only single-hop links between IS-IS neighbors.

Establishment and Deletion of BFD Sessions

The RM module provides related services for association with the BFD module for IS-IS. Through RM, IS-IS prompts BFD to set up or tear down BFD sessions by sending notification messages. In addition, BFD events are transmitted to IS-IS through RM.

Conditions for setting up a BFD session
  • Basic IS-IS functions are configured on each router and IS-IS is enabled on the interfaces of the routers.

  • BFD is globally enabled on each router, and BFD is enabled on a specified interface or process.

  • BFD is enabled on interfaces or processes, and the neighbors are Up. A DIS needs to be elected on a broadcast network.

Process of setting up a BFD session
  • P2P network

    After the conditions for setting up a BFD session are satisfied, IS-IS instructs BFD through RM to directly set up a BFD session between neighbors.

  • Broadcast network

    After the conditions for establishing BFD sessions are met, and the DIS is elected, IS-IS instructs BFD through RM to establish a BFD session between the DIS and each router. No BFD session is established between non-DISs.

    On a broadcast network, routers (including non-DIS routers) of the same level on a network segment can establish neighbor relationships. In the implementation of BFD for IS-IS, however, BFD sessions are established only between a DIS and a non-DIS. On a P2P network, BFD sessions are directly established between neighbors.

    If a Level-1-2 neighbor relationship is set up between two routers on a link, IS-IS sets up two BFD sessions for the Level-1 and Level-2 neighbors on a broadcast network, but sets up only one BFD session on a P2P network.

Conditions for tearing down a BFD session
  • P2P network

    When a neighbor relationship that was set up on P2P interfaces by IS-IS is down (that is, the neighbor relationship is not in the Up state) or when the IP protocol type of a neighbor is deleted, IS-IS tears down the BFD session.

  • Broadcast network

    When a neighbor relationship that was set up on P2P interfaces by IS-IS is torn down (that is, the neighbor relationship is not in the Up state), when the IP protocol type of a neighbor is deleted, or when the DIS is re-elected, IS-IS tears down the BFD session.

After dynamic BFD is globally disabled in an IS-IS process, the BFD sessions on all the interfaces in this IS-IS process are deleted.

IS-IS Responding to BFD Session Down Event

When detecting a link failure, BFD generates a Down event, and then notifies RM of the event. RM then instructs IS-IS to deletes the neighbor relationship. IS-IS recalculates routes to speed up route convergence on the entire network.

When both the local router and its neighbor are Level-1-2 routers, they establish two neighbors of different levels. Then IS-IS establishes two BFD sessions for the Level-1 neighbor and Level-2 neighbor respectively. When BFD detects a link failure, it generates a Down event and informs the RM module of the event. The RM module then instructs IS-IS to delete the neighbor relationship of a specific level.

IS-IS Auto FRR

With the development of networks, the services such as Voice over IP (VoIP) and online video services require high-quality real-time transmission. Nevertheless, if an IS-IS link fault occurs, traffic can be switched to a new link only after the processes, including fault detection, LSP update, LSP flooding, route calculation, and FIB entry delivery, are complete. As a result, it takes much more than 50 ms to rectify the fault, which cannot meet the requirement for real-time transmission services on the network.

Complying with RFC 5286 (Basic Specification for IP Fast Reroute Loop-Free Alternates), IS-IS Auto FRR protects traffic when links or nodes become faulty. IS-IS Auto FRR allows the forwarding system to rapidly detect such faults and take measures to restore services as soon as possible.

In most cases, you can bind BFD to IS-IS Auto FRR to ensure that the fault recovery time is within 50 ms. When BFD detects a link fault on an interface, the BFD session goes Down, triggering FRR on the interface. Subsequently, traffic is switched from the faulty link to the backup link, which protects services.

Principles

IS-IS Auto FRR pre-computes a backup link by using the Loop-Free Alternate (LFA) algorithm, and then adds the backup link and the primary link to the forwarding table. In the case of an IS-IS network failure, IS-IS Auto FRR can fast switch traffic to the backup link before routes on the control plane converge. This ensures normal transmission of traffic and improves the reliability of the IS-IS network.

The backup link is calculated through the LFA algorithm. With the neighbor that can provide the backup link being the root, the shortest path to the destination node is calculated by a device through the SPF algorithm. Then, the loop-free backup link is calculated according to the inequality defined in RFC 5286.

IS-IS Auto FRR can filter backup routes that need to be added to the IP routing table. Only the backup routes matching the filtering policy are added to the IP routing table. In this manner, users can flexibly control the addition of IS-IS backup routes to the IP routing table.

Applications

IS-IS Auto FRR support traffic engineering (TE) links, including the following types:

  • IP protecting TE

    As shown in Figure 7-80, the TE tunnel has the smallest IS-IS cost among the paths from RouterS to RouterD. Therefore, RouterS selects the TE tunnel as the primary path to RouterD. The path RouterS->RouterN->RouterD has the second smallest cost. According to the LFA algorithm, RouterS selects the path RouterS->RouterN->RouterD as the backup path. The outbound interface of the backup path is the interface that connects RouterS to RouterN.

    If the outbound interface of the backup link is the actual outbound interface of the TE tunnel, IP protecting TE fails.

    Figure 7-80 IP protecting TE

  • TE protecting IP

    As shown in Figure 7-81, the physical path RouterS-->RouterN-->RouterD has the smallest IS-IS metric among the paths from RouterS to RouterD. Therefore, RouterS prefers the path RouterS-->RouterN-->RouterD as the primary path from RouterS to RouterD. The IS-IS cost of the TE tunnel is 12, and the explicit path of the TE tunnel is the direct link from RouterS to RouterD. The IS-IS metric of the direct link from RouterS to RouterD is 13, which is greater than the IS-IS metric of the TE tunnel. Therefore, IS-IS selects the TE tunnel as the backup path. TE protecting IP is implemented.

    Figure 7-81 TE protecting IP

IS-IS Auto FRR traffic protection is classified into link protection and link-node dual protection.

Figure 7-82 IS-IS Auto FRR link protection

Figure 7-83 IS-IS Auto FRR link-node dual protection

Table 7-40 IS-IS Auto FRR traffic protection
Traffic Protection Type Object Protected Condition Application Example
Link protection Traffic passing through a specific link

The link cost must satisfy the following inequality:

Distance_opt(N,D) < Distance_opt(N,S) + Distance_opt(S,D)

In Figure 7-82, traffic is transmitted from RouterS to RouterD. The link cost satisfies the link protection inequality. When the primary link fails, RouterS switches the traffic to the backup link RouterS->RouterN so that the traffic can be further transmitted along downstream paths. This ensures that the traffic interruption time is within 50 ms.

Link-node dual protection Next-hop node or link from the local node to the next-hop node. Node protection takes precedence over link protection.
Link-node dual protection must satisfy the following conditions:
  • The link cost must satisfy the following inequality:

    Distance_opt(N,D) < Distance_opt(N,S) + Distance_opt(S,D)

  • The interface cost of the router must satisfy the following inequality:

    Distance_opt(N,D) < Distance_opt(N,E) + Distance_opt(E,D)

In Figure 7-83, traffic is transmitted along the path RouterS->RouterE->RouterD. The link cost satisfies the link protection inequality. When RouterE or the link between RouterS and RouterE fails, RouterS switches the traffic to the backup link RouterS->RouterN so that the traffic can be further transmitted along downstream paths. This ensures that the traffic interruption time is within 50 ms.

In Table 7-40, Distance_opt(X,Y) indicates the cost of the optimal path between node X and node Y. S indicates the source node of traffic; E indicates the faulty node; N indicates the node on the backup link; D indicates the destination node of traffic.

IS-IS Multi-Instance and Multi-Process

On a VPN-supporting device, you can associate multiple VPN instances with multiple IS-IS processes to implement IS-IS multi-instance. IS-IS multi-process allows you to create multiple IS-IS processes in the same VPN (or on the public network). These IS-IS processes are independent of each other. Route exchange between IS-IS processes is similar to route exchange between routing protocols.

Each IS-IS process can be bound to a specified VPN instance. A typical application is as follows: In a VPN, IS-IS runs between PEs and CEs and also runs on the VPN backbone network. On the PEs, the two IS-IS processes are independent of each other.

IS-IS multi-instance and multi-process have the following characteristics:
  • IS-IS multi-processes share an RM routing table. IS-IS multi-instances use the RM routing tables in VPNs, and each VPN has its own RM routing table.

  • IS-IS multi-process allows a set of interfaces to be associated with a specified IS-IS process. This ensures that the specified IS-IS process performs all the protocol operations only on this set of interfaces. In this manner, multiple IS-IS processes can work on a single router and each process is responsible for managing a unique set of interfaces.

  • When creating an IS-IS process, you can bind it to a VPN instance to associate the IS-IS process with the VPN instance. The IS-IS process accepts and processes only the events related to the VPN instance. When the bound VPN instance is deleted, the IS-IS process is also deleted.

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Updated: 2019-12-13

Document ID: EDOC1000041694

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