IS-IS Topology Structure

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 7-234 shows a network that runs IS-IS. The network is similar to an OSPF network topology with multiple areas. The backbone area contains all the routers in Area 1 and Level-1-2 routers in other areas.

Figure 7-234  IS-IS topology I

Figure 7-235 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 7-235  IS-IS topology II

• 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 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.

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 7-236  Pseudonode

As shown in Figure 7-236, 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.

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

Figure 7-237  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 10.10.1.1 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 10.10.1.1 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 010.010.001.001.

  • Divide the extended address 010.010.001.001 into three parts, each of which consists of four decimal digits. Then system ID 0100.1000.1001 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 itself and consists of an area ID and a system ID. 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.

  • 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 7-238 shows the TLV format. A TLV is also called a code-length-value (CLV).

Figure 7-238  TLV format

TLVs vary according to PDU types, as shown in Table 7-107.

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

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 7-239 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 7-239  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.

Process of Exchanging IS-IS LSPs

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.

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.

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.

Figure 7-240  Process of updating LSDBs on a broadcast link

1. As shown in Figure 7-240, 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.

Figure 7-241  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 7-241 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.

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 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 7-243  IS-IS Overload

As shown in Figure 7-243, 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.

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.

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 7-246  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 7-246, 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 7-246, 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).

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.

Concepts

IS-IS GR involves two roles, namely, GR restarter and GR helper.

• GR restarter: is a device that has the GR capability and restarts in GR mode.

• GR helper: is a device that has the GR capability and helps the GR restarter complete the GR process. The GR restarter must have the GR helper capability.

To implement GR, IS-IS uses TLV 211 (restart TLV) and three timers, T1, T2, and T3.

Restart TLV

The restart TLV is an extended part of an IS-to-IS Hello (IIH) PDU. All IIH packets of the router that supports IS-IS GR contain the restart TLV. The restart TLV carries the parameters for the protocol restart. Figure 7-247 shows the format of the restart TLV.

Figure 7-247  Restart TLV

Table 7-109 describes the fields of the restart TLV.

Timers

Three timers are introduced to enhance IS-IS GR: T1, T2, and T3.

• T1: If the GR restarter has already sent an IIH packet with RR being set but does not receive any IIH packet that carries the restart TLV and the RA set from the GR helper even after the T1 timer expires, the GR restarter resets the T1 timer and continues to send the restart TLV. If the ACK packet is received or the T1 timer expires three times, the T1 timer is deleted. The default value of a T1 timer is 3 seconds.

  Any interface enabled with IS-IS GR maintains a T1 timer. On a Level-1-2 router, broadcast interfaces maintain a T1 timer for Level-1 and Level-2 neighbor relationships.

• T2: is the time from when the GR restarter restarts until the LSDBs of all devices of the same level are synchronized. T2 is the maximum time that the system waits for synchronization of all LSDBs. T2 is generally 60 seconds.

  Level-1 and Level-2 LSDBs maintain their respective T2 timers.

• T3: is the maximum time during which the GR restarter performs GR. The T3 initial value is 65535 seconds. After the IIH packets that carry the RA are received from neighbors, the T3 value becomes the smallest value among the Remaining Time fields of the IIH packets. If the T3 timer expires, GR fails.

  The entire system maintains a T3 timer.

Session Mechanism

For differentiation, GR triggered by the master/slave switchover or the restart of an IS-IS process is referred to as restarting. In restarting, the FIB table remains unchanged. GR triggered by router restart is referred to as starting. In starting, the FIB table is updated.

The following describes the process of IS-IS GR in restarting and starting modes:

• Figure 7-248 shows the process of IS-IS restarting.

  Figure 7-248  IS-IS restarting
  
  

  1. After performing the protocol restart, the GR restarter performs the following actions:

     • Starts T1, T2, and T3 timers.

     • Sends IIH packets that contain the restart TLV from all interfaces. In such a packet, RR is set to 1, and RA and SA are set to 0.

  2. After receiving an IIH packet, the GR helper performs the following actions:

     • Maintains the neighbor relationship and refreshes the current Holdtime.

     • Replies with an IIH packet containing the restart TLV. In the packet, RR is set to 0; RA is set to 1, and the value of the Remaining Time field indicates the period from the current moment to the timeout of the Holdtime.

     • Sends CSNPs and all LSPs to the GR restarter.

     On a P2P link, a neighbor must send CSNPs.

     On a LAN link, only the neighbor of the DIS sends CSNPs. If the DIS is restarted, a temporary DIS is elected from the other routers on the LAN.

     If the neighbor does not have the GR helper capability, it ignores the restart TLV and resets the adjacency with the GR restarter according to normal IS-IS processing.

  3. After the GR restarter receives the IIH response packet, in which RR is set to 0 and RA is set to 1, from the neighbor, it performs the following actions:

     • Compares the current value of the T3 timer with the value of the Remaining Time field in the packet. The smaller value is taken as the value of the T3 timer.

     • Deletes the T1 timer maintained by the interface that receives the ACK packet and CSNPs.

     • If the interface does not receive the ACK packet or CSNPs, the GR restarter constantly resets the T1 timer and resends the IIH packet that contains the restart TLV. If the number of timeouts of the T1 timer exceeds the threshold value, the GR restarter forcibly deletes the T1 timer and turns to the normal IS-IS processing to complete LSDB synchronization.

  4. After the GR restarter deletes the T1 timers on all interfaces, the synchronization with all neighbors is complete when the CSNP list is cleared and all LSPs are collected. The T2 timer is then deleted.

  5. After the T2 timer is deleted, the LSDB of the level is synchronized.

     • In the case of a Level-1 or Level-2 router, SPF calculation is triggered.

     • In the case of a Level-1-2 router, determine whether the T2 timer on the router of the other level is also deleted. If both T2 timers are deleted, SPF calculation is triggered. Otherwise, the router waits for the T2 timer of the other level to expire.

  6. After all T2 timers are deleted, the GR restarter deletes the T3 timer and updates the FIB table. The GR restarter re-generates the LSPs of each level and floods them. During LSDB synchronization, the GR restarter deletes the LSPs generated before restarting.

  7. At this point, the IS-IS restarting of the GR restarter is complete.

• The starting device does not retain the FIB table. The starting device depends on the neighbors, whose adjacency with itself is Up before it starts, to reset their adjacency and suppress the neighbors from advertising their adjacency. The IS-IS starting process is different from the IS-IS restarting process, as shown in Figure 7-249.

  Figure 7-249  IS-IS starting
  
  
  1. After the GR restarter is started, it performs the following actions:

     • Starts the T2 timer for the synchronization of LSDBs of each level.

     • Sends IIH packets that contain the restart TLV from all interfaces.

       If RR in the packet is set to 0, a router is started.

       If SA in the packet is set to 1, the router requests its neighbor to suppress the advertisement of their adjacency before the neighbor receives the IIH packet in which SA is set to 0.

  2. After the neighbor receives the IIH packet that carries the restart TLV, it performs the following actions depending on whether GR is supported:

     • GR is supported.

       Re-initiates the adjacency.

       Deletes the description of the adjacency with the GR restarter from the sent LSP. The neighbor also ignores the link connected to the GR restarter when performing SPF calculation until it receives an IIH packet in which SA is set to 0.

     • GR is not supported.

       Ignores the restart TLV and resets the adjacency with the GR restarter.

       Replies with an IIH packet that does not contain the restart TLV. The neighbor then returns to normal IS-IS processing. In this case, the neighbor does not suppress the advertisement of the adjacency with the GR restarter. On a P2P link, the neighbor also sends a CSNP.

  3. After the adjacency is re-initiated, the GR restarter re-establishes the adjacency with the neighbors on each interface. When an adjacency set on an interface is in the Up state, the GR restarter starts the T1 timer for the interface.

  4. After the T1 timer expires, the GR restarter sends an IIH packet in which both RR and SA are set to 1.

  5. After the neighbor receives the IIH packet, it replies with an IIH packet, in which RR is set to 0 and RA is set to 1, and sends a CSNP.

  6. After the GR restarter receives the IIH ACK packet and CSNP from the neighbor, it deletes the T1 timer.

     If the GR restarter does not receive the IIH packet or CSNP, it constantly resets the T1 timer and resends the IIH packet in which RR and SA are set to 1. If the number of the timeouts of the T1 timer exceeds the threshold value, the GR restarter forcibly deletes the T1 timer and turns to the normal IS-IS processing to complete LSDB synchronization.

  7. After receiving the CSNP from the helper, the GR restarter synchronizes the LSDB.

  8. After the LSDB of this level is synchronized, the T2 timer is deleted.

  9. After all T2 timers are deleted, the SPF calculation is started and LSPs are regenerated and flooded.

  10. At this point, the IS-IS starting of the GR restarter is complete.

Classification of BFD for IS-IS

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

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.

• 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.

• 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.

• 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 delete 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.