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ME60 V800R010C10SPC500 Feature Description - WAN Access 01

This is ME60 V800R010C10SPC500 Feature Description - WAN Access
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Basic Concepts of OSPF

Basic Concepts of OSPF

This section describes the basic Open Shortest Path First (OSPF) concepts.

Router ID

A router ID is a 32-bit unsigned integer, which identifies a ME device in an autonomous system (AS). A router ID must exist before the ME device runs OSPF.

A router ID can be manually configured or automatically obtained.

If no router ID has been configured, the ME device automatically obtains a router ID using the following methods in descending order of priority.
  1. The ME device preferentially selects the largest IP address from its loopback interface addresses as the router ID.
  2. If no loopback interface has been configured, the ME device selects the largest IP address from its interface IP addresses as the router ID.

A ME device can obtain a router ID again only after a router ID is reconfigured for the ME device or an OSPF router ID is reconfigured and the OSPF process restarts.

Area

When a large number of ME devices run OSPF, link state databases (LSDBs) become very large and require a large amount of storage space. Large LSDBs also complicate shortest path first (SPF) computation and overload the ME devices. As the network grows, the network topology changes, which results in route flapping and frequent OSPF packet transmission. When a large number of OSPF packets are transmitted, bandwidth usage efficiency decreases, and each router on a network has to recalculate routes in case of any topology change.

OSPF resolves this problem by partitioning an AS into different areas. An area is regarded as a logical group, and each group is identified by an area ID. A ME device, not a link, resides at the border of an area. A network segment or link can belong only to one area. An area must be specified for each OSPF interface.

OSPF areas include common areas, stub areas, and not-so-stubby areas (NSSAs). Table 6-2 describes these OSPF areas.

Table 6-2 OSPF areas

Area Type

Function

Notes

Common area

By default, OSPF areas are defined as common areas. Common areas include:

  • Standard area: transmits intra-area, inter-area, and external routes.
  • Backbone area: connects to all other OSPF areas and transmits inter-area routes. The backbone area is area 0. Routes between non-backbone areas must be forwarded through the backbone area.
  • The backbone area must have all its devices connected.
  • All non-backbone areas must remain connected to the backbone area.

Stub area

A stub area is a non-backbone area with only one area border router (ABR) and generally resides at the border of an AS. The ABR in a stub area does not transmit received AS external routes, which significantly decreases the number of entries in the routing table on the ABR and the amount of routing information to be transmitted. To ensure the reachability of AS external routes, the ABR generates a default route and advertises the route to non-ABRs in the stub area.

A totally stub area allows only intra-area routes and ABR-advertised Type 3 link state advertisements (LSAs) carrying a default route to be advertised within the area.

  • The backbone area cannot be configured as a stub area.
  • An autonomous system boundary router (ASBR) cannot exist in a stub area. Therefore, AS external routes cannot be advertised within the stub area.
  • A virtual link cannot pass through a stub area.

NSSA

An NSSA is similar to a stub area. An NSSA does not advertise Type 5 LSAs but can import AS external routes. ASBRs in an NSSA generate Type 7 LSAs to carry the information about the AS external routes. The Type 7 LSAs are advertised only within the NSSA. When the Type 7 LSAs reach an ABR in the NSSA, the ABR translates the Type 7 LSAs into Type 5 LSAs and floods them to the entire AS.

A totally NSSA allows only intra-area routes to be advertised within the area.

  • An ABR in an NSSA advertises default routes in Type 7 LSAs within the NSSA.
  • All inter-area routes are advertised by ABRs.
  • A virtual link cannot pass through an NSSA.

Router Type

Routers are classified as internal routers, ABRs, backbone routers, or ASBRs by location in an AS. Figure 6-1 shows the four router types.

Figure 6-1 Router type layout
Table 6-3 describes the four router types.
Table 6-3 Router type descriptions

Router Type

Description

Internal router

All interfaces on an internal router belong to the same OSPF area.

ABR

An ABR connects the backbone area and non-backbone areas, and it can connect to the backbone area either physically or logically.

An ABR can belong to two or more areas, one of which must be a backbone area.

Backbone router

Backbone routers include internal routers in the backbone area and all ABRs.

At least one interface on a backbone router belongs to the backbone area.

ASBR

An ASBR exchanges routing information with other ASs.

An ASBR may not reside at the boundary of an AS, and it can be an internal router or an ABR.

LSA

OSPF encapsulates routing information into LSAs for transmission. Table 6-4 describes LSAs and their functions.

Table 6-4 LSAs and their functions

LSA Type

Function

Router-LSA (Type 1)

Describes the link status and cost of a ME device. Router-LSAs are generated by a ME device and advertised within the area to which the ME device belongs.

Network-LSA (Type 2)

Describes the link status of all routers on the local network segment. Network-LSAs are generated by a designated router (DR) and advertised within the area to which the DR belongs.

Network-summary-LSA (Type 3)

Describes routes on a network segment of an area. Network-summary-LSAs are generated by an ABR and advertised to other areas, excluding the totally stub area and totally NSSA. For example, an ABR belongs to both area 0 and area 1, area 0 has a network segment 10.1.1.0, and area 1 has a network segment 11.1.1.0. In this case, for area 0, the ABR generates Type 3 LSAs destined for the network segment 11.1.1.0; for area 1, the ABR generates Type 3 LSAs destined for the network segment 10.1.1.0.

ASBR-summary-LSA (Type 4)

Describes routes of an area to the ASBRs of other areas. ASBR-summary-LSAs are generated by an ABR and advertised to other areas, excluding the stub area, totally stub area, NSSA, totally NSSA, and area to which the ASBR of the route generation ABR belongs.

AS-external-LSA (Type 5)

Describes AS external routes, which are advertised to all areas, excluding the stub area, totally stub area, NSSA, and totally NSSA. AS-external-LSAs are generated by an ASBR.

NSSA-LSA (Type 7)

Describes AS external routes. NSSA-LSAs are generated by an ASBR and advertised only within NSSAs.

Opaque-LSA (Type 9/Type 10/Type 11)

Provides a general mechanism for OSPF extension. Different types of LSAs are described as follows:

  • Type 9 LSAs are advertised only on the network segment where the interface advertising the LSAs resides. Grace LSAs used in graceful restart (GR) are Type 9 LSAs.
  • Type 10 LSAs are advertised within an OSPF area. LSAs that are used to support traffic engineering (TE) are Type 10 LSAs.
  • Type 11 LSAs are advertised within an AS but have not been used in practice.

Table 6-5 describes whether a type of LSA is supported in an area.

Table 6-5 Support status of LSAs in different types of areas

Area Type

Router-LSA (Type 1)

Network-LSA (Type 2)

Network-summary-LSA (Type 3)

ASBR-summary-LSA (Type 4)

AS-external-LSA (Type 5)

NSSA-LSA (Type 7)

Common area (including standard and backbone areas)

Supported Supported Supported Supported Supported Not supported

Stub area

Supported Supported Supported Not supported Not supported Not supported

Totally stub area

Supported Supported Not supported Not supported Not supported Not supported

NSSA

Supported Supported Supported Not supported Not supported Supported
Totally NSSA Supported Supported Not supported Not supported Not supported Supported

Packet Type

OSPF packets are classified as Hello, Database Description (DD), Link State Request (LSR), Link State Update (LSU), or Link State Acknowledgment (LSAck) packets. Table 6-6 describes OSPF packets and their functions.

Table 6-6 OSPF packets and their functions

Packet Type

Function

Hello packet

Hello packets are periodically sent to discover and maintain OSPF neighbor relationships.

DD packet

DD packets contain the summaries of LSAs in the local LSDB. DD packets are used for LSDB synchronization between two ME devices.

LSR packet

LSR packets are sent to OSPF neighbors to request required LSAs.

A ME device sends LSR packets to its OSPF neighbor only after DD packets have been successfully exchanged.

LSU packet

LSU packets are used to transmit required LSAs to OSPF neighbors.

LSAck packet

LSAck packets are used to acknowledge received LSAs.

Route Type

Route types are classified as intra-area, inter-area, Type 1 external, or Type 2 external routes. Intra-area and inter-area routes describe the network structure of an AS. Type 1 or Type 2 AS external routes describe how to select routes to destinations outside an AS.

Table 6-7 describes OSPF routes in descending order of priority.

Table 6-7 OSPF routes

Route Type

Description

Intra-area route

-

Inter-area route

-

Type 1 external route

Type 1 external routes have high reliability.

Cost of a Type 1 external route = Cost of the route from a ME device to an ASBR + Cost of the route from the ASBR to the destination

When multiple ASBRs exist, the cost of each Type 1 external route equals the cost of the route from the local device to an ASBR plus the cost of the route from the ASBR to the destination. The cost is used for route selection.

Type 2 external route

Because a Type 2 external route has low reliability, its cost is considered to be much greater than the cost of any internal route to an ASBR.

Cost of a Type 2 external route = Cost of the route from an ASBR to the destination

If routes are imported by multiple ASBRs, the route with the smallest cost from the corresponding ASBR to its destination is selected. If the routes have the same cost from the corresponding ASBR to each route destination, the route with the smallest cost from the local router to the corresponding ASBR is selected. The cost of each Type 2 external route equals the cost of the route from the corresponding ASBR to the destination.

Network Type

Networks are classified as broadcast, non-broadcast multiple access (NBMA), point-to-multipoint (P2MP), or point-to-point (P2P) networks by link layer protocol. Table 6-8 describes the network types.

Table 6-8 OSPF network classification

Network Type

Link Layer Protocol

Graph

Broadcast

  • Ethernet
  • FDDI

NBMA

X.25

P2MP

Regardless of the link layer protocol, OSPF does not default the network type to P2MP. The network type must be manually changed to P2MP. In most cases, a non-fully connected NBMA network is changed to a P2MP network.

P2P

  • LAPB

DR and BDR

On broadcast or NBMA networks, any two ME devices need to exchange routing information. As shown in Figure 6-2, nME devices are deployed on the network. n x (n - 1)/2 adjacencies must be established. Any route change on a ME device is transmitted to other ME devices, which wastes bandwidth resources. OSPF resolves this problem by defining a DR and a backup designated router (BDR). After a DR is elected, all ME devices send routing information only to the DR. Then the DR broadcasts LSAs. ME devices other than the DR and BDR are called DR others. The DR others establish only adjacencies with the DR and BDR and not with each other. This process reduces the number of adjacencies established between ME devices on broadcast or NBMA networks.

Figure 6-2 Network topologies before and after a DR election

If the original DR fails, ME devices must reelect a DR and the ME devices except the new DR must synchronize routing information to the new DR. This process is lengthy, which may cause incorrect route calculations. A BDR is used to shorten the process. The BDR is a backup for a DR. A BDR is elected together with a DR. The BDR establishes adjacencies with all ME devices on the network segment and exchanges routing information with them. When the DR fails, the BDR immediately becomes a new DR. The ME devices need to reelect a new BDR, but this process does not affect route calculations.

The DR priority of a ME device interface determines its qualification for DR and BDR elections. The ME device interfaces with their DR priorities greater than 0 are eligible. Each ME device adds the elected DR to a Hello packet and sends it to other ME devices on the network segment. When both ME device interfaces on the same network segment declare that they are DRs, the ME device interface with a higher DR priority is elected as a DR. If the two ME device interfaces have the same DR priority, the ME device interface with a larger router ID is elected as a DR.

OSPF Multi-Process

OSPF multi-process allows multiple OSPF processes to independently run on the same ME device. Route exchange between different OSPF processes is similar to that between different routing protocols. A ME device's interface can belong only to one OSPF process.

OSPF multi-process is typically used on virtual private networks (VPNs) on which OSPF is deployed between provider edges (PEs) and customer edges (CEs). The OSPF processes on the PEs are independent of each other.

OSPF Default Route

A default route is the route whose destination address and mask are both all 0s. When no matching route is discovered, a ME device uses a default route to forward packets.

A default route generally applies to the following scenarios:

  • An ABR in an area advertises Type 3 LSAs carrying a default route within the area. The ME devices in the area use the received default route to forward inter-area packets.

  • An ASBR in an AS advertises Type 5 or Type 7 LSAs carrying a default route within the AS. The ME devices in the AS use the received default route to forward AS external packets.

OSPF routes are hierarchically managed. The priority of the default route carried in Type 3 LSAs is higher than the priority of the default route carried in Type 5 or Type 7 LSAs.

A ME device advertises LSAs carrying a default route by adhering to the following principles:

  • A ME device in an area can advertise LSAs carrying a default route only when the ME device has an interface connected to a device outside the area.
  • If a ME device has advertised LSAs carrying a default route, the ME device no longer learns the same type of LSA advertised by other ME devices, which carry a default route. That is, the ME device uses only the LSAs advertised by itself to calculate routes. The LSAs advertised by other ME devices are still saved in the LSDB.
  • If a ME device must use a route to advertise LSAs carrying an external default route, the route cannot be a route learned by the local OSPF process. A ME device in an area uses an external default route to forward packets outside the area. If the next hops of routes in the area are ME devices in the area, packets cannot be forwarded outside the area.
  • Before a ME device advertises a default route, it checks whether a neighbor in the full state is present in area 0. The ME device advertises a default route only when a neighbor in the full state is present in area 0. If no such a neighbor exists, the backbone area cannot forward packets and advertising a default route is meaningless. For the concept of the Full State, see OSPF Neighbor States.

Table 6-9 describes the principles for advertising default routes in different areas.

Table 6-9 Principles for advertising default routes in different areas

Area Type

Advertisement Principles

Common area

By default, a ME device in a common area does not generate a default route.

After being configured to do so, an ASBR generates a Type 5 LSA carrying a default route. The ME device then advertises the default route in the entire AS.

If no default route is generated on the ASBR, the ME device does not advertise a default route.

Stub area

Type 5 LSAs cannot be advertised within a stub area.

A ME device in the stub area must learn AS external routes from an ABR. The ABR automatically generates a Type 3 LSA carrying a default route and advertises it within the entire stub area. Then the ME device can learn AS external routes from the ABR.

Totally stub area

Neither Type 3 (except default Type 3 LSAs) nor Type 5 LSAs can be advertised within a totally stub area.

A ME device in the totally stub area must learn AS external and inter-area routes from an ABR. After you configure a totally stub area, an ABR automatically generates a Type 3 LSA carrying a default route and advertises it within the entire totally stub area. Then the ME device can learn AS external and inter-area routes from the ABR.

NSSA

A small number of AS external routes learned from the ASBR in an NSSA can be imported to the NSSA. Type 5 LSAs cannot be advertised within the NSSA. When at least a neighbor in Full status and an interface that is Up exist in the backbone area, the ABR automatically generates a Type 7 LSA carrying a default route and advertises it within the entire NSSA. A small number of AS external routes can be learned from the ASBR in the NSSA, and other inter-area routes can be learned from the ABR in the NSSA. Manual configurations must be performed on the ASBR to enable the ASBR to generate a Type 7 LSA carrying a default route and advertise the LSA within the entire NSSA.

An ABR does not translate Type 7 LSAs carrying a default route into Type 5 LSAs carrying a default route or flood them to the entire AS.

Totally NSSA

Neither Type 3 (except default Type 3 LSAs) nor Type 5 LSAs can be advertised within a totally NSSA.

A ME device in the totally NSSA must learn AS external routes from an ABR. The ABR automatically generates Type 3 and Type7 LSAs carrying a default route and advertises them to the entire totally NSSA. Then AS external and inter-area routes can be advertised within the totally NSSA.

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Updated: 2019-01-04

Document ID: EDOC1100059473

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