Cisco Systems IOS XR OSPF Process and Router ID

Implementing OSPF on Cisco IOS XR Software

Information About Implementing OSPF on Cisco IOS XR Software

RC-176

Cisco IOS XR Routing Configuration Guide

The OSPF network is composed of ABRs, ASBRs, and interior routers.

An area border routers (ABR) is a router with multiple interfaces that connect directly to networks in

two or more areas. An ABR runs a separate copy of the OSPF algorithm and maintains separate routing

data for each area that is attached to, including the backbone area. ABRs also send configuration

summaries for their attached areas to the backbone area, which then distributes this information to other

OSPF areas in the autonomous system. In Figure 10, there are two ABRs. ABR 1 interfaces Area 1 to

the backbone area. ABR 2 interfaces the backbone Area 0 to Area 2, a stub area.

An autonomous system boudary router (ASBR) provides connectivity from one autonomous system to

another system. ASBRs exchange their autonomous system routing information with boundary routers

in other autonomous systems. Every router inside an autonomous system knows how to reach the

boundary routers for its autonomous system.

ASBRs can import external routing information from other protocols like BGP and redistribute them as

AS-external (ASE) Type 5 LSAs to the OSPF network. If the CiscoIOS XR router is an ASBR, you can

configure it to advertise VIP addresses for content as autonomous system external routes. In this way,

ASBRs flood information about external networks to routers within the OSPF network.

ASBR routes can be advertised as a Type 1 or Type 2 ASE. The difference between Type 1 and Type 2

is how the cost is calculated. For a Type 2 ASE, only the external cost (metric) is considered when

multiple paths to the same destination are compared. For a Type 1 ASE, the combination of the external

cost and cost to reach the ASBR is used. Type 2 external cost is the default and is always more costly

than an OSPF route and used only if no OSPF route exists.

The interior routers (such as R1 in Figure 10) attached to one area (for example, all the interfaces reside

in the same area).

OSPF Process and Router ID

An OSPF process is a logical routing entity running OSPF in a physical router. This logical routing entity

should not be confused with the logical routing feature that allows a system administrator (known as the

Cisco IOS XR Owner) to partition the physical box into separate routers.

A physical router can run multiple OSPF processes, although the only reason to do so would be to

connect two or more OSPF domains. Each process has its own link-state database. The routes in the

routing table are calculated from the link-state database. One OSPF process does not share routes with

another OSPF process unless the routes are redistributed.

Each OSPF process is identified by a router ID. The router ID must be unique across the entire routing

domain. OSPFv2 obtains a router ID from the following sources, in order of decreasing preference:

OSPF attempts to obtain a router ID in the following ways (in order of preference):

•The 32-bit numeric value specified by the OSPF router-id command in router configuration mode.

(This value can be any 32-bit value. It is not restricted to the IPv4 addresses assigned to interfaces

on this router, and need not be a routable IPv4 address.)