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 16: OSPF Routing Components, on page 336, 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.
Autonomous System Boundary Routers (ASBR)
An autonomous system boundary 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 Cisco IOS 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.
Interior Routers
An interior router (such as R1 in Figure 16: OSPF Routing Components, on page 336) is 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 Software 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.
OSPF obtains a router ID from the following sources, in order of decreasing preference:
•
By default, when the OSPF process initializes, it checks if there is a router-id in the checkpointing
database.
•
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.)
•
The ITAL selected router-id.
•
The primary IPv4 address of an interface over which this OSPF process is running. The first interface
address in the OSPF interface is selected.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
338 OL-30423-03
Implementing OSPF
OSPF Process and Router ID
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