IPv6 Configuration Guide K/KA/KB.15.15

An ABR maintains a separate LSDB for each area to which it belongs. (All routers within the same
area have identical LSDBs.) The ABR is responsible for flooding inter-area-prefix-LSAs and inter-area
router LSAs between its border areas.
You can reduce this LSA flooding by configuring area ranges. An area range enables you to assign
an aggregate address to a range of IPv6 addresses. This aggregate address is advertised instead
of all the individual addresses it represents. You can assign up to eight ranges in an OSPFv3 area.
In Figure 24 (page 250), routers R2 and R5 are ABRs because they both have membership in more
than one area.
Figure 24 Example of deploying ABRs to connect areas to the backbone
R1
R2
R5
R3
R4
R
6
Area 0 (Backbone)
Area 1
A
r
e
a
2
Area Border Router (ABR)
Area Border Router
(ABR)
Autonomous system boundary router (ASBR)
This type of OSPFv3 router runs one or more Interior Gateway protocol and serves as a gateway
to other autonomous systems operating with interior gateway protocols. The ASBR imports and
translates different protocol routes into OSPFv3 through redistribution. ASBRs can be used in
backbone areas, normal areas, and NSSAs, but not in stub areas.
Designated routers
In an OSPFv3 network having two or more routers, one router is elected to serve as the designated
router (DR) and another router to act as the backup designated router (BDR). All other routers in
the area forward their routing information to the DR and BDR, and the DR forwards this information
to all routers in the network. This minimizes the amount of repetitive information that is forwarded
on the network by eliminating the need for each individual router in the area to forward its routing
information to all other routers in the network. If the area includes multiple networks, each network
elects its own DR and BDR.
In an OSPFv3 network with no DR and no BDR, the neighboring router with the highest priority is
elected as the DR, and the router with the next highest priority is elected as the BDR. If the DR goes
off-line, the BDR automatically becomes the DR, and the router with the next highest priority then
becomes the new BDR. If multiple HP routing switches on the same OSPFv3 network are declaring
themselves as DRs, both priority and router ID are used to select the designated router and backup
designated routers.
Priority is configurable by using the vlan vid ipv6 ospfv3 priority 0 - 255 command
at the interface level. You can use this parameter to help bias one router as the DR. If two neighbors
share the same priority, the router with the highest router ID is designated as the DR. The router
with the next highest router ID is designated as the BDR.
For example, in Figure 25 (page 251), the DR and BDR for the 2001:db8:0:5::/64 network in area
5 are determined as follows:
• Router A Priority: 0 Cannot become a DR or BDR.
• Router B Priority: 3 DR for the 2001:db8:0:5::/64 network.
250 OSPFv3 Routing