Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 585)

given area. The routers within the same area have identical topological databases. An ABR is
responsible for forwarding routing information or changes among its border areas.
Autonomous System Boundary Routers
An Autonomous System Boundary Router (ASBR) is a router that is running multiple protocols and
serves as a gateway to routers outside the OSPF domain and those operating with different protocols.
The ASBR is able to import and translate different protocol routes into OSPF through a process known
as redistribution. (For more information about redistribution, refer to the redistribute command in
Network OS Command Reference.)
Designated routers
In an OSPF broadcast network, OSPF elects one router to serve as the designated router (DR) and
another router on the segment to act as the backup designated router (BDR). This minimizes the
amount of repetitive information that is forwarded on the network. OSPF forwards all messages to the
designated router.
On broadcast networks such as LAN links, all routers on the LAN other than the DR and BDR form full
adjacencies with the DR and BDR and pass LSAs only to them. The DR forwards updates received
from one neighbor on the LAN to all other neighbors on that same LAN. One of the main functions of a
DR is to ensure that all the routers on the same LAN have identical LSDBs. Therefore, on broadcast
networks, an LSDB is synchronized between a DROther (a router that is not a DR or a BDR) and its DR
and BDR.
NOTE
In an OSPF point-to-point network, where a direct Layer 3 connection exists between a single pair of
OSPF routers, there is no need for designated or backup designated routers.
In a network with no designated router and no backup designated router, 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,
as shown in the figure below. Priority is a configurable option at the interface level; refer to the ip ospf
priority command in Network OS Command Reference.
FIGURE 62 Designated and backup router election
If the DR goes off line, the BDR automatically becomes the DR. The router with the next highest priority
becomes the new BDR.
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. The DR and BDRs are recalculated
after the OSPF protocol is disabled and re-enabled by means of the [no] router ospf command.
Autonomous System Boundary Routers
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