Chapter 2: About the HSIM-W6
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HSIM-W6 Installation Guide
Area Border Router
Area Border Routers (ABR) connect networks together. These routers have multiple interfaces and
participate in multiple areas. There must be at least one area border router in each area connecting
that area to a backbone. An ABR maintains a separate link-state database per area that it is attached
to.
Designated Router
Designated Routers (DR) are used by OSPF to reduce adjacencies. Other routers establish
adjacencies and synchronize databases only with the designated router and backup designated
router. These routers perform two key functions for the OSPF routing protocol:
•
The designated router creates a network links advertisement which lists the set of routers,
including the designated router, attached to the network.
•
The designated router is adjacent to all routers on the network and plays a central role in the
synchronization of link-state databases across adjacencies.
One DR is elected for each network and is elected by using the Hello Protocol.
If one or more routers have declared to be the DR in their Hello packets, the one with the highest
Router Priority becomes the DR. If more than one router has identical router priority, then the
router with the highest router ID becomes DR.
Backup Designated Router
Backup Designated Routers (BDR) are used for each multi-access network. The BDR is adjacent
to all routers on the network and becomes the Designated Router if the existing Designated Router
fails. The BDR does not generate network link advertisements and is also elected by the Hello
Protocol.
The Backup Designated Router is chosen from those routers who have not declared themselves to
be the Designated Router. If a one or more routers have declared itself to be the BDR in their Hello
packets, the one with the highest Router Priority becomes the BDR. If no routers have declared
themselves to be BDR, then the router with the highest router ID becomes BDR.
Detailed information on the OSPF protocol is documented in RFC 2178.
