Reference Guide

Designated and backup designated routers
OSPF elects a designated router (DR) and a backup designated router (BDR). The DR is responsible for generating LSAs for the entire
multiaccess network. Designated routers allow a reduction in network trac and in the size of the topological database.
Designated router Maintains a complete topology table of the network and sends updates to the other routers via multicast. All
routers in an area form a slave/master relationship with the DR. Every time a router sends an update, the router
sends it to the DR and BDR. The DR sends the update out to all other routers in the area.
Backup designated
router
Router that takes over if the DR fails.
Each router exchanges information with the DR and BDR. The DR and BDR relay information to other routers. On broadcast network
segments, the number of OSPF packets reduces by the DR sending OSPF updates to a multicast IP address that all OSPF routers on the
network segment are listening on.
The DRs and BDRs are congurable. If you do not dene DR or BDR, OS10 assigns them per the protocol. To determine which routers are
the DR and BDR, the OSPF looks at the priority of the routers on the segment —default router priority is 1. The router with the highest
priority is elected the DR. If there is a tie, the router with the higher router ID takes precedence. After the DR is elected, the BDR is elected
the same way. A router with a router priority set to zero cannot become the DR or BDR.
Link-state advertisements
A link-state advertisement (LSA) communicates the router’s routing topology to all other routers in the network.
Type 1—Router LSA
Router lists links to other routers or networks in the same area. Type 1 LSAs ood across their own area only. The
link-state ID of the Type 1 LSA is the originating router ID.
Type 2—Network
LSA
DR in an area lists which routers are joined within the area. Type 2 LSAs ood across their own area only. The link-
state ID of the Type 2 LSA is the IP interface address of the DR.
Type 3—Summary
LSA (OSPFv2),
Inter-Area Prex
LSA (OSPFv3)
ABR takes information it has learned on one of its attached areas and summarizes it before sending it out on other
areas it connects to. The link-state ID of the Type 3 LSA is the destination network's IP address.
Type 4—AS Border
Router Summary
LSA (OSPFv2),
Inter-Area-Router
LSA (OSPFv3)
In some cases, Type 5 External LSAs ood to areas where the detailed next-hop information may not be available,
because it may be using a dierent routing protocol. The ABR oods the information for the router—the ASBR
where the Type 5 originated. The link-state ID for Type 4 LSAs is the router ID of the described ASBR.
Type 5—AS-
External LSA
LSAs contain information imported into OSPF from other routing processes. Type 5 LSAs ood to all areas except
stub areas. The link-state ID of the Type 5 LSA is the external network number.
Type 7—NSSA-
External LSA
(OSPFv2), LSA
(OSPFv3)
Routers in an NSSA do not receive external LSAs from ABRs but send external routing information for
redistribution. They use Type 7 LSAs to tell the ABRs about these external routes, which the ABR then translates
to Type 5 external LSAs and oods as normal to the rest of the OSPF network.
Type 8—Link LSA
(OSPFv3)
Type 8 LSA carries the IPv6 address information of the local links.
Type 9—Link-Local
Opaque LSA
Link-local opaque LSA as dened by RFC2370 for OSPFv2. Intra-Area-Prex LSA carries the IPv6 prexes of the
router and network links for OSPFv3.
344 Layer 3