Information Technology Reference
In-Depth Information
OSPF Router Types
Figure 8-2
Area 3
ASBR
Internal
Area 0
Backbone
ABR
Area 1
Area 2
ABR
ASBR
Internal
Designated Routers
Although not an OSPF router type, some routers on multiaccess networks (such as Ethernet or
multipoint Frame Relay) are selected as DRs. The purpose of the DR is to collect all LSAs for
the multiaccess network and to forward the LSA to all non-DR routers. This reduces the amount
of LSA traffic generated on an Ethernet network. A router can be the DR for one multiaccess
network and not the DR in another attached multiaccess network.
The DR also floods the Network LSA (Type 2) to the rest of the area. A BDR is also selected,
which takes over the function of the DR if the DR fails. Both the DR and BDR become adjacent
to all routers in the multiaccess network. All routers that are not DR and BDR are classified
Drothers, and these routers are adjacent only to the DR and BDR. DRothers multicast packets
to the DR and BDR by using the multicast address 224.0.0.6 (AllDRouters). The DR floods
updates by using ALLSPFRouters (224.0.0.5).
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