OSPF adjacencies for multi-access networks

FIGURE 5 . 2
OSPF adjacencies for multi-access networks
Open Shortest Path First
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In Figure 5.2, you can see the dotted lines connecting the DROther routers to the DR and
BDR routers. Note also that there are no dotted lines between any of the DROther routers. The
dotted lines represent the formation of adjacencies. DROther routers form only two adjacencies
on a broadcast multi-access network—one with the DR and the other with the BDR. The following
router output indicates the assignments of routers connected via a broadcast multiaccess
network as well as three point-to-point network connections.
Note that the serial interface and subinterface connections displayed next do not
have DR/BDR/DROther assignments, because point-to-point and point-to-multipoint
links do not elect a DR and BDR. DR/BDR roles and election are covered
more fully in the following section, “DR and BDR Election Procedure.”
RouterA#
show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
172.16.22.101 1 FULL/DROTHER 00:00:32 172.16.22.101 FastEthernet0/0
172.16.247.1 1 FULL/DR 00:00:34 172.16.22.9 FastEthernet0/0
172.16.245.1 1 2WAY/DROTHER 00:00:32 172.16.12.8 FastEthernet1/0
172.16.244.1 1 2WAY/DROTHER 00:00:37 172.16.12.13 FastEthernet1/0
172.16.247.1 1 FULL/BDR 00:00:34 172.16.12.9 FastEthernet1/0
172.16.249.1 1 FULL/DR 00:00:34 172.16.12.15 FastEthernet1/0
172.16.248.1 1 2WAY/DROTHER 00:00:36 172.16.12.12 FastEthernet1/0
172.16.245.1 1 FULL/ - 00:00:34 172.16.1.105 Serial3/0.1
172.16.241.1 1 FULL/ - 00:00:34 172.16.202.2 Serial3/1
172.16.248.1 1 FULL/ - 00:00:35 172.16.1.41 Serial3/3.1
RouterA#
We need to bring up a few important points about this output. Notice that five different
interfaces are configured to use OSPF.
Interface FastEthernet 0/0 shows only a DROther and a DR. You know that there must always
be a DR and a BDR for each multi-access segment with two or more router interfaces. Deductively,
you can ascertain that RouterA must be the BDR for this segment. Furthermore, interface Fast-
Ethernet 1/0 shows neighboring DROthers, a DR, and a BDR, meaning that RouterA is also a
DROther on this network segment. What further proves this point is RouterA’s relationship with the
other DROthers. Remember that adjacencies are formed only by DRs and BDRs and their neighbors
on multi-access networks. Two DROthers will only go as far as the 2Way state with one another.
It’s also important to recognize that this command displays all OSPF neighbors and not
specific adjacencies. To learn adjacency formations, study the following summarization:

Valid point-to-point broadcast neighbors form adjacencies.
 Non-broadcast neighbors require special configuration (for example, neighbors on NBMA
or point-to-multipoint non-broadcast interfaces) for adjacency formation.
 Broadcast multi-access neighbors require the election of a DR and a BDR. All other routers
form adjacencies with only the DR and BDR.