Understanding the network.pdf - Back to Home

that provide a way for routers to exchange routing information. The
advantages of adjacencies are that they reduce the amount of link
calculation and route table construction needed for a router to build a
complete routing table. Routers become adjacent under different
circumstances:
• If a link type is point-to-point, the routers on each end will
become adjacent in order for routing information to be
distributed between the connected networks.
• If a router is a designated router or a backup designated router
on a broadcast or nonbroadcast multi-access network,
adjacency is formed with all of the routers within the common
network.
• If a link is a virtual link, an adjacency will be formed with the
backbone network, so backbone information can be exchanged.
The most common instance of adjacencies is in networks where a
designated router can be used.
On networks where more than one router is used to provide
connectivity, a designated router will be elected to represent the
router group. The designated router (see Figure 8.14) will compute
the routing table and exchange routing messages with other routers
within the area. There is also a backup designated router, which
performs the same functions if the primary fails.