Route Advertisements; Route Aggregation - Juniper J2300 User Manual

Route Advertisements

The routing table and forwarding table contain the routes for the routers
within a network. These routes are learned through the exchange of route
advertisements. Route advertisements are exchanged according to the
particular protocol being employed within the network.
Generally, a router transmits hello packets out each of its interfaces.
Neighboring routers detect these packets and establish adjacencies with
the router. The adjacencies are then shared with other neighboring
routers, which allows the routers to build up the entire network topology
in a topology database, as shown in Figure 57.
Figure 57: Route Advertisement
In Figure 57, router A sends out hello packets to each of its neighbors. Routers B
and C detect these packets and establish an adjacent relationship with router A.
Router B and C then share this information with their neighbors, routers D and E,
respectively. By sharing information throughout the network, the routers create a
network topology, which they use to determine the paths to all possible destinations
within the network. The routes are then distilled into the forwarding table of best
routes according to the route selection criteria of the protocol in use.

Route Aggregation

As the number of hosts in a network increases, the routing and forwarding tables
must establish and maintain more routes. As these tables become larger, the time
routers require to look up particular routes so that packets can be forwarded
becomes prohibitive. The solution to the problem of growing routing tables is to
group (aggregate) the routers by subnetwork, as shown in Figure 58.
D E
B C
A
Routing Overview
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Routing Overview