Distance Vector Routing Protocols 363
·
Periodic updates are expected to be received from neighboring routers at a specified
interval. Failure to receive updates from a neighbor in a timely manner results in the
removal of the routes previously learned from that neighbor.
·
A route learned from a neighboring router is assumed to be through that router.
·
A failed route is advertised for a time, with a metric that implies that the network is
"infinite" distance. This route is considered unusable. Infinity is defined by each routing
protocol to be some very large metric value. For instance, RIP's infinite metric is 16
because RIP's maximum valid hop count is 15.
Figure 6-2 demonstrates how Router A's directly connected subnets are advertised to Router B.
In this case, Router A advertises two directly connected routes.
Figure 6-2
Router A Advertising Directly Connected Routes
Table 6-3 shows the resulting routing table on Router B.
Table 6-3
Router B Routing Table, After Receiving Update in Figure 6-2
Group (Mask Is
255.255.255.0)
Outgoing Interface
Next Router
162.11.5.0
S0
162.11.8.1
162.11.7.0
E0
162.11.8.0
S0
162.11.9.0
S0
162.11.8.1
Router A
Router C
162.11.9.0
162.11.5.0
162.11.10.0
162.11.7.0
162.11.5.0
162.11.9.0
1
1
162.11.8.0
Router B
s0
s0
s1
E0
1
Routing Update
ch06.fm Page 363 Monday, March 20, 2000 5:11 PM