Understanding the network.pdf - Back to Home

MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Encapsulation PPP, loopback not set
Last input 00:01:38, output 00:01:38, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0 (size/max/drops); Total output drops: 0
Queuing strategy: weighted fair
Note that the two interfaces are both addresses from the same IP address subnet
(this is not permitted, unless dial backup is in use). Although these interfaces are
joined at the hip, so to speak, both interfaces must be configured appropriately, as
though they were each the primary interface. This is accomplished by setting the
default route for each interface, instead of just setting a single next hop address. If
this approach is not followed, when the primary interface fails, the default gateway
will be lost and the traffic will be dropped by the router because it does not know
where to send it.
NOTE
If a dynamic routing protocol is used, this will not occur because protocol will
readjust its forwarding behavior for the link failure.
Let's examine how the router behaves using this configuration. The following code
output illustrates the IP routing table of our example router, leibniz, under normal
operation, where the primary interface only is listed and is set as the gateway of last
resort. The router leibniz's routing table under normal conditions looks like this:
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate
default
U - per-user static route, o - ODR
T - traffic engineered route
Gateway of last resort is 172.168.44.4 to network 0.0.0.0
S 172.16.0.0/16 [1/0] via 172.16.44.0
C 192.168.191.0/24 is directly connected, Ethernet0
S* 0.0.0.0/0 [1/0] via 172.168.44.4, Serial0