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Chapter 5

IP Routing

Sending 5, 100-byte ICMP Echos to 172.16.50.1, timeout is

2 seconds:

.!!!!

Success rate is 80 percent (4/5), round-trip min/avg/max

= 64/66/68 ms

2621A#

Notice that the first response is a period. This is because the first ping

times out waiting for the ARP request and response. Once the ARP has
found the hardware address of the default gateway, the IP-to-Ethernet map-
ping will be in the ARP cache and will stay in the router's cache for four
hours. Any other IP connectivity to the next-hop router will not time out,
as no ARP broadcasts have to be performed.

From Router 2501C, a ping to 172.16.10.0 will test for good IP connec-

tivity. Here is the router output:

2501C#ping 172.16.10.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.16.10.1, timeout

is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max

= 64/67/72 ms

Notice that the first ping was not a time-out, since the ARP broadcasts are

sent only on a LAN, not a WAN. And, since we can ping from end-to-end
without a problem, our static route configuration was a success!

Default Routing

We use default routing to send packets with a remote destination network
not in the routing table to the next-hop router. You can only use default
routing on stub networks--those with only one exit port out of the network.

In the internetworking example used in the previous section, the only

routers that are considered to be in a stub network are 2621A and 2501C.
If you tried to put a default route on Routers 2501A and 2501B, packets
would not be forwarded to the correct networks because they have more
than one interface routing to other routers. However, even though Router
2501C has two connections, it does not have a router on the 172.16.50.0
network that needs packets sent to it. 2501C will only send packets to

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