Cisco CCNA Certification Test Prep:

Distance Vector Command Review

By Chris Bryant, CCIE #12933

A large part of your CCNA exam prep is learning Cisco router commands -- a lot of them! With all the commands and router configurations you're responsible for, it's easy to have a few of them slip past you. Here are five of the more commonly misunderstood commands that you'll need to master in order to earn your CCNA certfication!

Bandwidth

IGRP makes a default assumption that any Serial interface running IGRP is connected to a T1 line, which runs at 1544 KBPS. With equal-cost load-balancing enabled by default, this may be an undesirable assumption.

To alter IGRP’s assumption, use the bandwidth command on the serial interface in question. Note that this command does NOT actually affect the bandwidth available to the interface; it merely changes IGRP’s assumption of the bandwidth.

R2#conf t

R2(config)#int s0

R2(config-if)#bandwidth 512

Clear ip route *

This command clears your routing table of all non-static and non-connected routes. In a lab environment, it’s very handy because it forces your routers running routing protocols to send and request updates, rather than waiting for the regularly scheduled updates. An example of this command is shown with the debug ip igrp transactions command later in this review.

Debug ip igrp events

Debug ip igrp events allows you to see IGRP updates being sent and requested. Here, the debug is run and then the routing table is cleared. The router immediately broadcasts update requests via the IGRP-enabled interfaces.

R2#debug ip igrp events

IGRP event debugging is on

R2#clear ip route *

06:02:51: IGRP: broadcasting request on BRI0

06:02:51: IGRP: broadcasting request on Serial0.123

Debug ip igrp transactions

To configure IGRP unequal-cost load-sharing with the variance command, you’ve got to know the metric of the less-desirable routes. EIGRP keeps these in its topology table; IGRP has no such table.

To get the metrics of routes not in the routing table, run debug ip igrp transactions. To force IGRP updates, the routing table below was cleared with clear ip route *.

R2#debug ip igrp transactions

IGRP protocol debugging is on

R2#clear ip route *

06:05:33: IGRP: received update from 172.12.123.1 on Serial0.123

06:05:33: subnet 172.12.123.0, metric 10476 (neighbor 8476)

06:05:33: network 1.0.0.0, metric 8976 (neighbor 501)

06:05:33: IGRP: edition is now 3

06:05:33: IGRP: sending update to 255.255.255.255 via BRI0 (172.12.12.2)

06:05:33: network 1.0.0.0, metric=8976

06:05:33: IGRP: sending update to 255.255.255.255 via Serial0.123 (172.12.123.2) - suppressing null update

06:05:34: IGRP: received update from 172.12.12.1 on BRI0

06:05:34: subnet 172.12.13.0, metric 160250 (neighbor 8476)

06:05:34: network 1.0.0.0, metric 158750 (neighbor 501)

Debug ip rip

Run debug ip rip to troubleshoot routing update problems, RIP authentication problems, and to view the routing update contents. Clear ip route * was run to clear the routing table and to force a RIP update.

R2#debug ip rip

IP protocol debugging is on

R2#clear ip route *

6:14:53: RIP: received v2 update from 172.23.23.3 on Ethernet0

6:14:53: 1.0.0.0/8 via 0.0.0.0 in 16 hops (inaccessible)

6:14:53: 1.1.1.1/32 via 0.0.0.0 in 2 hops

6:14:53: 172.12.0.0/16 via 0.0.0.0 in 16 hops (inaccessible)

6:14:53: 172.12.12.2/32 via 0.0.0.0 in 2 hops

6:14:53: 172.12.13.0/30 via 0.0.0.0 in 1 hops

6:14:53: 172.12.123.0/24 via 0.0.0.0 in 1 hops

6:14:53: 172.23.0.0/16 via 0.0.0.0 in 16 hops (inaccessible)

ip route

To configure a static route to a given destination IP address, use the ip route command. The destination is followed by a subnet mask, and that can be followed by either the next-hop IP address or the exit interface on the local router.

R2#conf t

R2(config)#ip route 1.1.1.1 255.255.255.255 172.12.123.1

OR

R2(config)#ip route 1.1.1.1 255.255.255.255 serial0

ip route 0.0.0.0 0.0.0.0

To configure a default static route, use either of these two commands.

R2#conf t

R2(config)#ip route 0.0.0.0 0.0.0.0 172.12.123.1

OR

R2(config)#ip route 0.0.0.0 0.0.0.0 ethernet0

You could have any number for the first “0.0.0.0", since the second set of zeroes is the subnet mask. This means that any destination will match this route statement.

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com