CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

- IGRP configuration on RT2:
RT2(config)#router igrp 1
RT2(config-router)#network 192.168.2.0
RT2(config-router)#network 192.168.3.0
RT2(config-router)#network 192.168.4.0
RT2(config-router)#^Z
RT2#
- IGRP configuration on RT3:
RT3(config)#router igrp 1
RT3(config-router)#network 192.168.4.0
RT3(config-router)#network 192.168.5.0
RT3(config-router)#^Z
RT3#
- Verify the IGRP configuration on RT1, RT2 and RT3.
RT1#show ip route
Gateway of last resort is not set
C 192.168.1.0 is directly connected, FastEthernet1/0
C 192.168.2.0 is directly connected, Serial0/0
I 192.168.3.0 [100/8486] via 192.168.2.2, 00:00:09, Serial0/0
I 192.168.4.0 [100/10476] via 192.168.2.2, 00:00:09, Serial0/0
I 192.168.5.0 [100/10486] via 192.168.2.2, 00:00:09, Serial0/0
RT1#
- Q: If RIP and IGRP are running at the same time, routes from which protocol will be selected?
A: IGRP has a lower administrative distance value than RIP; hence IGRP routes will be selected.
Anyway, take note that if RIP is still running in the background, it can consume unnecessary
router CPU resources and network bandwidth.
- The information after the next hop IP address for a route indicates the invalid timer for a route
in hh:mm:ss format. The timer is reset to all 0s every time an update for a particular route is
received. When the invalid timer for a route is expired, the route will be marked as possibly
down and the router will start the holddown cycle for the particular route.
- A router will still use routes in holddown state when forward packets destined to the network.
This is a standard behavior of many IP routing protocols, which is based on the assumption that
temporary packet loss due to using routes to networks that might not be viable is better
than immediately accepting a less desirable route to the destination network.
- DV routing protocols do not support discontiguous networks. When a router running RIP or
IGRP receives a route, it checks if the routing update information contains the same major
network number that is configured on the receiving interface. If it does, it applies the subnet
mask that is configured on the receiving interface to the newly received route; else if it doesn’t
(the routing update information contains a different major network number than the receiving
interface), it applies the default classful subnet mask accordingly.
Ex: A router receives a route to 172.16.2.0 from an interface 172.16.1.1/24, it applies the /24
mask to the route and becomes 172.16.2.0/24. Else, it applies the default /16 mask to the route
and becomes 172.16.0.0/16.
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Copyright © 2008 Yap Chin Hoong
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