Case Study: EIGRP/IGRP Redistribution
One issue commonly faced with EIGRP is redistribution between IGRP and EIGRP for combining networks and for transitioning from IGRP to EIGRP. Use the network in Figure 7-17 as an example.
Figure 7-17. Redistribution between IGRP and EIGRP
In this network, Router A and Router B are redistributing between EIGRP AS 1 IGRP AS 2. They have similar configurations:
hostname A
!
router eigrp 1
redistribute igrp 2
network 10.0.0.0
!
router igrp 2
redistribute eigrp 2
network 10.0.0.0
!
Looking at the routing table, you can see that Router A prefers the IGRP route through Router C, rather than the EIGRP external site through Router B, which is actually the better route (through a T1 rather than a 56k link):
A#show ip route
10.1.1.0 [100/2000] via 10.1.5.2, 00:00:39, Serial0
Looking at the topology table on Router A, you can see the entry through Router B:
A#show ip eigrp topology 10.1.1.0 255.255.255.0
IP-EIGRP topology entry for 10.1.1.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 256000
Routing Descriptor Blocks:
10.1.5.2, from 10.1.5.2, Send flag is 0x0
Composite metric is (256000/25600), Route is External
The EIGRP metric is 256,000, which you can divide by 256 to directly compare to the IGRP metric. 256000/256 is 1000, so the EIGRP external metric is actually better. The redistribution is causing you to choose the worst route rather than the best.
You are choosing the IGRP route because of the administrative distance of the two protocols; whereas IGRP has an administrative distance of 100, EIGRP external sites have an administrative distance of 170.
If you reconfigure Router A and Router B so that EIGRP and IGRP are using the same AS, something odd happens in the routing table:
hostname B
!
router eigrp 1
network 10.0.0.0
!
router igrp 1
network 10.0.0.0
!
B#show ip route
DEX 10.1.1.0 [170/256000] via 10.1.10.2, 00:00:39, Ethernet0
Now the EIGRP external route learned from Router B is the preferred route! When comparing an EIGRP external (redistributed from IGRP in the same AS) and an IGRP route from the same AS, you ignore the administrative distances of the routes and compare only the metrics.