Eigrp built-in safety

 

Here’s an interesting case. I run eigrp on all routers. On R7 i check what path it has to the loopback of R6. In theory it should have one route but it should be aware in eigrp topology of two paths (an extra theoretical path via R3). But it is not. It only has one path to loopback of R6, that is through R6. Why is that?

Przechwytywanieine

 

Let’s check R3:

R3#show ip route 150.1.6.6
Routing entry for 150.1.6.6/32
Known via „eigrp 100”, distance 90, metric 435200, type internal
Redistributing via eigrp 100
Last update from 155.1.37.7 on Ethernet0/0.37, 00:05:27 ago
Routing Descriptor Blocks:
155.1.37.7, from 155.1.37.7, 00:05:27 ago, via Ethernet0/0.37
Route metric is 435200, traffic share count is 1
Total delay is 7000 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2
* 155.1.13.1, from 155.1.13.1, 00:05:27 ago, via Ethernet0/0.13
Route metric is 435200, traffic share count is 1
Total delay is 7000 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2

It load balances between R7 and R1. This is the answer. If I go through you, I won’t send you information that you could even in theory go through me because we might cause a loop.

It is enough for R3 to add some extra delay on the link to R7 and it will choose one path only (via R1). Then it will be able to send a routing update to R7 about the path through R1. Now R7 is aware of two possible paths: through R6 and R3.

Another solution would be to add a lot of delay on R7 on the link between R7 and R6. This will also cause R3 to stop load-balancing, send info to R7, and R7 will now have two possible paths in its eigrp topology table.

 

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