EIGRP Troubleshooting (summary-metric)

This exercise was first post by Arwin Reprakash (http://ithitman.blogspot.com/2015/01/eigrp-troubleshooting-lab-1.html#more). I took his post and found a way to resolve the problem

The diagrams and original idea belongs to Arwin Reprakash, please check the above link to see more details

 

 

Startup configuration file (VIRL)

 

 

THE PROBLEM

Everytime gi0/2 on R1 goes down, the metric to reach loopback 1 from R2 changes
The goal is to keep this metric the same

When gi0/2 in R1 is up

R2#sh ip route
 D 10.0.0.0 [90/3072] via 1.1.1.1, 00:03:59, GigabitEthernet1

When gi0/2 in R1 is down

R2#sh ip route
D 10.0.0.0 [90/130816] via 1.1.1.1, 00:00:05, GigabitEthernet1 


R1 advertises the route 10.0.0.0/23 as a summary address to R2, this summary addres groups R1 interfaces l0 and g0/2:

• l0/0 --> 10.0.0.1 255.255.255.0
• g0/2 --> 10.0.1.1 255.255.255.0

THE SOLUTION

For EIGRP, the smallest metric among the component routes (routes contained in the summary) will be employed as the summary route metric. That's why the metric changes when g0/2 is up or down

We need to find a way to fix the metric when the summary route is advertise. We can accomplish this with the command "summary-metric" inside the "router eigrp" process, but this command will ask us for the following values:

• Minimum bandwidth (Kbits)
• Total delay (tens of microseconds)
• Reliability
• Load
• Minimum MTU

To check which values is using the summary address (10.0.0.0/23) when gi0/2 i s up, we use the  command "show ip eigrp topology A.B.C.D":


#show ip eigrp topology 10.0.0.0 255.255.254.0

EIGRP-IPv4 Topology Entry for AS(100)/ID(10.0.0.1) for 10.0.0.0/23
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2816
  Descriptor Blocks:
  0.0.0.0 (Null0), from 0.0.0.0, Send flag is 0x0
      Composite metric is (2816/0), route is Internal
      Vector metric:
        Minimum bandwidth is 1000000 Kbit
        Total delay is 10 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 0
        Originating router is 10.0.0.1

Those are the values that the summary address use when g0/2 is up so we'll use them with the "summary-metric" command


R1(config)#router eigrp 100
R1(config-router)#summary-metric 10.0.0.0/23 1000000 1 255 1 1500

We had to divide by 10 the delay cause it has to be in ten microsends


That's it, everytime we shutdown the interface g0/2 on R1, the metric in R2 to reach 10.0.0.0/23 will remain the same


R2#sh ip route
 D 10.0.0.0 [90/3072] via 1.1.1.1, 00:03:59, GigabitEthernet1



I use cisco virl for these labs but i think they will work ok in GNS3 too.

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