Kevin Dorrell, CCIE #20765

16 Mar 2008

NMC Lab 12.7.5 : EIGRP uneven load-balancing

Filed under: EIGRP — dorreke @ 18:15

This section is a study in unequal load balancing in EIGRP using the variance command.  We have R5 that is being fed identical routes from R2 and R3.  It is load-sharing 1:1.  We have to change that ratio 6:1 in favour of R2.  The way they do it in the AK is to increase the delay on the interface towards R3 until it gets only 1/6 share of the traffic.  We end up with something like this on R5:

R5#show ip route 151.10.104.0 
Routing entry for 151.10.104.0/24 
  Known via "eigrp 100", distance 170, metric 20537600, type external 
  Redistributing via eigrp 100 
  Last update from 151.10.25.2 on Serial1/0, 00:14:14 ago 
  Routing Descriptor Blocks: 
  * 151.10.35.3, from 151.10.35.3, 00:14:14 ago, via Serial1/1 
      Route metric is 123225600, traffic share count is 1 
      Total delay is 4032250 microseconds, minimum bandwidth is 128 Kbit 
      Reliability 255/255, minimum MTU 1500 bytes 
      Loading 1/255, Hops 1 
    151.10.25.2, from 151.10.25.2, 00:14:14 ago, via Serial1/0 
      Route metric is 20537600, traffic share count is 6 
      Total delay is 21000 microseconds, minimum bandwidth is 128 Kbit 
      Reliability 255/255, minimum MTU 1500 bytes 
      Loading 1/255, Hops 1
R5#show ip eigrp topology 151.10.104.0/24 
IP-EIGRP (AS 100): Topology entry for 151.10.104.0/24 
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 20537600 
  Routing Descriptor Blocks: 
  151.10.25.2 (Serial1/0), from 151.10.25.2, Send flag is 0x0 
      Composite metric is (20537600/1732096), Route is External 
      Vector metric: 
        Minimum bandwidth is 128 Kbit 
        Total delay is 21000 microseconds 
        Reliability is 255/255 
        Load is 1/255 
        Minimum MTU is 1500 
        Hop count is 1 
      External data: 
        Originating router is 151.10.102.1 
        AS number of route is 12 
        External protocol is OSPF, external metric is 66 
        Administrator tag is 0 (0x00000000) 
  151.10.35.3 (Serial1/1), from 151.10.35.3, Send flag is 0x0 
      Composite metric is (123225600/1732096), Route is External 
      Vector metric: 
        Minimum bandwidth is 128 Kbit 
        Total delay is 4032250 microseconds 
        Reliability is 255/255 
        Load is 1/255 
        Minimum MTU is 1500 
        Hop count is 1 
      External data: 
        Originating router is 151.10.103.1 
        AS number of route is 12 
        External protocol is OSPF, external metric is 65 
        Administrator tag is 0 (0x00000000) 
R5# 

The original metric was 20537600 on both of these routes, but by adding to the delay on the interface, we have loaded the route via 151.10.35.3 until it is 6 times that through 151.10.25.2.

I thought I would be clever and try and do it another way, using an offset list.  So I removed the loading on the interface, and added offset-list 1 input 102688000, where access-list 1 defines the OSPF routes we want to re-balance.  That should bring the metric to 123225600.  Trouble is, it doesn’t work.  The route disappears altogether.  Why?

Well, when we add delay to our local interface, we add to the metric sure, but the “advertised distance” to the downstream router, R2, remains unaffected.  In this case it is 1732096.  OTOH, if we use an offset-list, it adds not only to the metric, but also to the “advertised distance”.  In this case, it becomes 104420096.  This is greater than the total metric of the route via R2, so we fail the “feasible successor” test.

Key point: Adding to the interface delay increases the local metric but leaves the neighbor’s apparent advertised distance unchanged.  An offset-list, OTOH, adds to the apparent advertised distance of the neighbor.

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