CCIE Pursuit Blog

September 15, 2008

Disabling EIGRP Unequal-Cost Load-Balancing

Filed under: Cisco,Cisco Certification,EIGRP,IOS — cciepursuit @ 6:53 am
Tags: , , , , ,

This may be old news to you guys but I had never seen the “traffic-share min across-interfaces” command before.  I was messing about with EIGRP unequal cost load-sharing the other day and came across it.

r1(config-router)#traffic-share ?
  balanced  Share inversely proportional to metric
  min       All traffic shared among min metric paths <-what’s this?

r1(config-router)#traffic-share min ?
  across-interfaces  Use different interfaces for equal-cost paths
r1(config-router)#traffic-share min across-interfaces

Weird.  It turns off unequal-cost load balancing by not allowing traffic over the Feasable Successor route(s) regardless of the variance command:

r1(config-router)#var 4
*Mar  2 00:05:02.000: %FIB-4-UNEQUAL: Range of unequal path weightings too large for prefix 150.1.6.0/24. Some available paths may not be used.

r1(config-router)#do sh ip route 164.1.26.0 255.255.255.0
Routing entry for 164.1.26.0/24
  Known via “eigrp 100”, distance 90, metric 3026432, type internal
  Redistributing via eigrp 100
  Last update from 164.1.12.2 on Serial0/0, 00:00:03 ago
  Routing Descriptor Blocks:
  * 164.1.13.3, from 164.1.13.3, 00:00:03 ago, via Serial0/1
      Route metric is 3026432, traffic share count is 1
      Total delay is 40100 microseconds, minimum bandwidth is 1280 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2
    164.1.12.2, from 164.1.12.2, 00:00:03 ago, via Serial0/0
      Route metric is 10514432, traffic share count is 0
      Total delay is 20100 microseconds, minimum bandwidth is 256 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1

r1(config-router)#var 40
r1(config-router)#do sh ip route 164.1.26.0 255.255.255.0
Routing entry for 164.1.26.0/24
  Known via “eigrp 100”, distance 90, metric 3026432, type internal
  Redistributing via eigrp 100
  Last update from 164.1.12.2 on Serial0/0, 00:00:02 ago
  Routing Descriptor Blocks:
  * 164.1.13.3, from 164.1.13.3, 00:00:02 ago, via Serial0/1
      Route metric is 3026432, traffic share count is 1
      Total delay is 40100 microseconds, minimum bandwidth is 1280 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2
    164.1.12.2, from 164.1.12.2, 00:00:02 ago, via Serial0/0
      Route metric is 10514432, traffic share count is 0
      Total delay is 20100 microseconds, minimum bandwidth is 256 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1

Here’s the explanation:

Similarly, when you use the traffic-share command with the keyword min, the traffic is sent only across the minimum-cost path, even when there are multiple paths in the routing table.

router eigrp 1
 network x.x.x.x
 variance 3
 traffic-share min across-interfaces

In this situation, EIGRP sends packets only through E-C-A, which is the best path to the destination network. This is identical to the forwarding behavior without use of the variance command. However, if you use the traffic-share min command and the variance command, even though traffic is sent over the minimum-cost path only, all feasible routes get installed into the routing table, which decreases convergence times. 

It turns out that this command is available for all routing protocols.

r1(config)#router os 100
r1(config-router)#traffic-share min ?
  across-interfaces  Use different interfaces for equal-cost paths
r1(config)#router rip
r1(config-router)#traffic-share min ?
  across-interfaces  Use different interfaces for equal-cost paths

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1 Comment »

  1. Nice explanation, hope your studies are going well 🙂

    Comment by galvezc — September 15, 2008 @ 9:40 am | Reply


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