EIGRP link load balancing

[sunilyk]

Hi,

We have 2 sites say A & B. These are connected by 2 routers each at every site. These sites are connected by 2 E1 links. We have EIGRP in place for routing.

We found that only one link is being utilized. EIGRP doesn't seem to be load balancing the traffic.

On both sides its a simple eigrp configuration, with networks defined and no auto summary.

Please let me know how I can achieve proper link load balancing.

Regards,

Sunil

[Georg Pauwen]

Hello Sunil,

how do you have your load balancing configured ? Are you using CEF or EIGRP variance ?

You might want to check these documents:

Troubleshooting Load Balancing Over Parallel Links Using Cisco Express Forwarding

http://www.cisco.com/en/US/partner/tech/tk827/tk831/technologies_tech_note09186a0080094806.shtml

How Does Unequal Cost Path Load Balancing (Variance) Work in IGRP and EIGRP?

http://www.cisco.com/en/US/partner/tech/tk365/technologies_tech_note09186a008009437d.shtml

Regards,

GP

[Kevin Dorrell]

The diagnosis all hinges on whether the two routes appear in the routing table show ip route. A couple of points to check:

1. Do both routes appear in the routing table? If not, then look at the configuration of routing protocol.

2. If both routes do appear in the routing table, are you testing with only one source-destination pair? If so, it is normal that the traffic goes down one link only.

This may sound pedantic, but I find it helps to remember that it is not the EIGRP itself that load-balances the traffic - it just populates the routing table and sets up the data structures that determine how the traffic is to be shared between the routes. It is then the switching path (CEF, fast-cache, process-switching etc.) that balances the traffic. If there are not two routes in the routing table, then no load balancing will take place.

Kevin Dorrell

Luxembourg

[sunilyk]

Hi,

Thanks for your feedback.

CEF is enabled globally.

But I can't see 2 routes in show ip route.

The scenario is as follows

At A site one L3 and 2 routers are in EIGRP

At B site one L3 and 2 routers are in EIGRP

EIGRP on L3 at A

router eigrp 110

network 10.31.0.0 0.0.255.255

no auto summary

EIGRP on R1 at A

router eigrp 110

network 10.31.2.0 0.0.0.255

and the serial links

The config is exactly similar on other routers.

Please let me know if anything is missing.

Regards,

Sunil

[Kevin Dorrell]

If you cannot see the two routes in show ip route, then they are probably coming in at unequal metric, so EIGRP is only putting the best one in the routing table. To allow load-sharing between unequal metric routes, you need something like variance 4 under the eigrp. Where 4 is the ratio of metrics that are acceptable to take part in the load-sharing. Here is the doc:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fiprrp_r/1rfeigrp.htm#wp1024988

(The doc is a bit naff because the example they give is for IGRP, even if the heading is specific for eigrp ;-( )

Kevin Dorrell

Luxembourg

[Richard Burts]

It might be helpful if you would post the output of show ip eigrp interface and of show interface from each of the routers.

It would also be helpful is you could be a little more specific about the topology of the network and about what destinations you are attempting to reach where you expect load balancing but are not getting it.

HTH

Rick

[Kevin Dorrell]

** duplicate posting, sorry **

[asd_asd_22]

EIGRP load balance in 6 equal cost paths (4 by default), and EIGRP metric is a composite metric, so there may be a diffrenece in the delay, load, reliability of the two paths. check show ip eigrp interface to chek the metrics. and to force the EIGRP process to laod balance over the 2 paths use the variance command.

router(config)#router eigrp [AS-number]

router(config-router)#variance 2

and the 2 multiplier means that the process will multiply the lowest cost by 2 and any path cost less or equal to this value will be included in the load balancing process.