Eigrp load balancing >4 links

[bboggs]

We are running EIGRP and have greater than 4 equal cost links (due to VLAN's). How does EIGRP decide which 4 to use? Is this stable or does it flap between the choices?

[rsissons]

EIGRP will attempt to load balance so will select the least utilised link. Load balancing however, is on a per session, rather than a per packet basis if fast switching is configured (the default).

It is possible to force the router to load balance on a per packet basis by disabling fast switching but this is NOT recommended as it can severely impact the performance of the router.

[Harold Ritter]

Just one precision. By default, Eigrp will select the best path based on the lowest BW and total delay across the path to destination, so the least utilized link will not necessarily be selected as the path.

Also, just to add to Kevin's comment, in some version version of IOS (12.0S) it is possible to have up to 8 concurrent paths installed in the RIB using the maximum-paths command.

Hope this helps,

[Kevin Dorrell]

I've not tried it, but I believe only the first four will make it into the routing table. You can increase this limit up to 6 if you want, with the command maximum-paths. Here is the documentation:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fiprrp_r/bgp_r/1rfbgp1.htm#wp1216639

As for how it distributes the packets between the routes once they are in the routing table, see the response from Rona Sissons above.

There is another thing to point out about per-session vs per-packet distribution: with per-session, packets within the session tend to arrive in their correct sequence. With per-packet, it is much more likely that the last (short) packets of a sequence will arrive at its destination before the penultimate one, adding a re-sequencing overhead at the destination.

Kevin Dorrell

Luxembourg

[bboggs]

Let me be more specific.

I know that the default limit for max paths is 4. We have 10 EQUAL COST paths between the routers. They will have to pick only 4. How do they decide which 4 paths to use? Is the decision stable or does it flap between the choices? Is the decision the same on both routers?

[ruwhite · Accepted solution]

The first four learned will be installed in the table. This is stable--if one route flaps, it will be replaced by another route, but the table will remain stable even if the flapping route comes bac. Having more successors than paths installed in the routing table can cause problems with EIGRP convergence, so be careful with this sort of configuration.

:-)

Russ.W

[quangnn]

Hi,

Use variance command to select links you want to have load balance. Make sure those are available by show ip eigrp topology.

[Kevin Dorrell]

The requirement was to balance between equal cost links, so the variance command is not relevant here. Sorry.

Kevin Dorrell

Luxembourg