1.3.d EIGRP load balancing

1.3.d i Equal-cost

Enabled by default, as long as the metrics match. Nothing really to do here.

1.3.d ii Unequal-cost

Use the variance command. The only trick is when you multiply your best path’s metric by the configured variance, the product has to be higher than any potential path’s metrics, otherwise they still won’t be considered. For example, if you have two paths, one with a metric of 200,000 and one with a metric of 500,000 and you set the variance to 2, it wouldn’t trigger unequal cost load-balancing. A variance of 2 would only trigger load-balancing for metrics of 400,000 or less. So we’d have to set the variance to 3.

R17
router eigrp 10
variance 3

1.3.d iii Add-path

Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_eigrp/configuration/xe-17/ire-xe-17-book/ire-add-path.pdf

The sequence of this blueprint topic is a little wonky, since Add-path if for DMVPN’s, which haven’t been covered yet, and it requires EIGRP Named-mode, which hasn’t been covered yet.

So what’s the deal with Add-path? We’ve come to expect equal cost load balancing by default with EIGRP. However, with EIGRP over DMVPN, you only get one path advertised. Add-path is a feature to advertise up to four paths to the same destination across the DMVPN. A couple notes about it:

• You have to use named mode.
• You can’t use the variance command.
• You configure it on the DMVPN hubs under the af-interface tunnel settings.
• You have to disable eigrp next-hop-self on those tunnel interfaces.
  • We want EIGRP to tell us the peer IP address to get to the next-hop, not tell us “use interface Tunnel 10”.
  • We need the no-ecmp-mode command because no next-hop-self by itself only evaluates the first entry in the topology table for that route. Using no-ecmp-mode will tell it to apply no next-hop-self to subsequent entries, as well.

R21
router eigrp DMVPN
address-family ipv4 autonomous-system 10
af-interface tunnel 10
no next-hop-self no-ecmp-mode
add-paths 4