ECMP

 

Cisco ECMP

ASA can uses up to 8 equal cost routes to the same destination network.  Traffic is distributed among the specified gateways based on an algorithm that hashes the source and destination IP addresses, incoming interface, protocol, source and destination ports. As opposed to round-robin load balancing, packets with the same source and destination pair are always sent towards the same next hop, as per the computed hash.

If you configure traffic zones to contain a group of interfaces, you can have up to 8 equal cost static or dynamic routes across up to 8 interfaces within each zone. For example, you can configure multiple default routes across three interfaces in the zone:

route outside1 0 0 10.1.1.2
route outside2 0 0 10.2.1.2
route outside3 0 0 10.3.1.2

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Asymmetric routing is allowed in ECMP zone. 

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Equal Cost Multiple Path (ECMP) enables the firewall to use up to four equal-cost routes to the same destination