Equal-cost multi-path routing
The ECMP feature allows OSPFv3 to add routes with multiple next-hop addresses and with equal costs to a given destination in the forwarding information base (FIB) on the routing switch. For example, if multiple, equal-cost, next-hop routes exist on a routing switch for a destination in a network with the prefix 2620:e::/64, these routes would appear similar to the following in the IPv6 Route Entries Table:
show ipv6 route command output with multiple next-hop routes
Switch(config)# show ipv6 route IPv6 Route Entries Destination : ::1/128 Gateway : lo0 Type: connected Sub-Type: NA Distance: 0 Metric: 1 Destination : 2620:c::/64 Gateway : 2620:e::55:2 Type: static Sub-Type: NA Distance: 200 Metric: 1 Destination : 2620:a::/64 Gateway : fe80::22:3%vlan22 Type: ospf3 Sub-Type: InterArea Distance: 110 1 Metric: 2 Destination : 2620:a::/64 Gateway : fe80::22:5%vlan22 Type: ospf3 Sub-Type: InterArea Distance: 110 Metric: 2 Destination : 2620:a::/64 Gateway : fe80::22:11%vlan22 Type: ospf3 Sub-Type: InterArea Distance: 110 Metric: 2 Destination : 2620:b::/64 Gateway : VLAN22 Type: connected Sub-Type: NA Distance: 0 Metric: 1
1 Multiple next-hop gateway addresses are displayed for the destination network 2620:a::/64
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Intra-area (routes to the destination in the same OSPFv3 area)
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Inter-area (routes to the destination through another OSPFv3 area)
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External (routes to the destination through another autonomous system)
Multiple ECMP next-hop routes cannot be a mixture of intra-area, inter-area, and external routes. In the above example, the multiple next-hop routes to network 2620:a::/64 are all inter-area.
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Intra-area routes are preferred to inter-area routes.
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Inter-area routes are preferred to external routes through a neighboring AS.
In addition, ECMP ensures that all traffic forwarded to a given host address follows the same path, which is selected from the possible next-hop routes.
ECMP load-sharing does not affect routed traffic to different hosts on the same subnet. That is, all traffic for different hosts on the same subnet will go through the same next-hop router. For example, if subnet 2001:db8:0:1f::/64 includes two servers at 2001:db8:0:1f::1ab:101 and 2001:db8:0:1f::1ab:93, all traffic from router "A" to these servers will go through the same next-hop router.