[Arista EOS] BGP maximum-paths x ecmp y

Tech Keypoint Summary

1. BGP maximum-paths control how many BGP path contributing into ECMP, for BGP table.

2. BGP ecmp control how many IP next-hop could be installed for the ECMP in IP RIB after recursively lookup.

3. If the maximum-path is configured but no ecmp specified, then ecmp is 32 by default.

The ECMP option in the maximum-paths command controls the maximum number of next-hops that get installed in the FIB after recursively looking up a next-hop for a prefix. By default this is already set to the platform maximum which you can check by running the command “show bgp instance | grep -i max”. In order to explain this command please see the following diagram.

BGP using loopback address for neighbor establish. SW1 configured static routes points to SW2/SW3 loopback address for IP reachability.

SW2 advertised 33.33.33.33/32 into BGP, and in this example, we take this routes for explanation.

Sw1 Baseline Configs

sw1#sh bgp instance | grep -i max
Maximum paths: 1 Maximum ECMP: 32
sw1#
sw1#sh ip bgp
BGP routing table information for VRF default
Router identifier 192.85.2.254, local AS number 65000
Route status codes: s — suppressed, * — valid, > — active, # — not installed, E — ECMP head, e — ECMP
 S — Stale, c — Contributing to ECMP, b — backup, L — labeled-unicast
Origin codes: i — IGP, e — EGP, ? — incomplete
AS Path Attributes: Or-ID — Originator ID, C-LST — Cluster List, LL Nexthop — Link Local Nexthop
Network Next Hop Metric LocPref Weight Path
 * > 33.33.33.33/32 3.3.3.3 0 100 0 65001 i
 * 33.33.33.33/32 2.2.2.2 0 100 0 65001 i
sw1#
sw1#sh ip route | egrep -A100 ^” C”
 C 1.1.1.1/32 is directly connected, Loopback0
 S 2.2.2.2/32 [1/0] via 10.1.0.1, Ethernet1
 via 10.1.0.3, Ethernet2
 S 3.3.3.3/32 [1/0] via 10.1.0.5, Ethernet3
 C 10.1.0.0/31 is directly connected, Ethernet1
 C 10.1.0.2/31 is directly connected, Ethernet2
 C 10.1.0.4/31 is directly connected, Ethernet3
 B E 33.33.33.33/32 [20/0] via 10.1.0.5, Ethernet3
 C 192.85.2.0/24 is directly connected, Ethernet21

Based on the above, when the default maximum-paths is used on this platform, we are seeing only a single route entry in the Routing Table.

B E 33.33.33.33/32 [20/0] via 10.1.0.5, Ethernet3

Now, we shall modify the maximum-paths in order to see both paths, via Sw2 and Sw3 and also add the ecmp option.

sw1#conf t
sw1(config)#router bgp 65000
sw1(config-router-bgp)#maximum-paths 2 ecmp 2
sw1(config-router-bgp)#end
sw1#sh run section bgp
router bgp 65000
 distance bgp 20 200 200
 maximum-paths 2 ecmp 2
 neighbor 2.2.2.2 remote-as 65001
 neighbor 2.2.2.2 update-source Loopback0
 neighbor 2.2.2.2 ebgp-multihop
 neighbor 2.2.2.2 maximum-routes 12000 
 neighbor 3.3.3.3 remote-as 65001
 neighbor 3.3.3.3 update-source Loopback0
 neighbor 3.3.3.3 ebgp-multihop
 neighbor 3.3.3.3 maximum-routes 12000

Now, if we look at the BGP Table we shall see both paths chosen and contributing towards ECMP and we shall see both of them in the Routing Table as different next-hops (a.k.a VIAs) for the route 33.33.33.33/32.

sw1#sh ip bgp
BGP routing table information for VRF default
Router identifier 192.85.2.254, local AS number 65000
Route status codes: s — suppressed, * — valid, > — active, # — not installed, E — ECMP head, e — ECMP
 S — Stale, c — Contributing to ECMP, b — backup, L — labeled-unicast
Origin codes: i — IGP, e — EGP, ? — incomplete
AS Path Attributes: Or-ID — Originator ID, C-LST — Cluster List, LL Nexthop — Link Local Nexthop
Network Next Hop Metric LocPref Weight Path
 * >Ec 33.33.33.33/32 2.2.2.2 0 100 0 65001 i
 * ec 33.33.33.33/32 3.3.3.3 0 100 0 65001 i
sw1#
sw1#sh ip route | egrep -A100 ^” C”
 C 1.1.1.1/32 is directly connected, Loopback0
 S 2.2.2.2/32 [1/0] via 10.1.0.1, Ethernet1
 via 10.1.0.3, Ethernet2
 S 3.3.3.3/32 [1/0] via 10.1.0.5, Ethernet3
 C 10.1.0.0/31 is directly connected, Ethernet1
 C 10.1.0.2/31 is directly connected, Ethernet2
 C 10.1.0.4/31 is directly connected, Ethernet3
 B E 33.33.33.33/32 [20/0] via 10.1.0.1, Ethernet1
 via 10.1.0.5, Ethernet3
 C 192.85.2.0/24 is directly connected, Ethernet21

However, the route to 2.2.2.2 which is the BGP next-hop for the prefix 33.33.33.33/32 has two next-hop entries in the Routing table based on the static route, but we are only seeing the one via 10.1.0.1 being installed.

S 2.2.2.2/32 [1/0] via 10.1.0.1, Ethernet1
 via 10.1.0.3, Ethernet2

Because we have the ecmp parameter set to 2, the other next-hop via 10.1.0.3 is not installed. We only see 33.33.33.33 reachable via 10.1.0.1 and 10.1.0.5

B E 33.33.33.33/32 [20/0] via 10.1.0.1, Ethernet1
 via 10.1.0.5, Ethernet3

This is what the ecmp parameter controls.
If we modify the maximum-paths command such that we do not specify the ecmp parameter and leave it to be the platform default, which in this case is 32 then we shall see that for the BGP route we have a total of 3 next-hops VIAs (next-hop entries) in the Routing Table.

sw1(config)#router bgp 65000
sw1(config-router-bgp)#no maximum-paths 2 ecmp 2
sw1(config-router-bgp)#maximum-paths 2
sw1(config-router-bgp)#sh active
router bgp 65000
 distance bgp 20 200 200
 maximum-paths 2
 neighbor 2.2.2.2 remote-as 65001
 neighbor 2.2.2.2 update-source Loopback0
 neighbor 2.2.2.2 ebgp-multihop
 neighbor 2.2.2.2 maximum-routes 12000 
 neighbor 3.3.3.3 remote-as 65001
 neighbor 3.3.3.3 update-source Loopback0
 neighbor 3.3.3.3 ebgp-multihop
 neighbor 3.3.3.3 maximum-routes 12000 
sw1(config-router-bgp)#
sw1(config-router-bgp)#end
sw1#sh bgp instance | grep -i max
Maximum paths: 2 Maximum ECMP: 32

We still see both paths as ECMP contributors in the BGP Table due to maximum-paths 2

sw1#sh ip bgp
BGP routing table information for VRF default
Router identifier 192.85.2.254, local AS number 65000
Route status codes: s — suppressed, * — valid, > — active, # — not installed, E — ECMP head, e — ECMP
 S — Stale, c — Contributing to ECMP, b — backup, L — labeled-unicast
Origin codes: i — IGP, e — EGP, ? — incomplete
AS Path Attributes: Or-ID — Originator ID, C-LST — Cluster List, LL Nexthop — Link Local Nexthop
Network Next Hop Metric LocPref Weight Path
 * >Ec 33.33.33.33/32 2.2.2.2 0 100 0 65001 i
 * ec 33.33.33.33/32 3.3.3.3 0 100 0 65001 i

However, now in the Routing Table we see all 3 next-hops because the ecmp parameter is set to the platform default of 32.

sw1#sh ip route | egrep -A100 ^” C”
 C 1.1.1.1/32 is directly connected, Loopback0
 S 2.2.2.2/32 [1/0] via 10.1.0.1, Ethernet1
 via 10.1.0.3, Ethernet2
 S 3.3.3.3/32 [1/0] via 10.1.0.5, Ethernet3
 C 10.1.0.0/31 is directly connected, Ethernet1
 C 10.1.0.2/31 is directly connected, Ethernet2
 C 10.1.0.4/31 is directly connected, Ethernet3
 B E 33.33.33.33/32 [20/0] via 10.1.0.1, Ethernet1
 via 10.1.0.3, Ethernet2
 via 10.1.0.5, Ethernet3
 C 192.85.2.0/24 is directly connected, Ethernet21