BGP Multipath As-Path Relax

In the previous post <link> we discussed the BGP multipath scenario and learnt how to configure the same. However in the last post router installed two parallel paths in the routing table from BGP as all the below parameters were matching on the two paths.

  • Weight
  • Local Preference
  • AS Path (both AS number and AS path length)
  • Origin code
  • MED
  • IGP metric

But what happens if the CE is dual homed to two different service providers and both the service providers are using different AS. In that case the two parallel paths won’t be the same as the AS path number would be different. In such a case to install two parallel paths from BGP in the routing table we have to use the BGP feature “bgp multipath as-path relax”.


Let us use the below topology to configure the same:

On R1 we have two EBGP neighbors to two different AS. R1 has two parallel paths to reach 4.4.4.4 i.e. R1>R2>R4 & R1>R3>R4. By default BGP will keep a single path to reach 4.4.4.4 on R1. Normal multipath command would also not work here as AS numbers of R2 and R3 are different.

Let us have a look at the configurations:

Currently on R1 we have the below configurations where we are still using the maximum path command under BGP. Since now the AS numbers are different for R2 and R3 we are seeing only a single best route in BGP table and also a single route in the routing table.

R1#sh runn | s r bgp

router bgp 100

bgp log-neighbor-changes

neighbor 9.9.12.2 remote-as 200

neighbor 9.9.13.3 remote-as 300

maximum-paths 2

R1#sh ip route

Codes: L – local, C – connected, S – static, R – RIP, M – mobile, B – BGP

D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area

N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2

E1 – OSPF external type 1, E2 – OSPF external type 2

i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2

ia – IS-IS inter area, * – candidate default, U – per-user static route

o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP

+ – replicated route, % – next hop override

 

Gateway of last resort is not set

 

1.0.0.0/32 is subnetted, 1 subnets

C        1.1.1.1 is directly connected, Loopback0

4.0.0.0/32 is subnetted, 1 subnets

B        4.4.4.4 [20/0] via 9.9.12.2, 00:03:40

9.0.0.0/8 is variably subnetted, 4 subnets, 2 masks

C        9.9.12.0/24 is directly connected, FastEthernet0/0

L        9.9.12.1/32 is directly connected, FastEthernet0/0

C        9.9.13.0/24 is directly connected, FastEthernet1/0

L        9.9.13.1/32 is directly connected, FastEthernet1/0

R1#sh ip bgp

BGP table version is 2, local router ID is 1.1.1.1

Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,

r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,

x best-external, a additional-path, c RIB-compressed,

Origin codes: i – IGP, e – EGP, ? – incomplete

RPKI validation codes: V valid, I invalid, N Not found

 

Network           Next Hop         Metric     LocPrf       Weight       Path

*   4.4.4.4/32       9.9.13.3                                                     0             300 400 i

*>                         9.9.12.2                                                   0            200 400 I   ####Single Best Route#####

Now let us configure the as-path relax on R1 and check the BGP and routing table again.

R1(config)#router bgp 100

R1(config-router)#bgp bestpath as-path multipath-relax

## Please note this command is a hidden command and not visible if you do a “?” for help.

Now let us check the routing table and BGP table again on R1 and we will be able to see both the parallel paths i.e. From R2 and R3 installed.

R1#sh ip route

Codes: L – local, C – connected, S – static, R – RIP, M – mobile, B – BGP

D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area

N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2

E1 – OSPF external type 1, E2 – OSPF external type 2

i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2

ia – IS-IS inter area, * – candidate default, U – per-user static route

o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP

+ – replicated route, % – next hop override

Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets

C        1.1.1.1 is directly connected, Loopback0

4.0.0.0/32 is subnetted, 1 subnets

B        4.4.4.4 [20/0] via 9.9.13.3, 00:02:43

                       [20/0] via 9.9.12.2, 00:02:43

9.0.0.0/8 is variably subnetted, 4 subnets, 2 masks

C        9.9.12.0/24 is directly connected, FastEthernet0/0

L        9.9.12.1/32 is directly connected, FastEthernet0/0

C        9.9.13.0/24 is directly connected, FastEthernet1/0

L        9.9.13.1/32 is directly connected, FastEthernet1/0

R1#sh ip bgp

BGP table version is 3, local router ID is 1.1.1.1

Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,

r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,

x best-external, a additional-path, c RIB-compressed,

Origin codes: i – IGP, e – EGP, ? – incomplete

RPKI validation codes: V valid, I invalid, N Not found

Network                   Next Hop            Metric       LocPrf         Weight         Path

 *m  4.4.4.4/32       9.9.13.3                                                        0              300 400 i

 *>                                 9.9.12.2                                                    0                200 400 i

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