Within the earlier put up <BGP Multipath State of affairs> we mentioned the BGP multipath situation and learnt how you can configure the identical. Nonetheless, within the final put up router put in two parallel paths within the routing desk from BGP as all of the under parameters have been matching on the 2 paths.
- Weight
- Native Desire
- AS Path (each AS quantity and AS path size)
- Origin code
- MED
- IGP metric
BGP Multipath As-Path Chill out
However what occurs if the CE is dual-homed to 2 totally different service suppliers and each the service suppliers are utilizing totally different AS. In that case, the 2 parallel paths gained’t be the identical because the AS path quantity can be totally different. In such a case to put in two parallel paths from BGP within the routing desk we now have to make use of the BGP function “bgp multipath as-path calm down”.
Allow us to use the under topology to configure the identical:
On R1 we now have two EBGP neighbours to 2 totally different AS. R1 has two parallel paths to succeed in 4.4.4.4 i.e. R1>R2>R4 & R1>R3>R4. By default, BGP will maintain a single path to succeed in 4.4.4.4 on R1. The traditional multipath command would additionally not work right here as AS numbers of R2 and R3 are totally different.
Associated – BGP AS Override
Allow us to take a look on the configurations:
At the moment, on R1 we now have the under configurations the place we’re nonetheless utilizing the utmost path command beneath BGP. Since now the AS numbers are totally different for R2 and R3 we’re seeing solely a single greatest route in BGP desk and likewise a single route within the routing desk.
R1#sh runn | s r bgp
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
D – EIGRP, EX – EIGRP exterior, O – OSPF, IA – OSPF inter space
N1 – OSPF NSSA exterior kind 1, N2 – OSPF NSSA exterior kind 2
E1 – OSPF exterior kind 1, E2 – OSPF exterior kind 2
i – IS-IS, su – IS-IS abstract, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter space, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP
+ – replicated route, % – subsequent hop override
Gateway of final resort shouldn’t be set
1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is immediately related, Loopback0
4.0.0.0/32 is subnetted, 1 subnets
B 4.4.4.4 [20/0] through 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 immediately related, FastEthernet0/0
L 9.9.12.1/32 is immediately related, FastEthernet0/0
C 9.9.13.0/24 is immediately related, FastEthernet1/0
L 9.9.13.1/32 is immediately related, FastEthernet1/0
R1#sh ip bgp
Standing codes: s suppressed, d damped, h historical past, * legitimate, > greatest, i – inner,
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 legitimate, I invalid, N Not discovered
Community Subsequent 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 Finest Route#####
Now allow us to configure the as-path calm down on R1 and verify the BGP and routing desk once more.
R1(config-router)#bgp bestpath as-path multipath-relax
## Please be aware this command is a hidden command and never seen when you do a “?” for assist.
Now allow us to verify the routing desk and BGP desk once more on R1 and we can see each the parallel paths i.e. From R2 and R3 put in.
R1#sh ip route
D – EIGRP, EX – EIGRP exterior, O – OSPF, IA – OSPF inter space
N1 – OSPF NSSA exterior kind 1, N2 – OSPF NSSA exterior kind 2
E1 – OSPF exterior kind 1, E2 – OSPF exterior kind 2
i – IS-IS, su – IS-IS abstract, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter space, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP
+ – replicated route, % – subsequent hop override
Gateway of final resort shouldn’t be set
1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is immediately related, Loopback0
4.0.0.0/32 is subnetted, 1 subnets
B 4.4.4.4 [20/0] through 9.9.13.3, 00:02:43
[20/0] through 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 immediately related, FastEthernet0/0
L 9.9.12.1/32 is immediately related, FastEthernet0/0
C 9.9.13.0/24 is immediately related, FastEthernet1/0
L 9.9.13.1/32 is immediately related, FastEthernet1/0
R1#sh ip bgp
Standing codes: s suppressed, d damped, h historical past, * legitimate, > greatest, i – inner,
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 legitimate, I invalid, N Not discovered
Community Subsequent 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
Associated – BGP Interview Questions
