Unlike most routing protocols, BGP only selects a single best path for each prefix. It doesn’t do ECMP (Equal Cost Multi-Path Routing) by default but it is possible to enable this.

In order for BGP to use the second path, the following attributes have to match:

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

Also, the next hop address for each path must be different. This comes into play when you are multihomed to the same router.

In this lesson, I’ll show you how to configure eBGP and iBGP to use more than one path.

Configuration

We’ll start with two eBGP scenarios.

eBGP

When it comes to eBGP, there are two options:

• Multiple paths to the same AS.
• Multiple paths to different ASes.

Same AS

Let’s look at a scenario where we have two paths to the same AS. Here’s the topology:

R1 is in AS 1  and connected to R2/R3 in AS23. R1 will will have paths to get to 192.168.23.0/24.

hostname R1
!
ip cef
!
interface GigabitEthernet0/1
 ip address 192.168.12.1 255.255.255.0
!
interface GigabitEthernet0/2
 ip address 192.168.13.1 255.255.255.0
!
router bgp 1
 neighbor 192.168.12.2 remote-as 23
 neighbor 192.168.13.3 remote-as 23
!
end

hostname R2
!
ip cef
!
interface GigabitEthernet0/1
 ip address 192.168.12.2 255.255.255.0
!
interface GigabitEthernet0/2
 ip address 192.168.23.2 255.255.255.0
!
router bgp 23
 network 192.168.23.0
 neighbor 192.168.12.1 remote-as 1
 neighbor 192.168.23.3 remote-as 23
!
end

hostname R3
!
ip cef
!
interface GigabitEthernet0/1
 ip address 192.168.13.3 255.255.255.0
!
interface GigabitEthernet0/2
 ip address 192.168.23.3 255.255.255.0
!
router bgp 23
 neighbor 192.168.13.1 remote-as 1
 neighbor 192.168.23.2 remote-as 23
!
end

Here’s the BGP table of R1:

R1#show ip bgp 
BGP table version is 2, local router ID is 192.168.13.1
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, 
              t secondary path, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *    192.168.23.0     192.168.13.3                           0 23 i
 *>                    192.168.12.2             0             0 23 i

R1 has two equal paths but decided to install the path to R2. We can enable load balancing with the maximum-paths command:

R1(config)#router bgp 1
R1(config-router)#maximum-paths 2

Let’s take another look at the BGP table:

R1#show ip bgp 
BGP table version is 3, local router ID is 192.168.13.1
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, 
              t secondary path, 
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   192.168.23.0     192.168.13.3                           0 23 i
 *>                    192.168.12.2             0             0 23 i

Now we have two entries. Note the “m” that stands for multipath. Both paths are installed in the routing table:

R1#show ip route bgp

B     192.168.23.0/24 [20/0] via 192.168.13.3, 00:13:02
                      [20/0] via 192.168.12.2, 00:13:02

That’s looking good.

Different AS

Let’s look at another eBGP scenario. This time, we have multiple AS numbers:

R1 can go through AS 3 or AS 2 to get to 4.4.4.4/32 in AS 4.

hostname R1
!
ip cef
!
interface GigabitEthernet0/1
 ip address 192.168.12.1 255.255.255.0
!
interface GigabitEthernet0/2
 ip address 192.168.13.1 255.255.255.0
!
router bgp 1
 neighbor 192.168.12.2 remote-as 2
 neighbor 192.168.13.3 remote-as 3
!
end

hostname R2
!
ip cef
!
interface GigabitEthernet0/1
 ip address 192.168.12.2 255.255.255.0
!
interface GigabitEthernet0/2
 ip address 192.168.24.2 255.255.255.0
!
router bgp 2
 neighbor 192.168.12.1 remote-as 1
 neighbor 192.168.24.4 remote-as 4
!
end

hostname R3
!
ip cef
!
interface GigabitEthernet0/1
 ip address 192.168.13.3 255.255.255.0
!
interface GigabitEthernet0/2
 ip address 192.168.34.3 255.255.255.0
!
router bgp 3
 neighbor 192.168.13.1 remote-as 1
 neighbor 192.168.34.4 remote-as 4
!
end

hostname R4
!
ip cef
!
interface Loopback0
 ip address 4.4.4.4 255.255.255.255
!
interface GigabitEthernet0/1
 ip address 192.168.24.4 255.255.255.0
!
interface GigabitEthernet0/2
 ip address 192.168.34.4 255.255.255.0
!
router bgp 4
 network 4.4.4.4 mask 255.255.255.255
 neighbor 192.168.24.2 remote-as 2
 neighbor 192.168.34.3 remote-as 3
!
end

Here’s the BGP table of R1:

R1#show ip bgp
BGP table version is 2, local router ID is 192.168.13.1
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, 
              t secondary path, 
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       192.168.13.3                           0 3 4 i
 *>                    192.168.12.2                           0 2 4 i

R1 has installed R2 as its next hop address. Let’s see if we can change that:

R1(config)#router bgp 1
R1(config-router)#maximum-paths 2

This command alone, however, doesn’t help:

R1#show ip bgp
*Mar 21 11:10:13.118: %SYS-5-CONFIG_I: Configured from console by console
BGP table version is 2, local router ID is 192.168.13.1
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, 
              t secondary path, 
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       192.168.13.3                           0 3 4 i
 *>                    192.168.12.2                           0 2 4 i

The problem here is that we have two different AS numbers, AS 2 and AS 3. We can tell BGP to “relax” its requirement of having the same AS path numbers and AS path length to only checking the AS path length. This can be done with the following hidden command: