BGP Next Hop Self

One potential issue with iBGP is that it doesn’t change the next hop IP address. Sometimes this can cause reachability issues. Let’s look at an example:

Above we have R1 and R2 in AS 12 running iBGP. R3 is in AS 3 and we use eBGP between R2 and R3. Once we advertise network 3.3.3.0 /24 on R3 in BGP then R2 will learn this prefix and stores it in its BGP table, the next hop IP adress will be 192.168.23.3.

Once R1 learns about prefix 3.3.3.0 /24 then the next hop IP address will remain 192.168.23.3. When R1 doesn’t know how to reach this IP address then it will fail to install 3.3.3.0 /24 in its routing table.

Let’s take a look at the configuration, I’ll show you two methods how we can deal with this issue.

Configuration

Here’s the BGP configuraton that we will use:

R1(config)#router bgp 12
R1(config-router)#neighbor 192.168.12.2 remote-as 12

R2(config)#router bgp 12
R2(config-router)#neighbor 192.168.12.1 remote-as 12
R2(config-router)#neighbor 192.168.23.3 remote-as 3

R3(config)#router bgp 3
R3(config-router)#neighbor 192.168.23.2 remote-as 12
R3(config-router)#network 3.3.3.0 mask 255.255.255.0

The configuration is pretty straight forward. We use iBGP between R1/R2 and eBGP between R2/R3. On R3 we advertised 3.3.3.0 /24 in BGP. Let’s take a look at the BGP tables:

R2#show ip bgp
BGP table version is 2, local router ID is 192.168.23.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 3.3.3.0/24       192.168.23.3             0             0 3 i

R2 has installed 3.3.3.0 /24 in its BGP table and it is a valid route, the next hop is 192.168.23.3. Let’s check R1:

R1#show ip bgp
BGP table version is 1, local router ID is 192.168.12.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
* i3.3.3.0/24       192.168.23.3             0    100      0 3 i

R1 learns the prefix but it’s unable to install it in the routing table:

R1#show ip route bgp

The problem here is that the next hop IP address is 192.168.23.3. Does R1 have any clue how to reach this address?

R1#show ip route
Codes: 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

Gateway of last resort is not set

C    192.168.12.0/24 is directly connected, FastEthernet0/0

R1 doesn’t know so it’s impossible to install 3.3.3.0 /24 in the routing table. How can we fix this? I’ll show you two different methods.

Advertise Network

The first solution is simple, we can advertise the network in iBGP (or an IGP if you use one) so that R1 is able to reach the next hop. Let’s advertise 192.168.23.0 /24 in BGP:

R2(config)#router bgp 12
R2(config-router)#network 192.168.23.0 mask 255.255.255.0

Now take a look at R1:

R1#show ip bgp
BGP table version is 3, local router ID is 192.168.12.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*>i3.3.3.0/24       192.168.23.3             0    100      0 3 i
*>i192.168.23.0     192.168.12.2             0    100      0 i

R1 learns about 192.168.23.0 /24 so now it knows how to reach the next hop for 3.3.3.0 /24. It can now install this network in the routing table:

R1#show ip route bgp
     3.0.0.0/24 is subnetted, 1 subnets
B       3.3.3.0 [200/0] via 192.168.23.3, 00:16:20
B    192.168.23.0/24 [200/0] via 192.168.12.2, 00:16:25

hostname R1
!
interface fastEthernet0/0
 ip address 192.168.12.1 255.255.255.0
!
router bgp 12
 neighbor 192.168.12.2 remote-as 12
!
end

hostname R2
!
interface fastEthernet1/0
 ip address 192.168.12.2 255.255.255.0
!
interface fastEthernet0/1
 ip address 192.168.23.1 255.255.255.0
!
router bgp 12
 neighbor 192.168.12.1 remote-as 12
 neighbor 192.168.23.3 remote-as 3
 neighbor 192.168.12.1 next-hop-self
 network 192.168.23.0 mask 255.255.255.0
!
end

hostname R3
!
interface fastEthernet0/0
 ip address 192.168.23.3 255.255.255.0
!
router bgp 3
 neighbor 192.168.23.2 remote-as 12
 network 3.3.3.0 mask 255.255.255.0
!
end

This will work but there is another solution that is easier. Let’s clean up before we continue:

R2(config)#router bgp 12
R2(config-router)#no network 192.168.23.0 mask 255.255.255.0

Now we can try something else…

Next Hop Self

Instead of advertising the network in between R2 and R3 we can configure R2 so that it will change the next hop IP address to it’s own address:

R2(config)#router bgp 12
R2(config-router)#neighbor 192.168.12.1 next-hop-self

From now on, when R2 advertises something to R1 then it will include it’s own IP address as the next hop. Let’s verify this:

R1#show ip bgp
BGP table version is 6, local router ID is 192.168.12.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*>i3.3.3.0/24       192.168.12.2             0    100      0 3 i

Above you can see that R1 learns 3.3.3.0 /24 with 192.168.12.2 as the next hop. Since this is directly connected, we can use this information:

R1#show ip route bgp
     3.0.0.0/24 is subnetted, 1 subnets
B       3.3.3.0 [200/0] via 192.168.12.2, 00:00:33

R1 has installed it in its routing table, problem solved!

hostname R1
!
interface fastEthernet0/0
 ip address 192.168.12.1 255.255.255.0
!
router bgp 12
 neighbor 192.168.12.2 remote-as 12
!
end

hostname R2
!
interface fastEthernet1/0
 ip address 192.168.12.2 255.255.255.0
!
interface fastEthernet0/1
 ip address 192.168.23.1 255.255.255.0
!
router bgp 12
 neighbor 192.168.12.1 remote-as 12
 neighbor 192.168.23.3 remote-as 3
 neighbor 192.168.12.1 next-hop-self
!
end

hostname R3
!
interface fastEthernet0/0
 ip address 192.168.23.3 255.255.255.0
!
router bgp 3
 neighbor 192.168.23.2 remote-as 12
 network 3.3.3.0 mask 255.255.255.0
!
end

I hope this lesson has been useful, if you have any questions feel free to leave a comment!