EBGP-Multihop command:
eBGP by default requires two Cisco routers to be connected directly in order to form the eBGP neighbor adjacency. eBGP multihop is used when eBGP neighbors are not directly connected.
Let’s consider the below scenario. We want to establish eBGP adjacency between the two routers Rtr1 and Rtr2. Rtr1 is assigned with AS number 100 while Rtr2 uses AS number 200.
First, let us configure the eBGP using the routers physical interfaces.
Now, let us use the same routers but try to establish the eBGP adjacency using the loopback adapters instead of physical ones.
IP Configuration:
| R1(config)#int gi0/0 R1(config-if)#ip address 172.16.10.1 255.255.255.0 |
| R3(config)#int gi0/0 R3(config-if)#ip address 172.16.10.2 255.255.255.0 |
Configure loopback interfaces:
| R1(config-if)#int lo0 *Apr 25 09:23:15.114: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0, changed state to up R1(config-if)#ip address 1.1.1.1 255.255.255.0 |
| R3(config-if)#int lo0 *Apr 25 09:25:38.713: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0, changed state to up R3(config-if)#ip address 2.2.2.1 255.255.255.0 |
I will configure the static routes so that routers Rtr1 and Rtr2 can reach each others loopback interfaces.
| R1(config)#ip route 2.2.2.0 255.255.255.0 192.168.10.2 |
| R3(config)#ip route 1.1.1.0 255.255.255.0 172.16.10.1 |
BGP Configuration on both routers:
| R1(config)#router bgp 100 R1(config-router)#neighbor 2.2.2.1 remote-as 200 R1(config-router)#neighbor 2.2.2.1 update-source lo0 R1(config-router)#network 1.1.1.0 mask 255.255.255.0 |
| R3(config)#router bgp 200 R3(config-router)#neighbor 1.1.1.1 remote-as 100 R3(config-router)#neighbor 1.1.1.1 update-source lo0 R3(config-router)#network 2.2.2.0 mask 255.255.255.0 |
