- In EBGP, by default only two directly connected Cisco IOS router can establish a neighbor adjacency.
- So as per BGP requirement, the external peers should be directly connected. "Neighbor EBGP-multihop" command breaks this barrier, it will allow the neighbor ship adjacency between indirectly connected EBGP peer.
- EBGP router has TTL value 1 by default, if BGP neighbor is more than one hop away, then TTL value will be decrease to 0 and it will be discarded.
- TTL range is from 1 to 255. We can set it manually via "neighbor ebgp-multihop" command.
- R1 and R3 are in different AS, and non BGP router R2 is available between them, so R1 and R3 are not directly connected.
- If R1 wants to establish the neighbor ship with R3 then TTL value should be 2, because R3 is 2 hop away from R1.
- When R1 will reach to R2 then TTL value will be decrease by 1 and it will be 1.
- When R1 will reach to R3 then TTL value will be decrease by 2 and it will be 0.
- After that neighbor ship will be establish between indirectly connected EBGP peer R1 and R3.
Without "neighbor ebgp-multihop" command
Configuration -
R1 Configuration -
interface FastEthernet0/0
ip address 192.167.22.1 255.255.255.0
duplex auto
speed auto
!
router bgp 100
no synchronization
bgp log-neighbor-changes
neighbor 192.167.23.3 remote-as 200
no auto-summary
!
ip route 192.167.23.0 255.255.255.0 192.167.22.2
R2 Configuration -
interface FastEthernet0/0
ip address 192.167.22.2 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet1/0
ip address 192.167.23.2 255.255.255.0
duplex auto
speed auto
R3 Configuration -
interface FastEthernet1/0
ip address 192.167.23.3 255.255.255.0
duplex auto
speed auto
!
router bgp 200
no synchronization
bgp log-neighbor-changes
neighbor 192.167.22.1 remote-as 100
no auto-summary
!
ip route 192.167.22.0 255.255.255.0 192.167.23.2
Verification -
R1#show ip bgp summary
BGP router identifier 192.167.22.1, local AS number 100
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
192.167.23.3 4 200 0 0 0 0 0 never Active
R3#show ip bgp summary
BGP router identifier 192.167.23.3, local AS number 200
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
192.167.22.1 4 100 0 0 0 0 0 never Active
You can see in above output, configuration is correct but there is no neighbor-ship adjacency, because both EBGP peer R1 and R3 are indirectly connected.
With "neighbor ebgp-multihop" command
Configuration -
Now we configured "neighbor ebgp-multihop" command on both Router R1 and R3
R1 Configuration -
interface FastEthernet0/0
ip address 192.167.22.1 255.255.255.0
duplex auto
speed auto
!
router bgp 100
no synchronization
bgp log-neighbor-changes
neighbor 192.167.23.3 remote-as 200
neighbor 192.167.23.3 ebgp-multihop 2
no auto-summary
!
ip route 192.167.23.0 255.255.255.0 192.167.22.2
R3 Configuration -
interface FastEthernet1/0
ip address 192.167.23.3 255.255.255.0
duplex auto
speed auto
!
router bgp 200
no synchronization
bgp log-neighbor-changes
neighbor 192.167.22.1 remote-as 100
neighbor 192.167.22.1 ebgp-multihop 2
no auto-summary
!
ip route 192.167.22.0 255.255.255.0 192.167.23.2
Verification -
Now check the below output
R1#show ip bgp summary
BGP router identifier 192.167.22.1, local AS number 100
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
192.167.23.3 4 200 15 15 1 0 0 00:11:42 0
R3#show ip bgp summary
BGP router identifier 192.167.23.3, local AS number 200
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
192.167.22.1 4 100 15 15 1 0 0 00:11:49 0
Now we can see in above output, now after using "neighbor ebgp-multihop" command neighor-ship established between both indirectly connected EBGP peer R1 and R2.
Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.
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