I show you in the previous blog how to establish neighbourship between BGP using the Physicl Interface of the Router but today we will use a different appproach with a technology called {LOOPBACK}.
Using loopback interfaces for BGP neighbors provides stability by ensuring the session stays up even if physical links fail, as loopbacks are always on, but requires an Interior Gateway Protocol (IGP) like OSPF to advertise them and the update-source command to source BGP packets from the loopback.

from the picture above you can see my scenario is an iBGP neighbourship so let’s start
First i configure the IP Addresses on both Routers with making sure an extra step which is Reachability.
The loopback IP address must be reachable via an IGP (OSPF, EIGRP, IS-IS) or static routes.

Repeat the same steps on Router 2
Now i will configure basic BGP neighbourship command and sharing my network using the network command

i will repeat the same steps on Router 2
Now we will check the status of our neighborship and ….
oops [ACTIVE]
no one want to see this
BGP Neighborship in the ACTIVE state means the router is actively trying to establish a TCP connection with its peer by initiating the connection on port 179, but the handshake hasn’t completed.

and let me see the detail
you can see in the last line [No active TCP Connection]
we are missing the most important command [update-source loopback 0]
telling the BGP Router to use the loopback interface as the source of conenction

Repeat the same step on Router2
YESSS and the neighbor cames up
And also im receiving a prefix from my neighbor
Issue the command [show ip bgp]
i can see the network i received

That was the iBGP case
but what if we had an eBGP situation and we are using the loopback

so its different when both router in different AS and the line before the last shows a new error
[External BGP neighbor not directly connected]

so we need to add a different command
[ebgp-multihop] which increases this TTL value as the default is 1

Repeat the same on Router 2
and YESSS the nighborship cames up
and now im receiving the prefix again

and i can ping my neighbor internal network

Note:

• By default, eBGP uses a TTL of 1, which means routers must be directly connected. If a BGP router is more than one hop away, the TTL decreases from 1 to 0, and the packet is discarded. The ebgp-multihop command increases this TTL value, enabling BGP neighbors to be multiple hops apart.

Welcome Back
in the previous blog i show you how to establish BGP Neighbourship using direct connection BUT what if the neighbor were not directly connected the solution is (Multihop). By default eBGP uses a TTL of 1 (multihop) increases this TTL value which enable BGP neighbors to be multiple hops apart.

this is my GNS3 Scenario:
Now this is the configuration on R2 but also in each router i need an [IP Route] to the other Router
Then i will configure the basic BGP neighbourship which i explain in the previous blog

Repeat the steps for R3

Now when issue the command (show ip bgp summary) you can see the neighbor stuck on status (idle)
to see more detail i will issue the command (show ip bgp neighbor) you see in the last line in the picture it mention (External BGP neighbor not directly connected)

so now let me add the magic touch (ebgb-multihop 2)
2 is the number of the hop
also in the other neighbor
I add the debug command so i can see the result
and as you can see the neighbor is up
and now as you can see the status change and i receive 1 prefix from my neighbor
and everything is perfect

Note:

• By default, eBGP uses a TTL of 1, which means routers must be directly connected only and If a BGP router is more than one hop away, the TTL decreases from 1 to 0, and WAK WAK WAK the packet is discarded.
• in case the connection is direct between eBGP that is TTL of 1 so it will check if neighbor in the same subnet but im using loopback then configure (neighbor x.x.x.x disable-connected-check) on both router