iBGP and eBGP Configuration

iBGP and eBGP Configuration
In this network, we will configure iBGP and eBGP. The loopback address of each router participating
in iBGP is used in the neighbor statement when referring to the router. The directly
connected interface addresses are used for the eBGP connections.
Using a loopback address for an iBGP session is a good idea. A loopback interface is always
up and will never go down unless administratively shut down. This way, if an iBGP speaker has
more than one path to its iBGP neighbor and one of the paths goes down, the connection will
stay up. The reason the TCP connection stays up is because of the existence of another route to
the same destination. When using loopback interfaces for BGP sessions, the following additional
command needs to be entered in router configuration mode:
neighbor address update-source interface
address - IP address of the remote device.
interface - the interface to use as the source for the BGP session.
Configuring BGP 263
Without this command, the BGP speakers will never become peers with one another.
The reason is because the remote BGP speaker expects to receive the packet from the BGP
speaker’s loopback address, but without the update-source keyword, the BGP packet will
use the address of the outbound interface of the BGP speaker. The remote BGP speaker
will receive the packet and ignore it because that is not the address it is expecting to see.
With the update-source keyword, the packet will contain the address of the loopback
interface and not the outbound interface.
Now that you understand how this works, take a look at Figure 8.13.
We’re now going to do a step-by-step configuration of the network in Figure 8.13. When
configuring this network, we do not want BGP to be synchronized with the IGP. In order to disable
synchronization, you will need to enter the command no synchronization on each router
in AS 200. The loopback addresses for routers R2, R3, and R4 are as follows:
R2 Lo0-2.2.2.2
R3 Lo0-3.3.3.3
R4 Lo0-4.4.4.4