Refer to the exhibit. It is desired to set up a BGPneighbor relationship between routers R1
and R4. BGP packets between them could travel through R2 or R3. What is the simplest
configuration that will allow for failover? Select the best response.

A.
Configure BGP neighbor relationships between all interfaces on R1 and R4.

B.
Install a direct connection between R1 and R4.

C.
Configure loopback interfaces on R1 and R4 to provide the update source address for
BGP packets.

D.
Configure only one neighbor relationship between R1’s 192.168.1.2 interface and R4’s
172.16.10.2 interface.

Explanation:
Assume R4 only uses this configuration:
R4(config)#router bgp 65000
R4(config-router)#neighbor 192.168.1.2 remote-as 65000
Then if R1 sends BGP packets to R4 via R3, the source IP address of the packets is
192.168.2.2. But R4 does not recognize this IP address because 192.168.2.2 is not
configured in the “neighbor” command of R4. Therefore the IBGP session between R1 & R4
is not established -> these packets will be dropped.
The simplest configuration to allow R1 and R4 communicate via both R2 & R3 is to use a
loopback interface address rather than a physical interface address as the source IP
address for all BGP packets. To do this, use the command:
R4(config-router)#neighbor 1.1.1.1 update-source loopback0
(In which 1.1.1.1 is the loopback interface of R1).In practical, we should establish
neighborship with the loopback interface rather than the physical interface because if the
physical interface goes down, the neighborship would be lost while a loopback interface
never goes down.
Also, when configuring “1.1.1.1″as the neighbor, you must configure on R1 the “neighbor
4.4.4.4 update-source loopback0” command so that the source IP address of packets sent
from R1 (loopback0 – 1.1.1.1) will be matched with the neighbor command configured on
R4.