Which of the following statements is TRUE about the communication occurring between rtrA and rtrB in the given exhibit?

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Last Updated on July 31, 2021 by InfraExam

Which of the following statements is TRUE about the communication occurring between rtrA and rtrB in the given exhibit?

300-410 Part 05 Q20 073
300-410 Part 05 Q20 073
  • The only loopback interface used in the communication is the loopback 0 interface of rtrA.
  • The only loopback interface used in the communication is the loopback 1 interface of rtrB.
  • Both loopback 0 and loopback 1 interfaces are used for the communication between rtrA and rtrB.
  • Neither loopback 0 nor loopback 1 interface is used for the communication between rtrA and rtrB.
Explanation:
The only loopback interface used in the communication is the loopback 0 interface of rtrA. The configuration on the rtrA router indicates that BGP is enabled on rtrA in the autonomous system number (ASN) 450. The neighbor 131.78.45.2 remote-as 450 command establishes a connection with the rtrB interface having the 131.78.45.2 address. The Gi0/1 interface of rtrB has the address 131.78.45.2, which is directly connected to the Gi0/0 interface (132.78.45.1) of rtrA. The next line, neighbor 131.78.45.2 update-source loopback 0, specifies that the 131.78.45.2 interface (Gi0/1) of rtrB communicates with the loopback 0 interface on rtrA.

In the configuration of rtrB, the neighbor 10.144.1.1 remote-as 450 command establishes a neighboring relationship with the interface having the address10.144.1.1. The loopback 0 interface of rtrA has the address 10.144.1.1. The loopback 1 interface on rtrB is assigned an IP address but not used in establishing BGP connections between rtrA and rtrB

Loopback 1 interface of rtrB only would only be used in the communication between rtrA and rtrB if you configured rtrA and rtrB as follows:

rtrA(config)#router bgp 450
rtrA(config-router)# neighbor 131.78.1.1 remote-as 450
rtrB(config)#router bgp 450
rtrB(config-router)# neighbor 131.78.45.1 remote-as 450
rtrB(config-router)# neighbor 131.78.45.1 update-source loopback 1

Both loopback 0 and loopback 1 interfaces are NOT used for communication between rtrA and rtrB. Only the loopback 0 interface of rtrA is used. Both of the loopback interfaces would be used in the communication between rtrA and rtrB only if you changed the configuration of rtrA and rtrB, as given below:

rtrA(config)# router bgp 450
rtrA(config-router)# neighbor 131.78.1.1 remote-as 450
rtrA(config-router)# neighbor 131.78.1.1 update-source loopback 0
rtrB(config)#router bgp 450
rtrB(config-router)# neighbor 10.144.1.1 remote-as 450
rtrB(config-router)# neighbor 10.144.1.1 update-source loopback 1

Because the loopback 0 interface of rtrA is used in communication, is incorrect to state that neither loopback 0 nor loopback 1 interface is used. To ensure that neither of the loopback interfaces are be used for communication, you would configure rtrA and rtrB as follows:

rtrA(config)# router bgp 450
rtrA(config-router)# neighbor 131.78.45.2 remote-as 450
rtrB(config)# router bgp 450
rtrB(config-router)# neighbor 131.78.45.1 remote-as 450

Objective:
Layer 3 Technologies
Sub-Objective:
Configure and verify eBGP (IPv4 and IPv6 address families)

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