Routers R1 and R3 are configured as following to be BGP neighbor. And router R2 has two static routes so that R1 is able to ping the interface loopback0 of R3 and R3 is able to ping the loopback0 of R1.
R1 |
R2 |
interface loopback0 ip route 0.0.0.0 0.0.0.0 12.1.1.2 |
interface loopback0 ip route 0.0.0.0 0.0.0.0 23.1.1.2 |
The "show ip bgp neighbors" result of R1 is displayed as following:
The BGP neighbors are not successfully established. Please select TWO possible reasons:
A. Missed the ebgp multi-hop configuration.
B. Missed the update-source configuration.
C. The no synchronization command was configured.
D. Default route was used to access the BGP neighbor.
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Answer: B,D
Explanation:
A: The ebgp multi-hop command is used for eBGP neighbors only. By default, the hop limitation is set to 1 when connecting to an eBGP neighbor. The ebgp multi-hop allows you to change the TTL default value (1) of the TCP packet in order to skip the eBGP hop limitation. However, there is no hop limitation in iBGP session.
B: The update-source command causes the BGP messages to be sourced from the IP address of the loopback interface rather than from the physical interface the message is sent on. Without it, the TCP source of the TCP sessions would be the outgoing interface address. The end points of the TCP sessions would not match and would therefore not come up. Without it, the address is unreachable, and the IBGP session is not created.
C: The no synchronization command is used to disable the BGP synchronization. However, it affects the BGP route learning and route table installation only, and does not concern the neighbor establishment.
D: A default route is never going to be used to establish a BGP session (iBGP/eBGP). You will see the no route output in the debugs though you will be able to ping the BGP neighbor.
In an internal BGP environment where you have more control, you can propagate the route dynamically using a routing protocol. In an external situation, it is recommended to configure a static route to reach the next hop.