Troubleshooting BGP Idle State

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When establishing a new external BGP connection to a peer's loopback interface, it is important to configure the necessary parameters correctly to ensure that the connection is established successfully. In this scenario, the fact that the peer's loopback interface is reachable but the BGP connection remains in Idle state suggests that there may be a configuration issue with one of the required BGP parameters.

Of the four parameters listed in the answers, the parameter that is required to establish the BGP connection is "multihop".

The "multihop" parameter is used to configure BGP to use multiple hops to reach the peer's IP address, which is necessary when the peer's IP address is not directly connected to the local router. In this case, the fact that the peer's loopback interface is reachable suggests that it may be located beyond a router or switch that requires multiple hops to reach.

To configure "multihop", the "neighbor" command is used with the "ttl-security hops" parameter. This specifies the number of hops that BGP is allowed to make to reach the peer's IP address.

For example, to configure "multihop" with a maximum of 3 hops, the following configuration could be used:

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set protocols bgp group <group-name> neighbor <peer-ip> multihop ttl-security hops 3 

This would allow BGP to make up to 3 hops to reach the peer's IP address and establish the BGP connection successfully.

In contrast, the other three parameters listed in the answers are not directly related to establishing a BGP connection to a peer's loopback interface:

• "multipath" is used to enable the installation of multiple BGP paths in the routing table for the same prefix. This is not relevant to establishing a BGP connection to a loopback interface.
• "as-override" is used to override the local AS number with a different AS number when advertising routes to a specific peer. This is also not directly relevant to establishing a BGP connection to a loopback interface.
• "advertise-inactive" is used to advertise inactive BGP routes to a specific peer. This is also not directly relevant to establishing a BGP connection to a loopback interface.