What would happen if BGP identifier field of both the routers are identical

Question

Say we

• have two routers R1 and R2. R1 has a router id configured as 1.1.1.1 R2 also has the same router id configured i.e. 1.1.1.1

• R1 and R2 are connected to each other via a P2P link. This has an ip of 192.168.1.1 and 192.168.1.2 respectively.

My Query is ,

• When BGP neighbourship is formed between these two routers, and Open message is sent. If BGP Identifier field contains 1.1.1.1 as value on open messages sent by R1 and R2, what would happen ?
• Would the neighborship form ?
• If an error would be detected and notification sent, which one would it be ?

Thanks in advance

Answer 1

RFC 4271 did not define explicitly what happened in the case of duplicate BGP identifiers, they were simply assumed/designed to be globally unique.

However, to support networks without IPv4, RFC 6286 relaxes the requirement, saying that BGP identifiers only have to be unique within an AS. It revises the open message error handling and collision detection rules to support this.

Under RFC 6286 rules connecting two routers with the same router ID and different AS numbers is acceptable, while connecting two routers with the same router ID and the same AS number is an error. The collision detection rules are revised to say that in the event the router IDs are equal, the AS number is used as a tie breaker.

Answer 2

RFC 4271 defines:

BGP Identifier

A 4-octet unsigned integer that indicates the BGP Identifier of the sender of BGP messages. A given BGP speaker sets the value of its BGP Identifier to an IP address assigned to that BGP speaker. The value of the BGP Identifier is determined upon startup and is the same for every local interface and BGP peer.

Since each (unicast) IP address needs to be uniquely assigned, the identifier needs to be unique as well. Duplicate identifiers may cause false connection collision detection (clause 6.8), leading to instabilities and other general mess. Since this scenario isn't well defined by the RFC, the outcome depends on the actual implementations.