BGP Attributes Modified Between External BGP Peers

BD

When external BGP (eBGP) peers exchange routing information, certain BGP attributes are modified by default. Two such attributes are the AS-Path and Next Hop.

1. AS-Path:

The AS-Path attribute is a mandatory attribute in BGP that indicates the sequence of Autonomous Systems (AS) that the BGP update has traversed. When a BGP update is sent from one AS to another, the receiving AS prepends its own AS number to the AS-Path attribute before forwarding the update to the next AS. This is done to prevent routing loops in the BGP network.

When an eBGP peer receives a BGP update, it removes its own AS number from the AS-Path attribute and replaces it with the AS number of the sending AS. This ensures that the AS-Path attribute reflects the correct sequence of ASs that the BGP update has traversed.

2. Next Hop:

The Next Hop attribute in BGP indicates the IP address of the next hop router that should be used to reach the destination network. When an eBGP peer receives a BGP update from another eBGP peer, it modifies the Next Hop attribute to reflect its own IP address as the next hop.

This is done because eBGP peers are typically not directly connected to each other, and the original Next Hop IP address in the BGP update may not be reachable from the receiving eBGP peer. By modifying the Next Hop attribute to its own IP address, the receiving eBGP peer ensures that the BGP update will be forwarded to the correct next hop router and ultimately reach the destination network.

In contrast, when a BGP update is exchanged between internal BGP (iBGP) peers, neither the AS-Path nor the Next Hop attribute is modified by default. This is because iBGP peers are typically directly connected to each other, and the original AS-Path and Next Hop information can be preserved without causing any routing issues.