4.2. BGP Peering Segments

In the context of BGP Egress Peer Engineering (EPE), as described in [SR-CENTRAL-EPE], an EPE-enabled egress node MAY advertise segments corresponding to its attached peers. These segments are called BGP peering segments or BGP peering SIDs. They enable the expression of source-routed inter-domain paths.

An ingress border router of an Autonomous System (AS) may compose a list of segments to steer a flow along a selected path within the AS towards a selected egress border router C of the AS and through a specific peer. At a minimum, a BGP peering engineering policy applied at an ingress node involves two segments: the Node-SID of the chosen egress node and the BGP peering segment for the chosen egress node peer or peering interface.

Three types of BGP peering segments/SIDs are defined: PeerNode SID, PeerAdj SID, and PeerSet SID.

• PeerNode SID: a BGP PeerNode segment/SID is a local segment. At the BGP node advertising it, its semantics are:

  • SR operation: NEXT.
  • Next-Hop: the connected peering node to which the segment is related.

• PeerAdj SID: a BGP PeerAdj segment/SID is a local segment. At the BGP node advertising it, the semantics are:

  • SR operation: NEXT.
  • Next-Hop: the peer connected through the interface to which the segment is related.

• PeerSet SID: a BGP PeerSet segment/SID is a local segment. At the BGP node advertising it, the semantics are:

  • SR operation: NEXT.
  • Next-Hop: load-balance across any connected interface to any peer in the related group.

A peer set could be all the connected peers from the same AS or a subset of these. A group could also span across AS. The group definition is a policy set by the operator.

The BGP extensions necessary in order to signal these BGP peering segments are defined in [BGPLS-SR-EPE].