bgpd: Implement EBGP-OAD peering type

bgpd: Implement EBGP-OAD peering type

At each EBGP boundary, BGP path attributes are modified as per [RFC4271], which includes stripping any IBGP-only attributes.

Some networks span more than one autonomous system and require more flexibility in the propagation of path attributes. It is worth noting that these multi-AS networks have a common or single administrative entity. These networks are said to belong to One Administrative Domain (OAD). It is desirable to carry IBGP-only attributes across EBGP peerings when the peers belong to an OAD.

This document defines a new EBGP peering type known as EBGP-OAD, which is used between two EBGP peers that belong to an OAD. This document also defines rules for route announcement and processing for EBGP-OAD peers.

https://datatracker.ietf.org/doc/html/draft-uttaro-idr-bgp-oad

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>