When BGP peers set up their session between
them, they send an OPEN message possibly containing optional parameters.
One optional parameter is capabilities.
Possible capabilities are Multiprotocol extensions, route refresh, outbound
route filtering (ORF), and so on. When the BGP peers exchange the Multiprotocol
extension capability, they exchange AFI and SAFI numbers and thus identify what
the other BGP speaker is capable of.
IPv6 in BGP is implementated via
Multi-Protocol BGP (MPBGP) (RFC 2283), as is MPLS and VPN’s through two new
attributes: MP_UNREACH_NLRI and MP_REACH_NLRI. The first two values in these
two attributes contain the Address Family Identifier (AFI) and the Subsequent
Address Family Identifier (SAFI).
|
AFI
|
Meaning
|
|
1
|
IPv4
|
|
2
|
IPv6
|
.
|
SAFI
|
Meaning
|
|
1
|
Unicast
|
|
2
|
Multicast
|
|
3
|
Unicast and
multicast
|
|
4
|
MPLS Label
|
|
128
|
MPLS-labeled VPN
|
If BGP is carrying IPv4 traffic, AFI equals 1,
SAFI equals 1 for Unicast, or SAFI equals 4 for MPLS.
If BGP is carrying IPv6 traffic, AFI equals 2, SAFI equals 1 for Unicast, and SAFI equals 2 for multicast.
If BGP is carrying IPv6 traffic, AFI equals 2, SAFI equals 1 for Unicast, and SAFI equals 2 for multicast.
This is best seen during BGP session
negotiation by using the “debug bgp all” command. Obviously use this command
with caution and not on full-feed internet peerings.
In the above output you can clearly see these
two BGP peers are capable of supporting IPv4 and IPv6 with Unicast and
MPLS/VPN’s, along with Route-Refresh.
