Operation Manual – IPv4 Routing
H3C S5500-EI Series Ethernet Switches
Chapter 5 BGP Configuration
5-18
4)
After the restart, the GR Restarter will reestablish a GR session with its peer and
send a new GR message notifying the completion of restart. Routing information is
exchanged between them for the GR Restarter to create a new routing table and
forwarding table with stale routing information removed. Thus the BGP routing
convergence is complete.
5.1.7 MP-BGP
I. Overview
The legacy BGP-4 supports IPv4, but does not support other network layer protocols
like IPv6.
To support more network layer protocols, IETF extended BGP-4 by introducing
Multiprotocol Extensions for BGP-4 (MP-BGP), which is defined in RFC2858.
Routers supporting MP-BGP can communicate with routers not supporting MP-BGP.
II. MP-BGP extended attributes
In BGP-4, the three types of attributes for IPv4, namely NLRI, NEXT_HOP and
AGGREGATOR (contains the IP address of the speaker generating the summary route)
are all carried in updates.
To support multiple network layer protocols, BGP-4 puts information about network
layer into NLRI and NEXT_HOP. MP-BGP introduced two path attributes:
z
MP_REACH_NLRI: Multiprotocol Reachable NLRI, for advertising feasible routes
and next hops
z
MP_UNREACH_NLRI: Multiprotocol Unreachable NLRI, for withdrawing
unfeasible routes
The above two attributes are both optional non-transitive, so BGP speakers not
supporting multi-protocol ignore the two attributes and do not forward them to peers.
III. Address family
MP-BGP employs address family to differentiate network layer protocols. For address
family values, refer to RFC 1700 (Assigned Numbers). Currently, the system supports
multiple MP-BGP extensions, including IPv6 extension. Different extensions are
configured in respective address family view.