BGP Overview 379
The deficiency of confederation is: when changing an AS into a confederation,
you need to reconfigure your routers, and the topology will be changed.
In large-scale BGP networks, both route reflector and confederation can be used.
BGP GR
n
For GR (Graceful Restart) information, refer to “GR Overview” on page 247.
1 To establish a BGP session with a peer, a BGP GR Restarter sends an OPEN message
with GR capability to the peer.
2 Upon receipt of this message, the peer is aware that the sending router is capable
of Graceful Restart, and sends an OPEN message with GR Capability to the GR
Restarter to establish a GR session. If neither party has the GR capability, the
session established between them will not be GR capable.
3 The GR session between the GR Restarter and its peer goes down when the GR
Restarter restarts BGP. The GR capable peer will mark all routes associated with the
GR Restarter as stale. However, during the configured GR Time, it still uses these
routes for packet forwarding, ensuring that no packet will be lost when routing
information from its peer is recollected.
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.
MP-BGP 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.
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:
■ MP_REACH_NLRI: Multiprotocol Reachable NLRI, for advertising feasible routes
and next hops
■ MP_UNREACH_NLRI: Multiprotocol Unreachable NLRI, for withdrawing
unfeasible routes
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