3Com Switch 8800 Configuration Guide Chapter 31 MBGP Multicast Extension Configuration
31-2
I. MP_REACH_NLRI attribute
MP_REACH_NLRI is an optional non-transitive attribute, and can be used to:
z Send the routing information of a new reachable protocol.
z Send the next hop information about the new protocol with the same coding mode
as that of NLRI.
z Enable the router to report part or all of the SNPAs (Sub-network Points of
Attachment) saved in the local system.
II. MP_UNREACH_NLRI attribute
The MP_UNREACH_NLRI is an optional non-transitive attribute that can be used for
the purpose of withdrawing one or multiple unfeasible routes from service. It includes
the following fields:
z AFI and SAFI.
z Withdrawn Routes: Contains one or multiple NLRIs, in which are the unreachable
destination addresses.
An UPDATE packet that contains the MP_UNREACH_NLRI is not required to carry any
other path attributes.
These two attributes enables MBGP to carry multi-protocol information. MSGP
therefore supports both unicast and multicast by constructing different topology maps
to implement appropriate policies. Besides, MBGP may construct different inter-domain
routes for unicast and multicast under a same policy.
31.1.3 MBGP Operating Mode and Message Type MBGP runs on a router in the following two modes:
z IBGP (Internal BGP)
z EBGP (External BGP)
MBGP running in an autonomous system is called IBGP; MBGP running across
autonomous systems is called EBGP.
MBGP offers four types of messages:
z Open Message
z Update Message
z Notification Message
z Keepalive Message
Open Message is the first message sent after the TCP connection is established. It is
used to establish MBGP peer relationship. Notification Message is used to notify errors.
Keepalive message is used to check the validity of a connection. Update Message is
the most important information in the MBGP system, used to exchange routing
information among peers. It consists of three parts at the most: MP_UNREACH_NLRI,
Path Attributes and MP_REACH_NLRI.