3Com Switch 8800 Configuration Guide Chapter 31 MBGP Multicast Extension Configuration31-2I. MP_REACH_NLRI attributeMP_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 modeas that of NLRI.z Enable the router to report part or all of the SNPAs (Sub-network Points ofAttachment) saved in the local system.II. MP_UNREACH_NLRI attributeThe MP_UNREACH_NLRI is an optional non-transitive attribute that can be used forthe purpose of withdrawing one or multiple unfeasible routes from service. It includesthe following fields:z AFI and SAFI.z Withdrawn Routes: Contains one or multiple NLRIs, in which are the unreachabledestination addresses.An UPDATE packet that contains the MP_UNREACH_NLRI is not required to carry anyother path attributes.These two attributes enables MBGP to carry multi-protocol information. MSGPtherefore supports both unicast and multicast by constructing different topology mapsto implement appropriate policies. Besides, MBGP may construct different inter-domainroutes for unicast and multicast under a same policy.31.1.3 MBGP Operating Mode and Message TypeMBGP 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 acrossautonomous systems is called EBGP.MBGP offers four types of messages:z Open Messagez Update Messagez Notification Messagez Keepalive MessageOpen Message is the first message sent after the TCP connection is established. It isused 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 isthe most important information in the MBGP system, used to exchange routinginformation among peers. It consists of three parts at the most: MP_UNREACH_NLRI,Path Attributes and MP_REACH_NLRI.
