3com 3031 Instruccion De Instalación

At present, the most widely used inter-domain unicast routing protocol is BGP-4. 
Because the multicast topology may be different from the unicast topology, BGP-4 
must be modified in order to implement the transmission of inter-domain 
multicast routing information. Some routers in the network may only support 
unicast rather than multicast and may not forward multicast packets since the 
particular policy requires that to construct inter-domain multicast trees, the 
knowledge of multicast-supporting parts of the network (or the multicast network 
topology) is also necessary, other than that of the unicast routing information.

BGP-4 has been proved to be an effective and stable inter-domain unicast routing 
protocol. Therefore, it is more rational to enhance and extend the BGP-4 protocol 
ithan to construct a new protocol. RFC2858 specifies the methods of 
multi-protocol BGP extension. The extended BGP (Multi-protocol BGP, MBGP or 
BGP-4+) carries not only the IPv4 unicast routing information but also routing 
information of other network layer protocols (such as multicast and IPv6 etc). 
Carrying the multicast routing information is only one of the extension functions. 
This chapter describes mainly MBGP extension for multicast.

MBGP enables unicast and multicast routing information to be exchanged through 
the same process but stored in different routing tables. As MBGP is an enhanced 
version of BGP-4, all the common policies and configuration methods that BGP-4 
supports can be applied to multicast.

To enable MBGP to support multicast, RFC2858 defines two new path attributes 
MP_REACH_NLRI (Multiprotocol Reachable NLRI) and MP_UNREACH_NLRI 
(Multiprotocol Unreachable NLRI) in UPDATE packets. They are all optional 
non-transitive attributes, that is, routers that do not support MBGP can ignore the 
information in the attributes and not forward the attributes.

Among the information carried by MP_REACH_NLRI and MP_UNREACH_NLRI, AFI 
(Address Family Identifier) and SAFI (Subsequent Address Family Identifier) can 
identify for which address family the information is. SAFI is a complement to NLRI 
(Network Layer Reachability Information) with1 for the unicast mode of NLRI , and 
2 for the multicast mode of NLRI.

MP_REACH_NLRI is an optional non-transitive attribute, and can be used to:

■

Send the routing information to a new protocol.

■

Send the next hop information about the new protocol with the same coding 
mode as that of NLRI.