3com 8807 User Guide
46
MBGP M
ULTICAST
E
XTENSION
C
ONFIGURATION
MBGP Multicast
Extension Overview
Extension Overview
Introduction
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 routing trees,
you need to know the unicast routing information as well as the information of
multicast-supporting parts of the network, namely, the multicast network
topology.
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 routing trees,
you need to know the unicast routing information as well as the information of
multicast-supporting parts of the network, namely, the multicast network
topology.
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
than to construct a new protocol. RFC2858 provisions the multi-protocol
extension method for BGP. The extended BGP (MBGP, also written as BGP-4+) can
not only carry IPv4 unicast routing information but also the routing information of
other network layer protocols (such as multicast, IPv6). Carrying multicast routing
information is only one of the extended functions.
protocol. Therefore, it is more rational to enhance and extend the BGP-4 protocol
than to construct a new protocol. RFC2858 provisions the multi-protocol
extension method for BGP. The extended BGP (MBGP, also written as BGP-4+) can
not only carry IPv4 unicast routing information but also the routing information of
other network layer protocols (such as multicast, IPv6). Carrying multicast routing
information is only one of the extended functions.
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.
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.
This chapter describes mainly MBGP extension for multicast.
MBGP Extension
Attributes for Multicast
To make MBGP support multicast, RFC2858 defines two new route attributes in
the UPDATE message: MP_REACH_NLRI (multiprotocol reachable NLRI) and
MP_UNREACH_NLRI (multiprotocol unreachable NLRI). 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.
the UPDATE message: MP_REACH_NLRI (multiprotocol reachable NLRI) and
MP_UNREACH_NLRI (multiprotocol unreachable NLRI). 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), with value 1 for the unicast mode of
NLRI, and value 2 for the multicast mode of NLRI.
(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), with value 1 for the unicast mode of
NLRI, and value 2 for the multicast mode of NLRI.
MP_REACH_NLRI attribute
MP_REACH_NLRI is an optional non-transitive attribute, and can be used to: