network protocols handbook.pdf

83Protocols GuideTCP/IP - Network Layer ProtocolsProtocol NameMBGP: Multiprotocol BGPProtocol DescriptionThe multiprotocol BGP (MBGP) feature adds capabilities to BGPto enable multicast routing policy throughout the Internet and toconnect multicast topologies within and between BGP autonomoussystems. In other words, multiprotocol BGP (MBGP) isan enhanced BGP that carries IP multicast routes. BGP carriestwo sets of routes, one set for unicast routing and one set formulticast routing. The routes associated with multicast routingare used by the Protocol Independent Multicast (PIM) to builddata distribution trees.Multiprotocol BGP is useful when a link is required to be dedicatedto multicast traffic, perhaps to limit which resources areused for which traffic, or if all multicast traffic exchange at onenetwork access point (NAP) is required. Multiprotocol BGP allowsa unicast routing topology different from a multicast routingtopology.The only three pieces of information carried by BGP-4 that areIPv4 specific are (a) the NEXT_HOP attribute (expressed as anIPv4 address), (b) AGGREGATOR (contains an IPv4 address),and (c) NLRI(expressed as IPv4 address prefixes). Any BGPspeaker, including an MBGP speaker, has to have an IPv4 address,which will be used, among other things, in the AGGRE-GATOR attribute. To enable BGP-4 to support routing for multipleNetwork Layer protocols the only two things that have tobe added to BGP-4 are (a) the ability to associate a particularNetwork Layer protocol with the next hop information, and (b)the ability to associate a particular Network Layer protocol withNLRI.There are two attributes defined in the MBGP regarding NLRI:1) MP_REACH_NLRI for the purpose of advertising a feasibleroute to a peer, permitting a route to advertise the network layeraddress of the router to be used as the next hop and allowinga given router to report some or all of the subnetwork points ofattachment (SNPAs) and 2) MP_UNREACH_NLRI for the purposeof withdrawing multiple unfeasible routes from service.To provide backward compatibility, as well as to simplify introductionof the multiprotocol capabilities into BGP-4, two newattributes, Multiprotocol Reachable NLRI (MP_REACH_NLRI),and Multiprotocol Unreachable NLRI (MP_UNREACH_NLRI)are used in the MBGP. MP_REACH_NLRI is used to carry theset of reachable destinations together with the next hop informationto be used for forwarding to these destinations. MP_UN-REACH_NLRI is used to carry the set of unreachable destinations.Both of these attributes are optional and non- transitive.This way a BGP speaker that doesn’t support the multiprotocolcapabilities will just ignore the information carried in these attributes,and will not pass it to other BGP speakers.Protocol StructureMultiprotocol Reachable NLRI - MP_REACH_NLRI (Type Code14): The attribute is encoded as follows:2 Bytes 1Byte 1ByteAddress Family IdentifierNumber ofSNPAsSecond SNPA(variable)SubsequentAddress FamilyIdentifierNetwork Address of Next Hop (variable)Length of firstSNPALength of LastSNPA (1 Byte)First SNPA(variable)Last SNPA(variable)Length of NextHop NetworkAddressLength ofsecond SNPA(1 Byte)Network LayerReachabilityInformation(variable)• Address Family Identifier - carries the identity of theNetwork Layer protocol associated with the NetworkAddress that follows.• Subsequent Address Family Identifier - provides additionalinformation about the type of the NetworkLayer Reachability Information carried in the attribute.• Length of Next Hop Network Address - expresses thelength of the “Network Address of Next Hop” field asmeasured in octets.• Network Address of Next Hop - a variable length fieldthat contains the Network Address of the next routeron the path to the destination system.• Number of SNPAs - contains the number of distinctSNPAs to be listed in the following fields. The value0 may be used to indicate that no SNPAs are listed inthis attribute.• Length of Nth SNPA – expresses the length of the“Nth SNPA of Next Hop” field as measured in semioctets• Nth SNPA of Next Hop - contains an SNPA of therouter whose Network Address is contained in the“Network Address of Next Hop” field.• Network Layer Reachability Information - lists NLRIfor the feasible routes that are being advertised inthis attribute.Multiprotocol Unreachable NLRI - MP_UNREACH_NLRI: Theattribute is encoded as follows:Address Family Identifier(2 Bytes)SubsequentAddress FamilyIdentifier(1 Byte)Withdrawn Routes(variable)• Address Family Identifier - carries the identity of theNetwork Layer protocol associated with the NLRI thatfollows.