Multicast BGP

 Salaaaam, 

Today, i'm explaining "Tough" MulticastBGP, because several Network Engineers complain that they don't get what the motivation for MBGP and what's the benefit from using Multicfast BGP instead of normal BGP in a Multicast Environnement between several AS.

Multicast BGP Extension is used to control the Multicast Path between 2 AS. 

In fact, the control of Multicast Path is like this:

Normally, RPF check is the method used to establish the Path that multicast flow will follow, for example, Join are sent from downstream routers to Source according to the RPF interface, thus establishing the RPT or SPT..   We should notice that , the RPF check is based on Unicast Routing Table,

The process below lead that if we have several Paths between 2 AS, so multicast path will be similar to other Unicast flows paths, thus it’s difficult to control the multicast differently.

One solution to this issue, is using MulticastBGP, in fact MulticastBGP is like MPBGP, in fact it fill a speciall BGP table , called the “multicast address family” table, filling that table is like filling with MBGP a table of a VPN (VRF)..   so the Benefit is that, when you are using the address family multicast,  So , When RPF find the Mutlticast BGP address family , so it use this table instead of Unicast Routing Table!!!

So by this table, we can control multicast path, independly from unicast traffic..!!

CONFIGURATION

+ Configuration of PIM, RP, AutoRP, BSR ,… all is normal

+ Configuration of MulticastBGP part:

à Configure BGP with 2 parts: the ipv4 family part, and the multicast family part

Example:

       

Before configuration, Just Normal BGP:

!

router bgp 100

 neighbor 150.1.7.7 remote-as 100

 neighbor 150.1.7.7 update-source Loopback0

 neighbor 150.1.7.7 next-hop-self

 neighbor 155.1.146.4 remote-as 200

 redistribute ospf 1

!

Configuration Changes with MulticastBGP (delete the general neighbor relation, and configure 2 independants families : unicast family and multicast family)

router bgp 100

 bgp log-neighbor-changes

 neighbor 150.1.7.7 remote-as 100

 neighbor 150.1.7.7 update-source Loopback0

 neighbor 155.1.146.4 remote-as 200

 !

 address-family ipv4

  redistribute ospf 1

  neighbor 150.1.7.7 activate

  neighbor 150.1.7.7 next-hop-self

  neighbor 155.1.146.4 activate

  no auto-summary

  no synchronization

 exit-address-family

 !

 address-family ipv4 multicast

  redistribute ospf 1

  neighbor 150.1.7.7 activate

  neighbor 150.1.7.7 next-hop-self

  neighbor 155.1.146.4 activate

  no auto-summary

  no synchronization

 exit-address-family