Interdomain Multicast Routing Protocols

 

Interdomain Multicast Routing Protocols

When multicast group members are located in different Autonomous Systems (ASs), an intra-domain protocol such as DVMRP, MOSPF, or PIM is not sufficient. We need an interdomain multicast routing protocol to exchange multicast-routing information between different ASs.

The important protocols discussed in the text are:

  1. MBGP — Multicast Border Gateway Protocol
  2. MSDP — Multicast Source Discovery Protocol
  3. BGMP — Border Gateway Multicast Protocol

1. Why do we need Interdomain Multicast Routing?

Consider multicast group G1 whose members are distributed across three autonomous systems:

             AS 1
          Source S
              |
              |
        +-----+-----+
        |           |
       AS 2        AS 3
     G1 members   G1 members

An intra-domain protocol such as PIM can construct the multicast tree inside each AS, but we also need a mechanism to:

  • exchange multicast information between ASs,
  • identify paths between different ASs,
  • discover multicast sources located in other ASs,
  • construct a multicast tree spanning multiple ASs.

This is the role of interdomain multicast routing.

2. MBGP — Multicast Border Gateway Protocol

MBGP is an extension of BGP, which is used for interdomain unicast routing.

The important idea is that MBGP provides separate paths for unicast and multicast traffic.

How it works

Border routers belonging to different ASs exchange multicast-routing information.

             AS 1
        Border Router
             |
             | MBGP
             |
        Border Router
             AS 2

Thus, MBGP allows the border routers to determine the appropriate path for multicast traffic between autonomous systems.

Shared-group tree

MBGP uses a shared-group multicast routing approach.

For each multicast group, a router in each AS is selected as a Rendezvous Point (RP).

             RP (AS 1)
              /     \
             /       \
          AS 2       AS 3
           RP         RP
          /            \
       G1 members    G1 members

So, multicast routing information is exchanged between the border routers of different ASs, while the RPs act as representatives for the multicast groups.

3. Problem with MBGP

A major problem with MBGP is:

How does an RP learn about multicast sources located in other autonomous systems?

For example:

AS 1                  AS 2
Source S  --------->  RP
                       |
                       |
                    Group G1

The RP in AS 2 needs to know that Source S in AS 1 exists and is sending to group G1.

MBGP itself has difficulty providing this source-discovery information.

This leads to the use of MSDP.

4. MSDP — Multicast Source Discovery Protocol

MSDP (Multicast Source Discovery Protocol) was proposed to solve the source discovery problem in interdomain multicast routing.

The basic idea is:

Each AS has a source representative router that informs RPs in other ASs about multicast sources existing in its AS.

For example:

        AS 1                         AS 2
   Source S                         RP
      |                              |
      |                              |
 Source Representative  -------->   RP
              MSDP

The source representative tells other RPs:

"There is a multicast source for group G in my AS."

This allows RPs in different ASs to learn about multicast sources located outside their own AS.

The text describes MSDP as a suggested protocol for addressing the difficulty MBGP has in informing RPs about sources in other ASs.

5. BGMP — Border Gateway Multicast Protocol

Another protocol discussed in the text is BGMP (Border Gateway Multicast Protocol).

BGMP was considered as a possible replacement for MBGP.

Its main idea is different:

For each multicast group, construct one shared multicast tree whose root is located in one AS and whose leaves can be located in different ASs.

For example:

                  Root AS
                    |
                   AS 1
                 /      \
               AS 2      AS 3
                |          |
              G1 members  G1 members

Here:

  • There is one shared tree for a multicast group.
  • The root is located in one particular AS.
  • The leaves can be networks in different ASs.
  • Multicast sources can use this shared tree to reach members.

6. Problems with BGMP

The text identifies two important issues:

Problem 1: Selecting the root AS

For each multicast group, we need to decide:

Which AS should contain the root of the shared multicast tree?

Problem 2: Informing sources

Once the root AS is selected, all multicast sources need to know:

Where is the root of the multicast tree?

Sources can then tunnel their multicast packets toward that root.

7. MBGP vs MSDP vs BGMP

ProtocolMain purposeImportant idea
MBGPInterdomain multicast routingExtension of BGP; provides separate unicast and multicast paths
MSDPSource discoveryInforms RPs about multicast sources in other ASs
BGMPInterdomain multicast tree constructionOne shared tree with a root in one AS

8. Overall Picture

The three protocols can be understood as follows:

                 INTERDOMAIN MULTICAST
                         |
          +--------------+--------------+
          |              |              |
        MBGP           MSDP           BGMP
          |              |              |
   Multicast paths   Discover       Shared tree
   between ASs       sources        across ASs
          |              |              |
          +--------------+--------------+
                         |
                Multicast communication
                    across different ASs

Easy way to remember

MBGP → Multicast paths between ASs
MSDP → Multicast source discovery between ASs
BGMP → Shared multicast tree across ASs

This is the key conceptual distinction for an examination question on Interdomain Multicast Routing Protocols.

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