Multicast

Terms

Multicast

• A one-to-many service using UDP packets destined to group IP address
• Hosts subscribe to the group, routers and switches replicate for the group

IGMP — Internet Group Management Protocol

• Receivers send IGMP to LHR to request multicast streams
• Switches see IGMP (for snooping), and the LHR uses this to build the MDT

PIM — Protocol Independent Multicast

• Multicast capable routers communicate to each over via PIM

IIF — Incoming Interface

• AKA, the RPL interface
• Part of the MDT

OIL — Outgoing Interface List

• Part of the MDT

MDT — Multicast Distribution Tree

• The full set of links participating in multicast, via PIM, IGMP, including IILs, and OILs

RP — Rendezvous Point

• A router designated as the root of a shared tree

(*, G)

• Pronounced as “Star comma Gee”
• AKA, a shared tree
• Require a RP
• Called Star comma Gee, because typing “show ip mroute” … this is what shows up

(S, G)

• Pronounced as “Ess comma Gee”
• AKA a source tree. These do not require a RP

Source Tree

• AKA, SPT, or shortest path tree. SPT is best tree.

RPT — Rendezvous Point Tree

• *,G that points towards the RP.

ASM — Any Source Multicast

• The host only knows the group it wants to receive (239.10.10.10)

SSM — Source Specific Multicast

• The host already knows the source, and group address (10.0.0.1, 232.10.10.10)

Upstream

• Towards the source

Downstream

• Towards group members

FHR — First hop router

• This router receives a multicast stream

LHR — Last Hop Router

• Receives IGMP messages from receivers, which are translated into PIM join messages

MRIB — Multicast Routing Information Base

• Shows RPTs, SPTs, RPFIs, OILs, and IILs

MFIB — Multicast Forwarding Information Base

• AKA MFIB
• Used the program the ASICS

RIB — Routing Information Base

DF — Designated Forwarder

• Used in PIDIR-PIM

RPF - Reverse Path Forwarding

PIM is protocol independent, in the sense, that if a stream turns on, it must have a source, so it takes the form (0.0.0.1, 239.1.1.1), a (S,G).

If we do show ip route 10.0.0.1, we’ll see the interface the router intends to send any traffic towards that source address. This is the upstream interface.

As multicast traffic flows from 10.0.0.1, it should flow into the upstream interface, and out of any downstream interfaces the OIL.

Tracing the traffic back to the source this way is called reverse path forwarding and the interface along this path is the RPF.

The PIM neighbor on the RPF is called the RPF neighbor.

Any multi-cast traffic from any given source, not received on the RPF is discarded. This prevents loops.

Shared Trees

(*,G) entries in the mroute table require fewer resources, since multiple sources can use the same tree.

(*,G) entries in the mroute table represent a security risk, because any source can send to this shared tree.

Theory (in V4)

Multicast is always TO a group, a destination, or a set of destinations.

Multicast comes from an older time. Unlike Unicast addresses, you can tell via bits if a v4 address is multicast.

A multicast address always start with 1110

Common L3 Addresses

Same Broadcast Domain

Can Be Forwarded

PIM

PIM forms adjacencies in only one direction

The multicast source is the root of the tree. Packets flow downstream from the source. Control plane traffic like PIM joins flow upstream to the RP, or to the reciever.

IANA Assignments

PIM

PIM Message Types

Shared-Tree (*,G)

• Shared trees are essential for multiple senders to the same group

• A single tree is built for each group, regardless of source

  • 3 sources, 1 tree

• Selects a router as the root of the tree

• If a receiver is on the same subnet as the sending host, it will need to revert to PIM Dense for that segment

• This isn’t always better. Shared trees will typically take suboptimal paths through a network

• Source trees are better distributed, hence they are more robust

• RP Selection is a hassle

Source Based Multicast (S,G)

• PIM dense uses a separate tree for each multicast source and destination group.
• Groups do not share trees.
  • 3 Sources 3 trees.

Commands

show mrib route
show ip mroute
!
! PIM
!
show pim rpf hash
show pim range-list
show pim topology
!
! What interface should I receive this host traffic from?
!
show ip rpf 10.0.0.0
show ip mfib
!
! See if multicast even works
!
show ip pim stats
!
! PIM traffic
!
show ip pim interface detail
!
! DF election
!
show ip pim interface df

FLAGS
 A - Accepting. This interface is accepting data
 F - Forwarding. Where to send multicast traffic

Nexus 7K

show forwarding multicast route group <>

Lab Stuff

BPF - Capture all PIM, but not PIM hello messages.

ip proto 103 and not ether[34] == 0x20

References

IP Multicast - Cisco