Multicast Part 1

Multicast Formats

Dense - A push method, where a multicast feed gets pushed to all network devices.
Sparse - A pull method, where a multicast feed gets requested in by the network devices.
Sparse-Dense -

Sparse:

common config commands used;

• ip pim sparse-mode
• Used on all interfaces running multicast in SPARSE mode. Interface level command
• ip igmp join-group [multicast group address]
• used on the receiver  
• 
  
• ip pim announce-rp 
• ip pim announce candidat
• ip pim dr-priority [priority value]
• The designated router (DR) is the device that is responsible for sending the (S,G) entry to the RP. 
• The DR is the router with the highest priority in the LAN segment where the source is connected to. By default the DR priority is "ONE". The router with the highest IP address is in the LAN segment is chosen as the DR. Using the " ip pim dr-priority [priority value]" you can influence the DR choice.
• caveat: A router and/or switch that is a DR and the source for a particular group will not register source. Basically, the router and/or switch cannot be the source and DR at the same time in a LAN segment 
• ip mroute [network][mask][next-hop] 
• used to create a static multicast route

Shortest Path Tree (SPT) Vs Shared Tree;

In the beginning SP mode builds a shared tree from the Source and the Client to the RP. At this stage the flow will traverse via the RP. Once both shared trees are built (Client to RP and Source to RP) the flow will optimise itself (triggered by the last hop router) to run on the SPT. This would mean the flow will uses the shortest route between Source and Client. This flow need NOT be via the RP

PIM Assert;

In a broadcast domain where two or more routers having the same path costs (AD and Metric) to the source will flood Multicast traffic, as these routers will have a valid RPF. However, to avoid traffic duplication only one router will be PERMITTED to flood traffic. When a router sees another router having the same priority to the SOURCE it will insert a PIM Assert message.  This message contains the source IP, the group and the path cost to the source.

DR Priority;

A PIM Designated Router (DR) is elected on every multiple-access segment, i.e. every segment where multiple routers share the same medium/subnet. This election process is based on the highest priority and highest IP address – the router with the numerically higher value wins the election. This process is preemptive

and every new router with a better priority will preempt the previous DR.

One purpose of a DR is signaling multicast delivery trees using PIM messages when it sees interested receivers on the shared segment by means of IGMP. Another purpose is registering active sources on the segment with the regional RP.

Verification: show ip pim interface [xxx] detail


Rack1R1#show ip pim interface fastEthernet 0/0 detail
FastEthernet0/0 is up, line protocol is up
Internet address is 155.1.146.1/24
Multicast switching: fast
Multicast packets in/out: 4624/811
Multicast TTL threshold: 0
PIM: enabled
PIM version: 2, mode: sparse-dense
PIM DR: 155.1.146.1 (this system)
PIM neighbor count: 2
PIM Hello/Query interval: 30 seconds
PIM Hello packets in/out: 603/304
PIM State-Refresh processing: enabled
PIM State-Refresh origination: disabled
PIM NBMA mode: disabled
PIM ATM multipoint signalling: disabled
PIM domain border: disabled
Multicast Tagswitching: disabled

Auto RP;

Cisco Proprietary product

User a cRP (Candidate RP)  and MA (Mapping Agent)

Operate in PIM Sparse-Dense

NBMA Mode;

First of all, it works only with PIM SM (sparse mode groups), as it relies on PIM Join messages to function. Secondly, it treats the “monolithic” Frame-Relay interface as a collection of point-to-point links connected to other nodes. It does so by tracking the PIM Join

messages received from those neighbors. For every neighbor that sends a Join message the router creates a separate (S,G) state bound to this neighbor’s IP address. Now this state emulates a point-to-point link connected to this neighbor. All PIM Prune messages received from this neighbor will only affect its own (S,G) state, leaving other neighbors’ states intact. Also, any multicast traffic received from this neighbor is treated like it has been received from a point-to-point link. Thus, the RPF procedure will allow forwarding the multicast packets back out of the same interface to all other interested neighbors. Finally, PIM NBMA mode replaces the ineffective pseudo-broadcast replication on all PVCs with fastswitched packet distribution only to the PVCs with subscribed neighbors. This saves router CPU resources and WAN bandwidth.

Notice that you may enable NBMA mode on a PIM SM/DM interface, even though this is not the recommended configuration. It will only work for groups signaled via sparse mode.

NOTE: Configuration applied ONLY on the hub device interface facing the spokes

Common show commands used;

• sh ip mroute - Tells you where the Feed (source) is coming from (incoming interface from the source)  and going out of (out-going interface towards the client).  
• sh ip pim nei
• sh ip pim mpacket
• sh ip mcount  - Should be tested on the last hop or first hop devices. This verifies the actual traffic flow from the SOURCE coming IN. This is the (S,G) entries coming in. **** Very useful ****

Common debug commands used;

• debug ip pim mpackets
• debug ip pim

SSM