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Ask the Expert: Enterprise Design and Deployment of Multicast

Welcome to this Cisco Support Community Ask the Expert conversation. This is an opportunity to learn and ask questions about Cisco enterprise design and deployment of multicast solutions.

The enterprise world is evolving to be overcome with large throughput capacity and record numbers of users connecting to the network. Mechanisms such as multicast, which allows for a minimization of throughput for multiple users subscribing to the same stream, are a welcome addition. Applications such as enterprise all-hands video streaming, trading applications, mass operating system deployment, and custom implementations can put a strain on the network if done via unicast. Multicast can minimize this strain by replicating a single stream for subscription by multiple parties who would like to receive the same information. For this Ask the Expert event, Patrick Lloyd, CCIE R&S no. 39750 and a network consulting engineer with Cisco’s Enterprise Advanced Services Delivery Team, will answer questions about multicast design and implementation based on best practices and prior experience with large enterprise deployments.

Patrick Lloyd is a network consulting engineer for Cisco’s Enterprise East Advanced Services team, working to support and lend his expertise to a number of financial, insurance, healthcare, and consulting customers. In his four years of experience, he has lent design expertise to multicast networks ranging from 500 Cisco devices and 20K users to upward of 4500 Cisco devices and 50K users. Patrick is certified with his Cisco Certified Internetworking Expert no. 39750 in the Routing and Switching track and also has achieved certification in CCNA Security and Securing Cisco Routers and Switches as part of the CCNP Security track. Patrick received his MS degree in networking and systems administration from Rochester Institute of Technology in Rochester, NY, and his BS degree in computer science from Eastern Connecticut State University. He frequently gives customer-based knowledge transfers.

Remember to use the rating system to let Patrick know if you have received an adequate response.

Because of the volume expected during this event, Patrick might not be able to answer every question. Remember that you can continue the conversation in Network Infrastructure under the subcommunity WAN, Routing & Switching shortly after the event. This event lasts through September 12, 2014. Visit this forum often to view responses to your questions and the questions of other Cisco Support Community members.

Everyone's tags (2)
New Member

hi Are there any scenarios

hi Are there any scenarios that would dictate a deployment of multicast dense mode as opposed to sparse mode?

Cisco Employee

Thanks for the question! 

Thanks for the question!  This is actually a good one that I've encountered with a couple customers in the past, the tradeoff between a flood and prune type design, as opposed to the shared tree -> shortest path tree sequence.  As per Cisco best practice, we are actively trying to get customers to implement sparse mode, going so far as to not support PIM dense mode in our data center products.  And for good reason!  The last thing you want is a chatty protocol within the data center which is flooding traffic out to receivers who may or may not be interested in it every 3 minutes.  Instead, you're much better off having interested receivers join a stream, have your RP connect the interested senders and receivers, and then transition to the shortest path between source and destination.

That being said, if you're studying for CCIE or looking to get experience in how multicast works, dense mode should at least be a lab exercise!

Links for reference as to the difference in PIM modes:

Dense Mode Operation:

Pim Modes and explanation of each:

A great slide deck to learn the operation of multicast:

Troubleshooting Multicast:


Let me know if this is the answer you're looking for!

Hi Patrick! I am looking for

Hi Patrick!


I am looking for a way to leak multicast between VRFs. I have a network with remote sites running routers such as the 1921. These routers are running VRF lite and connect to a service provider running MPLS to transport our routes. We run BGP towards the service provider and each remote site has its own AS.

At a remote site, multicast is generated in VRF A. I then want to spread this multicast to the receivers that are located in VRF B and/or C. So far the only feature I have found is Extranet VPN but to the best of my knowledge it seems to require MPLS which we are not running.

My colleague and I had a labbing session today which could have been a CCIE lab :P We tried to do import of VRF A routes into VRF B which got the routes imported but not the multicast state.  We tried to run MSDP from a router to itself in different VRFs! 

So far the only working solutions I've seen are to connect router with VRF A to router with VRF B and run MSDP. Either that or put a physical loop between two interfaces where one interface resides in VRF A and the other resides in VRF B and then put the appropiate mroutes to only affect multicast traffic flow as opposed to unicast.

I can't really think of anything else at this time. Maybe something with multicast service reflection.

Hope you can steer me in the right direction here. Thanks.


Daniel Dib

CCIE #37149

Cisco Learning Network VIP

Cisco Champion

Daniel Dib CCIE #37149 Please rate helpful posts.
Cisco Employee

Hi Daniel,I personally haven

Hi Daniel,

I personally haven't done this, but it is possible.  The caveat that you ran into was you were using MSDP, a more common multicast feature, as opposed to Multicast Dynamic Tunnel (MDT).  You would use the tunnel to do the import of the multicast routes into the require VRF.  See the following documentation links showing how this would be done:

Thanks Patrick but we are not

Thanks Patrick but we are not running MPLS (no VPNv4) so I don't believe it's possible. Also each remote site is in a different AS.

So far the only working solution has been a physical loop cable to leak between VRFs.

Daniel Dib CCIE #37149 Please rate helpful posts.
New Member

Hi there.. How does multicast

Hi there.. How does multicast work in my Layer 2 network? Can I enable multicast, or is there anything specific that needs to be done?

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