redundant multicast

Answered Question
Jul 19th, 2008
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Hi there. I have a pair of redundant 4507R's with all of my servers dual-homed to both switches. I bring in multiple multicast feeds on both switches, with each feed typically having an A side and a B side. I also receive some of these feeds from more than one provider. In my current setup, I have mroute's defined on the second 4507R, that points each multicast feed back to the correct source. My concern is that if I lose that 2nd switch, I will lose all of my multicast feeds with it. How can I setup better redundancy for multicast so that the loss of one switch doesn't make me completely lose all my multicast feeds?


Thanks in advance!


--Brandon

Correct Answer by Giuseppe Larosa about 8 years 9 months ago

Hello Brandon,

the answer is yes

Switch1 -- Router1--\

---Provider network

Switch2 -- Router2--/


you can do this as explained in previous post.

Thanks for the good marks.


Multicast is becoming more and more important.

I studied this subject for Cisco Certifications


Hope to help

Giuseppe



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Steve Lyons Sat, 07/19/2008 - 07:09
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To provide redundancy for each source some customers install dual network interface cards within the source devices and or configure multiple sources. Also some customers use a dynamic routing protocol and advertise the source networks within their interior gateway routing protocol, instead of static mroutes, to ensure reverse path forwarding checks pass when topology changes occur.


Steve Lyons - Cisco

branfarm1 Sat, 07/19/2008 - 09:25
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thanks for the information. to clarify, do I need to setup mroutes on the primary switch and then do the redistribution?

Giuseppe Larosa Sat, 07/19/2008 - 23:21
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Hello Nyle,

if you use dynamic unicast routing for the multicast sources + IP PIM on your devices you should be able to get a working multicast without using multiple mroute in simple topologies.


the mroute is used to pass the RPF check on the source : a multicast routing router accepts traffic only if it is received on the interface that is outgoing interface to reach the source.

Using an unicast dynamic routing protocol is enough if the network topology does not provide parallel links as the outgoing interface to the source.

In that case unicast routing can take advantage of multiple paths using load balancing but RPF will choice only one path as the best one.

To avoid this you can :

disable ip multicasting on physical links

use a tunnel GRE with ip pim configured and you can also use mroute pointing to the GRE tunnel to allow the RPF check to pass.

In this case unicast routing provides redundancy in routing the GRE tunnel's packets and the GRE tunnel assures that traffic will arrive always on a single logical interface even if topology provides multiple paths to the source of the multicast stream.


I don't think you need redistribution of mroutes in your case (are you using multiprotocol BGP for address family ip4 multicast ? )


Hope to help

Giuseppe

branfarm1 Mon, 07/21/2008 - 08:39
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Thanks for your help Giuseppe. I'm still a little confused though. Here's what I've got configured on my secondary switch:


ip multicast-routing


Then, on the vlan interfaces that receive the multicast:


ip pim dense-mode

ip mroute x.x.x.x next-hop


In this case, the next-hop is a router on my network that is connected to the providers network.


On my primary switch I don't have anything related to multicast configured. I'm running OSPF on both switches, and the primary switch has a connection to the providers network via another redundant router.


How do I tell OSPF about the multicast sources, if only one of my devices has mroute's in place?


Thanks in advance.

Giuseppe Larosa Mon, 07/21/2008 - 12:41
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Hello,

if I understanc correctly up to now your switch1 has no role in multicast.


PIM uses the unicast routing table to get information about the sources of multicast flows.

OSPF in Cisco implementation is not able to provide multicast forwarding (MOSPF LSA tpye 6). So let OSPF do its job.


If your Switch1 is connected to the same router what you need is simply the following:


sWitch1

ip multicast-routing


ip mroute x.x.x.x next-hop


int vlan to router


ip pim dense-mode


int vlan access-vlan

ip pim dense-mode


Switch1 and Switch2 will elect a PIM DR, an IGMP querier and only one of them will forward the multicast group in the access vlan(s).

If SWitch2 fails Switch1 will take place as PIM DR, igmp querier, and multicast group forwarder.


Hope to help

Giuseppe

branfarm1 Mon, 07/21/2008 - 13:42
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That's very helpful -- I had no idea PIM would elect a designated router. I really need to do some more reading on multicast... as you're painfully aware of now.


One more question though... what if my two switches are connected to two different routers from the provider? Like this:


Switch1 -- Router1--\

---Provider network

Switch2 -- Router2--/


Can I still set it up the way you've indicated?


Thanks again,


--Brandon

Correct Answer
Giuseppe Larosa Tue, 07/22/2008 - 03:08
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Hello Brandon,

the answer is yes

Switch1 -- Router1--\

---Provider network

Switch2 -- Router2--/


you can do this as explained in previous post.

Thanks for the good marks.


Multicast is becoming more and more important.

I studied this subject for Cisco Certifications


Hope to help

Giuseppe



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