Quick question on multicast with VOIP

[gp1200x]

                   We are looking into paging on our VOIP network and I just started reading about it. The App that was purchased uses a single IP address for the multicast and they create groups under a control point. My question is this...if the product uses a single IP address for the multicasting are there groups within that which determine which phones receive pages based on a group the phone is in? So does the switch know which group each port (phone) is associated with and how does the port know this. I am being told by people that there is nothing cofigured on the phone itself as for being a member of a specific group for paging. So how does a selected group of phones receive a page without it being broadcasted to every phone?  Or does the paging get blasted to every phone and somehow only the phones for that specific page group play it on their speakers?  I've been reading but I haven't found this type of information yet.  Thanks

[George Thomas · Accepted solution]

As far as which phones receive the page, the paging server will push down XML commands to the phones and instruct it to listen in on the Multicast IP. The commands will only be pushed down to phones that are part of the paging group. The instance where you need multiple multicast addresses is when you need multiple pages at the same time and the server increases the multicast address by IP address rather than port however incrementing by IP address is what is preferred.

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[islam.kamal]

Hi

Multicast enable is the most important to enable Paging over your network, Please find the below:-

1-Nothing should be done for phones.

2-Only access swithces which connected to your phones , should be enable with igmp protocol to join multicast.

3-you should enable mutltcast routing between L3 devices and between routers  or L3 switches ,as below

ip multicast-routing

/under interface between l3 devices enable PIM/

interface g1/0

ip pim sparse-dense-mode

int vlan 100

description voice vlan

ip pim sparse-dense-mode

Thank you

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[gp1200x]

Thanks, both answers helped me but let me ask a couple more questions to understand. So it sounds like the phones dynamically join the multicast group by the application contacting the selected group of phones just before the page is started. There is only one group per IP mulitcast address (usually) and the groups setup in the software are just a way of logically selecting or grouping which phones will receive the page and have no real significance to the actual multicast group. Since there is one group per multicast IP address, only one organization can send a page at any one time to prevent multiple voices on the paging. Multiple simultaneous pages would require their own additional IP mc address. I will have to ask the programmers about this since they said they only needed one address. We will set it up for additional IP mc addresses if we can since I am sure multiple paging will be wanted.

Our network has 6500s as a core with 3750x/e and 4500s as connected at the L3 with OSPF. We then have additional L2 3750x stacks attached with phones. If no phone on the switch was contacted by the app for the page, will the switch not receive the multicast packets sent out for the page?   

At the end of the page  does the software again contact the phone to tell it to disjoin from the mc group? If it doesn't what would stop the phone from receiving pages that may follow the original?

If  simultanous paging is allowed on a network, what happens if a phone is in both paging groups, hence multiple multicast groups that are undergoing simultaneous pages?

Thanks  

[islam.kamal · Accepted solution]

Hi

1- There are many types of  multicast , Please check the below link:-

http://www.cisco.com/en/US/tech/tk828/technologies_tech_note09186a0080094821.shtml

2-Multicast is consists of source which will be source of multicast and groups.Only switches which run IGMP which will join the group.Multicast will be running on your netwrok , when you request paging you will join multicast other not join multicast.

3-I can suggest a solution for you:-

A-Enable SSM on your Core switch , this is the best solution nowadays , and enable PIM on the interfaces.

B-Enable PIM between L3 devices which will transfer multicast between each other.

C-Enable IGMP on L2 switches or access switches.

Thank you

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