How do find the IP address assigned by a DHCP in my cisco catalyst 6513?

[mayrac]

Hi

I am having trouble finding what DHCP is assigning the IP address to our new phones, I have a DHCP that i have access to that assigns IP address to all other devices, however the IP range of the phones is not listed and that might be because i have a second DHCP  most likely configured in one of my core switches. I am not sure where i can confirm this and i am needing to configure our phone system with a system set for multicast so we can send out alert notifications on all phones and intercom system in case of an emergency. please help ! 

[chrihussey]

Some things to check:

1. Check the 6513 configuration itself to see if the switch is indeed providing DHCP to the phones.
2. Check the ip-helper addresses configured on the switch interfaces. These will point to the DHCP servers.
3. If 1 & 2 don't show anything, then the DHCP serve may be on the specific VLAN. Suggest spanning a port to a sniffer, connect a phone and look for the responding DHCP server.

Hope this helps

[mayrac]

Thank you i did find (DHCP) in the catalyst configuration and it does have an ip helper address that i am familiar with. It looks like the DHCP i am looking for is set on the vlan assigned to voice. Is there a command that can confirm that the voice vlan is issuing these addresses to my phones? 

[chrihussey]

Please clarify, is the 6513 configured to provide DHCP to the voice VLAN, or is there an IP-helper address on the voice VLAN?

If you think the 6513 itself is providing the DHCP (meaning issuing the addresses), then you can verify with the following command:

!

sh ip dhcp binding

!

[mayrac]

Yes i do see that the Cisco 6513 has a voice vlan configured with an ip helper, when i run the command provided i do see some assigned addresses but none match the addresses assigned to the phones.

[chrihussey]

So the ip-helper IP on the voice vlan should be the DHCP server.......no?

[mayrac]

That is where i get confused as i am not too familiar. 

[chrihussey]

I think it would be safe to assume the ip-helper address on the voice VLAN interface is the IP of the DHCP server.

To further elaborate; a DHCP request is a broadcast on the local LAN (or VLAN) and does not get forwarded anywhere else. If the DHCP server is not directly on the local LAN, the IP designated in the ip-helper address is where the DHCP request broadcast packet will be forwarded in unicast fashion for transport across the network. 

Hope that explains things and is of some help.

[mayrac]

yes thank you that helps understand it better, If i wanted my phones to receive a multicast would i need to tag the unit that is sending the multicast message to the same vlan as the phones for it to work? to help explain better we have a device called ALGO that is supposed to send out a message through our intercom system and our phones when i dial a specific extension to initiate our (lock down) message. The problem we are having is that the message isn't going through on any of the phones but the message is working through the intercom system which is where the ALGO is connected too. The ALGO is on a different VLAN the phones are on the VLAN you just helped me confirm. hope this makes sense. 

[chrihussey]

So it's a multicast issue then. By default, multicasts won't cross VLANs, but if you enable multicast routing it should work. Is all the routing occurring on the 6513 and it is just the ALGO and voice VLAN? If so:

!

conf t

!

ip multicast-routing

!

ip pim rp-address x.x.x.x (IP of local loopback interface or ALGO VLAN interface)

!

interface vlan (ALGO VLAN interface)

 ip pim sparse-mode

!

interface vlan (VoIP VLAN)

 ip pim sparse-mode

!

Hopefully this will help, if not, just let me know.

Regards

[mayrac]

yes i had both issues the first we found by finding the DHCP on the 6513, The next is getting the ALGO and Phones all on the same VLAN for multicast, I have 2 6513 switches but they connect to each other and both show the same voice VLAN 100. What i am trying to do is have the ALGO on the same voice VLAN as the phones for the message to work. This is where i am not sure how to do so. 

[chrihussey]

As far as the multicasting is concerned you have two choices:

1- Putting them all on the same VLAN would certainly work, the thing you need to determine is whether the ALGO and intercom system require DHCP and is it compatible with the DHCP requirements of the VoIP phones? If the ALGO and intercom system is/can all be statically defined (no DHCP) then it definitely won't be an issue. Their addresses would just have to be excluded from the DHCP scope. The only caveat would be if you have enough address space available for both.

 If so, it would just be a matter of re-IPing the ALGO and intercom to the VoIP VLAN and move those ports / interfaces into the voice VLAN.

2- If it needs to work across two VLANs you'll have to implement multicast routing. So with two 6513 the configuration would be the same on both with the exception being the RP address should be the same on both.

[mayrac]

Thank you again for all of your help, I want to say that i just need to re-IPing the ALGO to the Voip Vlan the ALGO is just the relay that connects to the intercom system to make the messaging work. 

[chrihussey]

Ahhh. The yes, that would be the way to go. Just make sure it doesn't use an IP in the DHCP scope for the phones.

Regards

[mayrac]

yeah the ALGO has an IP from another ip range on a different vlan. Can I move the ip with it when i tag it to the voip vlan? I had someone who was trying to help me and he did the tag but we didnt know what address the ALGO was given when we moved it because we didnt know where the DHCP on the switch was coming from or what IP the Voip Vlan had assigned the ALGO.