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Polycom and multiple DHCP servers

  • 1.  Polycom and multiple DHCP servers

    Top 25 Contributor
    Posted 12 days ago

    Hi All,

    Just thought I'd through the question out there to see if anyone has any insights into how DHCP works with Polycom devices and whether you've faced any issues in a dual DHCP environment.

    We think we've come across an issue just with Polycom phones where they seem to favour/preference a single DHCP server - and when one server is down for maintenance they don't communicate with the other DHCP server and then don't get an IP address.

    In our network, we have two MS DHCP servers in a load-balanced config (50/50), and our Voice VLAN's configured with a "ip helper-address" entry for each DHCP servers.

    After performing some packet captures, can see when a PC sends a DHCP Discover - it always gets a DHCP Offer back from each of the DHCP servers (so two offers in total - which is expected behaviour), and the first response received is used (also expected behaviour). This is on our Data VLAN (i.e. 172.16.x.x)

    However, when a Polycom phone sends a DHCP Discover, it only ever receives a DHCP Offer from one of the two DHCP servers. It also then consistently seems to get the offer from the same server. Then whilst one of our DHCP servers was out for patching, it seems the phone would not get an IP from the other DHCP server and just came 'DHCP failed'. This is on our Voice VLAN (i.e. 10.19.x.x)

    To rule out any VLAN or firewall config issues, I temporarily disabled CDP/LLDP to force the phone onto the Data VLAN (i.e. getting a 172.16.x.x address) - and the packet capture showed only 1 DHCP Offer received for the Polycom broadcast.

    So if you can help with any of the following questions, or have any other insights:
    - if you have a similar setup (i.e. multiple DHCP servers with ip helper-address configured on VLAN), do you see multiple DHCP Offer's from all DHCP servers?
    - does the Polycom have some form of 'DHCP cache'
    - do Microsoft DHCP servers only send single offers for specific devices?
    - something else I haven't considered?

    Just to add - there's no issues with the phones actually functioning on the network (provisioning, registering and calling is all OK) - it's just getting the IP address first.