DHCP Discovery Packet Size Limit

[slayer925]

I am having problems trying to get a DHCP address lease for an Avaya 4610SW VoIP telephone. I am using a Cisco 2621 router. The DHCP is configured. However, the Avaya VoIP just won't get an IP address. A test laptop and a Polycom SIP 6000 conference phone do get IP addresses from the same router without any issues. The one thing that I have found is that the bootp UDP packet coming from the Avaya VoIP telephone is 1308 bytes in size. The laptop and Polycom generate 344 bytes and 342 bytes bootp UDP packets. Is the Cisco router sticking to the RFC size of 576 bootp maximum packet size? Is this why the router ignores the Avaya but not the laptop or Polycom?

 

[vipergg]

It probably has something to do with the options of the scope of the phone address range . think you have use option 242 or soemthing of thast nature within the scope for the phones. I would do a search on dhcp and avaya phones. Don't think it has anything to do with dhcp packet size. also using a router as my dhcp server for a voip install would not be my first choice.

 

[slayer925]

Option 242 is used for newer 9xxx series Avaya phones. Option 176 is used for the older 4xxx series phones. Actually, the DHCP options are fine. The problem is the initial DHCP Discover request packet is never heard by the router. We normally use Windows 2003 servers as DHCP servers. This setup is a special case. I have also tried the 4610SW Avaya phone behind a Fortinet device which I know is a strict RFC compliant appliance. It fails to get an IP address there as well (FortGate device setup as DHCP server). It still looks like packet size is the problem here (breaking RFC size limit for bootp UDP).