CICD 210-060

CICD 210-060

Troubleshooting IP Phone Registration Process Failure

Let's say that an IP phone does successfully get a DHCP response, it successfully downloads a configuration file via TFTP, but it still isn't registering. What might be going on here? Well, we might want to check the contents of that configuration file. Let’s make sure that appropriate IP addresses for our call agent or agents are listed. Again this could be the IP address of a Communications Manager Express router or a Communications Manager server. We might also need to check that call agent and see if there's an entry for this phone, if there's not, the registration might be rejected because it could not find a configuration file that was for that phone-specific MAC address. But even if we had not previously configured that phone in the call agent we might've enabled auto-registration. We might want to make sure that if we think auto-registration should be working and the phone should automatically register even without a preconfiguration, you might want to check and make sure that auto registration is indeed enabled.

Divide-and-Conquer Methodology

When troubleshooting an IP phone registration, it's important to understand the different elements involved in the registration.

Cisco IP Phone Registration Process

First we have our phone that is attempting to register and it connects into a Catalyst switch. And that Catalyst switch should be configured with a voice VLAN of which that phone is a member, if that's misconfigured that could be our issue right there. Or perhaps the port is not configured appropriately for that phone. The switch then gets us out to the rest of the network which might be through other routers and switches. And it gets us to a DHCP server because when the phone boots up it goes out and attempts to get IP address information, subnet mask, default gateway information, and the IP address of a TFTP server from the DHCP server. In order to make an intelligent DHCP request, however, when the phone sends a DHCP request it's going to do so as a member of a certain subnet, a certain VLAN. How did the phone know what VLAN it's a member of? Well that goes back to the Catalyst switch. The Catalyst switch uses CDP Version 2 specifically to tell the IP phone, "hey, you belong to this VLAN so when you're making your DHCP request do so as a member of this VLAN". Then after the DHCP response comes to the phone and the phone now knows its IP address, its subnet mask, its default gateway, the IP address of a TFTP server, it can then reach out to that TFTP server and attempt to download its configuration file. If there's something going wrong with the TFTP server, that could be preventing the phone from registering. But if the configuration file is downloaded then the phone can look in that configuration file to find the IP address of as many as three different call agents with which it can attempt to register.

Troubleshooting: No IP Address

Based on our discussion of the elements involved in an IP phone registration, let's take a look at a few troubleshooting scenarios, beginning with a phone not getting an IP address.

One reason that a phone might not get an IP address is that when it sent its DHCP request it did so as a member of an incorrect VLAN. Remember the Catalyst switch needs to tell the phone which VLAN it belongs to, what is the voice VLAN in other words, and it's going to that via CDP Version 2. Let's make sure CDP Version 2 is enabled, and let's make sure the DHCP server is configured for the appropriate default gateway, it has the appropriate scope and DHCP Option 150. Not something that's very common in non-IP Telephony networks, but DHCP Option 150 that's the way that the DHCP server can hand out the IP address of a TFTP server to an IP phone.

Troubleshooting: TFTP Download Fails

Another IP phone registration issue could be that the TFTP download fails. One reason that a phone might not be able to retrieve its configuration file from a TFTP server is that it doesn't know the IP address of that TFTP server. Maybe Option 150 was not specified or an incorrect Option 150 was specified in the Communications Manager. Or maybe Option 66 was used.

Option 66 is the DNS name of the TFTP server, but if the phone doesn't have DNS server information it would be unable to resolve Option 66. Typically though we don't use Option 66 we use Option 150 and you can go into the network settings menu on the phone to see if it has contacted a DHCP server and to see if it knows the IP address of a TFTP server. If it does we can go to that TFTP server and make sure that the TFTP server is running.

Troubleshooting: Not Registered

Let's say that an IP phone does successfully get a DHCP response, it successfully downloads a configuration file via TFTP, but it still isn't registering. What might be going on here? Well, we might want to check the contents of that configuration file. Let's make sure that appropriate IP addresses for our call agent or agents are listed. Again this could be the IP address of a Communications Manager Express router or a Communications Manager server. We might also need to check that call agent and see if there's an entry for this phone, if there's not, the registration might be rejected because it could not find a configuration file that was for that phone-specific MAC address. But even if we had not previously configured that phone in the call agent we might've enabled auto-registration. We might want to make sure that if we think auto-registration should be working and the phone should automatically register even without a preconfiguration, you might want to check and make sure that auto registration is indeed enabled.