Archive for the ‘Training’ Category

One of the more tedious parts of any phone system deployment is configuring the access layer switches to support said phones.  The configuration in and of itself isn’t complicated, but every port that may receive a phone needs to be setup correctly.  In Cisco parlance, this is accomplished with the switchport voice vlan <ID> command.  I’ve typed that into the CLI a thousand times and never really knew what it did besides “make the phones work”.  After a little research, I finally found some answers.  I thought I’d share them with you.

In the old days, before the Catalyst 2950, configuring a switch port for use by a phone involved creating an explicit 802.1q trunk.  This made sense from the perspective that it allowed traffic from multiple VLANs to pass on a single link.  It also allowed the 802.1p priority bits for Quality of Service (QoS) tagging to be sent with the frames.  The downside is that it was very difficult for phone mobility.  You either needed to provision every phone-facing switchport in your organization to be an 802.1q trunk or you had to leave the phones were they were.  While the latter is usually the case in most of my deployments, the mobility provided by the ability to plug a phone in anywhere in the network and not worry about extra configuration is key to some clients.  Thankfully, Cisco fixed this starting in the 2950 with a little concept known as the Auxiliary VLAN.

The Auxiliary VLAN (AUX VLAN) is a specialized VLAN that sits beside a regular access VLAN configured on a switch (sometimes called a “normal” VLAN).  The purpose of the AUX VLAN is to allow IP phones to transmit their payloads along with the untagged data coming from a PC that might be plugged into a switchport on the back of the phone.  The AUX VLAN allows these two devices to transmit on the same port without the need to use an explicit trunk on the link.  In addition, since the port is not configured explicitly as an 802.1q trunk, extraneous VLANs will not be flooded over the port.  In essence, the port becomes a two VLAN trunk.  All the phone traffic is tagged with the ID of the AUX VLAN and the PC traffic is untagged.  Curiously, according to this document, the traffic in the AUX VLAN must also carry a Class of Service (CoS) of 5 along with the AUX VLAN ID.  Otherwise, the traffic is dropped.  So how does the phone get the ID of the AUX VLAN so it can start sending the traffic?  Ah, that’s where CDP comes in.

Cisco Discovery Protocol (CDP) is very crucial in the operation of a Cisco IP phone.  It not only provides the AUX (Voice) VLAN ID for the phone to being sending traffic on the AUX VLAN, it also allows the phone to automatically negotiate power settings.  This allows the phone to use less than the maximum 15.4 watts of power under the 802.3af PoE standard.  If you disable CDP on the port facing the phone/PC you will likely start pulling your hair out.  Even though the phone might have already assigned itself in the Voice VLAN, removing CDP from the switchport in question causes it to forget where to find the voice VLAN.  You’ll need to re-enable CDP and reboot the phone.  You could also statically configure an 802.1q trunk to fix the issue, but where’s the fun in that?

One other curious note is that I’ve always been told that the connection between the phone and the switch when switchport voice vlan is configured is a “special 802.1q trunk”.  Not that I’ve ever been able to see that configuration, as show interface trunk seems to think that the port isn’t trunking and show interface switchport says that it’s an access port.  The key is in Cisco’s documentation.  The correct term for a port with switchport voice vlan configured is a “multi-VLAN access port”.  The distinction between the two is that only the two vlans (voice and access) configured on the switchport will be accepted on the link.  If you were to do something silly like, oh I don’t know, plug another switch into the back of the phone and configure an access port on that switch to be in a different VLAN than the voice or PC access VLAN, traffic will not pass through the phone port to the switch.  Once again, that’s because this isn’t a real trunk.  The switch will only accept tagged frames from the Voice (AUX) VLAN.

Thanks Networking Nerd for this article.


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Wow, am I old or is Cisco moving at about 3 revisions a year? I think the first was correct.. Oh well. I have a huge 8.x project coming up for a client and so I have been installing a lab version to play around with. Last night I tried to MAP my lab UC appliance from a Windows machine, then I remember that this changed as of CCM 5.x….

So I’m sure you remember the “good old days” of CCM 4.x? You can do almost everything on the box. Because it’s in fact a Windows 2000 box. However, this brings security and supportability issues.

With the introduction of Linux-based Unified Communication appliance (CUCM 5.x – 8.x), Cisco locked down the box. You can only access the box via admin web page or a tailored command line.

One of the inconveniences is to review log files. On the old-school CCM 4.x, you may just view the logs in C:\Program Files\Cisco\Trace. On the new UC appliance, you’ll have to use RTMT (RealTime Monitoring Tool). This is especially annoying if you’re testing your system. For each test, you’ll have to download a new set of logs to your computer. (though you may use ‘Remote Browse’ in RTMT, its function is very limited)

What if we can go back to the “good old days” and view the file system just like a Windows drive? I think I discovered a way to do it on a forum site.

Take a look at the screenshot below. It’s a CUCM 6.1.4 mapped to my Windows XP laptop. You can read/write files on CUCM just like a local hard drive. For those people who are not a fan of VI, you may use your favorite editor (such as Notepad++/UltraEdit). And you may use any Windows tools, such as Windows search, WinGrep, WinZip, etc. How’s that? 🙂

To achieve this, you need two things: a root account on CUCM and a software who can map a SFTP server to a network drive (such as sFTPdrive).

 BAM issue solved. In closing I must admit, I was one of the Cisco CCVP’s at Cisco Live asking developers WHEN ARE YOU GOING TO STOP USING WINDOWS? – Soon my friend, soon.. So I would trade this inconvenience any day to keep from having to deal with Microsoft.

Cisco 642-416 Test Prep

Posted: May 22, 2012 in Training
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Cisco E-Books

Posted: May 12, 2012 in Training
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I’ve added some Cisco Press e-books to the Download page. These are linked the Google E-Reader. I will upload more once I have gathered them up.