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Ethernet cable pairs and VOIP phone questions

Hello all once again;

We have an office that is doing A LOT of moving of PC's and VOIP phones. Most of the phones are 7941s or 7965s, but there are a couple 7911s.

There are several issues at play here that we are trying to address. First, we use port-security and mac-address sticky for port security. We also use violation restrict (I hate it would prefer shutdown myself).

Anyway, there have been a bunch of instances in which phones have been moved, but they aren't working. They aren't configuring, or they dont power on. Note all are PoE, and the switches are either 3750G PoE or 4500 with 3 Gigabit PoE blades.

The issue we notice is that the phone will populate the access switchport with it's mac address. But the distro switch won't arp the device, so it never pulls an IP. But the distro gets the mac address...

We then do a test cable-diagnostics tdr interface g1/1/1, and after it runs, we are noticing that the problem phones have 1 of the 4 pairs, or 2 of the 4 pairs, failing.

We've done further research; the low end voip phones are 100 Mb switchports in the back. The 7965s and such are 1Gb switchports. If we got this understood well, you'll need all 4 pairs to work for the 1Gb. So, the q's we have are:

1. How accurate is that "test cable-diagnostics tdr" command?

2. If the phone is a 1Gb, and  1 or two of the pairs aren't working, will the phone function still? Or does it need all 4 pairs to work?

3. If the phone is a 100Mb, it only needs two pair to work, correct?

Also, if PC is plugged into a phone, and both are plugged into a Gb port...and the phone is only a 100Mb phone, then the PC will only have the 100 Mb connection, correct?

Bottom line, what pairs are needed, and is this the cause of the mac address not populating into the distro and arping corre         

4 REPLIES
Hall of Fame Super Gold

Re: Ethernet cable pairs and VOIP phone questions

1 accurate.

2 depends which pairs are broken. If the  "10/100 mbps" , phone will not power.

3 and 4: yes.

Hall of Fame Super Gold

Re: Ethernet cable pairs and VOIP phone questions

1. How accurate is that "test cable-diagnostics tdr" command?

Accurate.  But I hate doing TDR on a 3560 or on a 4500/6500 chassis.  The reason is because it takes a minimum of 61 seconds to fully finish a TDR on these models as compared to a 2960/G/S a 3750G/E/X (5 to 7 seconds).  I use this in our network a lot.  The cable contractors are not happy, though.

But I would exercise caution when doing TDR on the 4K/6K switches because of the "lack of information" with the result.  TDR on these chassis will tell you whether or not there's a fault.  It won't tell you what kind of fault and how far away is the fault.  It'll tell you how long the cable run is. 

2. If the phone is a 1Gb, and  1 or two of the pairs aren't working, will the phone function still? Or does it need all 4 pairs to work?

Yes and no.  For a VoIP phone to work, you need a minimum of three pairs, A, B and C.  Pair "D" is GigabitEthernet.   Even if you have a GigabitEthernet port and you have a cable run with Pair "D" not working, your clients (VoIP or not) will not work UNLESS you specify the port to operate in 10/100 Mbps (Command:  auto speed 10 100).

Read this:  How to use Time-Domain Reflectometer (TDR)

Hall of Fame Super Gold

Re: Ethernet cable pairs and VOIP phone questions

Leo Laohoo wrote:

Yes and no.  For a VoIP phone to work, you need a minimum of three pairs, A, B and C.  Pair "D" is GigabitEthernet.   Even if you have a GigabitEthernet port and you have a cable run with Pair "D" not working, your clients (VoIP or not) will not work UNLESS you specify the port to operate in 10/100 Mbps (Command:  auto speed 10 100).

Read this:  How to use Time-Domain Reflectometer (TDR)

Actually, 10/100 Ethernet uses only two pairs.

http://en.wikipedia.org/wiki/100BaseTX

Hall of Fame Super Gold

Re: Ethernet cable pairs and VOIP phone questions

Actually, 10/100 Ethernet uses only two pairs.

No arguments there.

However, here's a flaw:  Let us say that you have a GigabitEthernet client and you plug it into a GigabitEthernet port.  The link doesn't go up.  You do a TDR and you see that Pair "D", the GigabitEthernet pair, doesn't work.

The only way to get this to work, aside from moving the connection to a FastEthernet port, is to configure the GigabitEthernet from triple speed to a "dual" speed.

Don't know why the switch ain't "smart" enough to detect that.  Maybe it's an IETF specification.  

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