RTP/UDP traffic being blocked from Netgear to cisco

[jordyhall]

We have used to have a network consisting almost entirely of Netgear switches.

We recently introduced a Cisco 3750X into the network as our main switch.

since we have done this our IP phones have had issues of sound only working one way and some have no sound either way.

mainly the sound from the PBX to the phones is being blocked. I have run wiresharks that show the packets are reaching the Cisco switch then going no further. It only seems to be RTP traffic being blocked though. While we are using multiple VLANs the problem is occuring only on the one VLAN.

some of the Netgear switches have basic default settings i.e all ports in VLAN1but some of the switches are layer 3. I have turned QOS off and can see no other reason that RTP traffic in perticular would be blocked.

Does anyone know of any debugs or show commands I could use to see what is happening to these packets or have any ideas why a switch is not doing it's job of mobing packets.

The phone system we use is Toshiba and we have tried turning off COS from the PBX but this did not change anything.

The odd this seems to be if the phone and connection to the PBX are both plugged into a Netgear layer 3 switch all works fine but if say the phone is connected to a netgear layer 2 'Smart Switch' then the voice is 'blocked' from the IP phone.

Thanks

Jordy

[gogasca]

Hi Jordy

Based on your description, normally those cases are security devices or routing problem.

When you mention this is a new switch, lets consider source and destination network and VLAN

From which network to which other network the RTP flow is routed?

Is only RTP being block ?

If you source an ICMP or other traffic from same source interface to same destination same issue occurs?

example:

IP Phone1 network a 10.1.1.X, destiantion IP Phone  in network b 10.1.2.x

can u ping from network a to network b

You can configure an ACL to see matches from UDP traffic to destination network.

Can you give us more details about the topology and your VLANS can be a simple routing issue.

Thanks

[jordyhall]

Hi Gogasca,

Ok how to start? The new Cisco switch we installed is now our main Router/Switch we have 3 vlans 2,3 and 5.

vlan 5 (192.168.219.0) is the main vlan in that this goes out to the firewall and internet.

We have the following routes setup:

192.168.219.0 vlan 5

192.168.218.0 vlan 2

192.168.217.0  vlan 3

0.0.0.0 0.0.0.0 192.168.219.1 (firewall)

Vlans have the IP addresses:

192.168.219.9 vlan 5

192.168.218.1 vlan 2

192.168.217.1 vlan 3

We wanted to use QoS but we turned that off when we had the problem.

We have IP Routing enabled

The odd thing is we have no issues with traffic going between the VLANs.

The issue we are having is only on Vlan 5 in that the PBX card are on the 219 network and so are most of the phones. The next strange thing is that if I plug a phone into one of our Layer 3 netgear switches the phones work fine and I have moved all the PBX links to Layer 3 switches as well to get it to work. but still if I plug a phone into a layer 2 switch or straight into the Cisco switch then the traffic from the phone is blocked (or lost). I have run packet traces and I can see 2 way traffic up to the Cisco then one way traffic. some of the traffic for the phones is UDP and some is RTP I'm not sure why it's different.

I thought it may have to do with native VLANs as most of the netgear switches have the default VLAN 1 but I didn't think that should matter as they or only using 1 vlan and are hanging off an access port of the Cisco switch.

Not sure wht else I can say, I can't work out why it's only UDP traffic within the network.

I will look into your idea of ACLs

Thanks

Jordy

[paolo bevilacqua]

We wanted to use QoS but we turned that off when we had the problem.

Check configuration again, if QoS is not completely turned off in every aspect, you can have packet loss as you're experiencing.

[jordyhall]

I've run show mls qos and get the response:

QoS is disabled
QoS ip packet dscp rewrite is enabled

could the dscp rewrite be it? Why would that stop traffic I'll try disabling it.

[paolo bevilacqua]

I've seen strange things happening related to incomplete QoS configurations.

Just make sure there are no QoS command anywhere.

[jordyhall]

We have found that the issue is to do with 802.1p we had this set on the PBX and turning it off fixed the problem.

I also found I need to set on the interfaces the voice VLAN. I didn't realise this needed to be set.

I take it all interfaces in VLAN 5 will need to be set to 'switchport voice vlan 5'?

The thing is we don't have a set Voice VLAN for example VLAN 5 is used for phones and data.

I guess I will need to set the vlan on each interface and then set as voice VLAN as well.

Does anyone know if this will work ok?

[jordyhall · Accepted solution]

The phone system is using 802.1p and turning this off allowed the voice from the phone to go through.

Also setting the 'switchport voice vlan...' command on the interface allowed it to work.

[jordyhall]

The issue is fixed how do I say  this is answered?