Port Configuration With IP Phone and PC On Same Port

[FoxtrotRomeo]

Hello all,

We just installed a hosted VOIP system using Cisco 7900 series IP phones.  We are having a strange issue with a few computers where they pull DHCP information from our VOIP provider's DHCP server on the Internet and not our LAN DHCP server. 

The switchports areconfigured as:

switchport mode access

My rationale behind this is that the phones would use CDP to get their VLAN info from the providers Cisco router and the PCs would just ride on the default VLAN.  But this is not the case.  Computers randomly keep getting DHCP info from the provider's router.  Do I have to use voice vlan x and make the switchports trunks?

[ALIAOF_]

Here is an example of how our ports are configured.  I'm assuming you have the phone connected to the jack and your computer is connected to the phone, note: vlan 10 is the VoIP network:

switchport access vlan 10

switchport trunk native vlan 20

switchport trunk allowed vlan 10,20

switchport mode dynamic desirable

switchport voice vlan 20

srr-queue bandwidth share 10 10 60 20

queue-set 2

priority-queue out

mls qos trust cos

auto qos voip trust

spanning-tree portfast

[FoxtrotRomeo]

Thanks for the prompt response. 

My native vlan is the default vlan.  In that case isn't switchport mode access the same as switchport access vlan 1?

[ALIAOF_]

That is correct.

[liju1488]

With newer IOS it is not required that a switchport connecting a phone piggybacking a PC to be configured as a trunk specifically.

[Anilsingh]

Hi Mohammad Ali,

i have 100 users with same configuration Port Configuration With IP Phone and PC On Same Port.

connection setting Firewall Sonicwall to Core switch SG550 to Access switch 5 Netgear and 1 cisco sg300.

When we joined the office in morning the our phones are in hanged mode, after restart 
Core switch all phones working fine and again second days same issue. 

Please could you help me what is the reason to hang the phone. when phone hanged that time Access switch latency is very high 2000 ms. pls help

[Jarrod DeMotts · Accepted solution]

You don't need to make them trunks.

Switchport mode access

switchport access vlan (data)

switchport voice (voice vlan)

spanning-tree portfast

CDP will figure out if it is a phone or not and allow clients plugged in behind the phone (through the phone) to use the data vlan

[ALIAOF_]

That is true with the new Cisco switches and IOS you do not need to setup the trunks any more.

[FoxtrotRomeo]

My problem is a "rogue" DHCP server that actually belongs to my hosted VOIP provider handing out IP addresses to PCs.  So it looks like I have to take the PCs out of that broadcast domain and put them in a different vlan.  So it looks like I'll be doing what DeMotts suggested.  Thanks guys.

[paul driver]

Hello

Or you try and negate the voice dhcp server via a mac /vlan  acl

1) mac address-table static xxxx.xxxx.xxxx vlan x drop auto-learn

or

2) mac access-list extended rouge-dhcp

permit host xxxx.xxxx.xxxx any

vlan access-map drop-dhcp 10

Action drop

Match MAC address rouge-dhcp

Vlan access-map drop-dhcp 99

action forward

Vlan filter drop-dhcp vlan x x

Res

paul

[Izac ICT]

Hello Jarrad,

I have same issue. I'll create vlan10 for PCs and vlan20 for Ip phones. I'll use below configuration, they will get IP from same internal DHCP server but from different pools. How can I configure them to get the IP from right pool?

Thank you.

Switchport mode access

switchport access vlan 10

switchport voice 20

spanning-tree portfast

Cheers,

Isac

[Georg Pauwen]

Hello,

assuming that you are using Cisco IP Phones, the phones will use CDP to determine which VLAN they belong to. Your switchport configuration looks fine. If you are using local pools, make sure that option 150 points to the CUCM. Here is an example:

ip dhcp excluded-address 192.168.20.1

ip dhcp pool VOICE

network 192.168.20.0 255.255.255.0

domain-name yourdomain

dns-server 8.8.8.8 8.8.4.4

default-router 192.168.20.1

option 150 ip 172.16.1.1 <-- this is the IP address of the CUCM

lease 5

[Izac ICT]

Thank you for the prompt reply. I have to use the MS DHCP server for both VLANs so it means if I assign IP address to VLAN and IP helper address, Devices can get IP from right IP pool? How can I be sure?

BTW Phones are SNOM not Cisco.

Thanks again.

ISac

[Georg Pauwen]

Hello,

SNOM phones work differently as far as I remember. You have to use trunk ports. If Vlan 10 is your data Vlan, and Vlan 20 your voice Vlan, the ports should look like this:

interface FastEthernet0/1
switchport trunk encapsulation dot1q
switchport mode trunk
switchport trunk allowed vlan 10,20
switchport trunk native vlan 10

You also need IP Helper addresses. Obviously make sure your SNOM phones are set to DHCP.