VLANS URGENT

[carl_townshend]

I can get my dhcp server to see my phones, the server is in vlan1 the phones are in 200, we have about 10 switches, 1 server, 9 clients. Do I just need to enable ip routing on the core switch ?

please help

[Josef Oduwo]

Because VLANs are considered logical broadcast domains, communicating between VLANs requires inter-VLAN routing see http://www.cisco.com/univercd/cc/td/doc/product/software/ios113ed/113ed_cr/switch_c/xcvlan.htm for an overview of inter-VLAN routing and http://www.cisco.com/univercd/cc/td/doc/product/lan/cat5000/rel_5_2/layer3/routing.htm#wp42311 for working with inter-VLAN routing and subinterfaces.

Cheers,

Josef.

[carl_townshend]

if use intervlan routing, if I get a broadcast say from vlan 1, will this affect vlan 200 ?

[Kevin Dorrell]

No it will not. You will need to tell the router to forward the broadcasts from VLAN 200 to VLAN 1. On the logical interface that controls VLAN 200, you will need ip helper-address a.b.c.d, where a.b.c.d is the IP address of your DHCP server. You will also need service dchp so that the router knows to tell the DHCP server which scope to allocate the address in (although it is actually enabled by default).

http://www.cisco.com/en/US/products/ps6441/products_command_reference_chapter09186a00804a9579.html#wp1140104

Kevin Dorrell

Luxembourg

[Josef Oduwo]

Broadcast traffic is limited by broadcast domains - logical (VLANS) or otherwise (routers).

To let traffic thru you need a 'helper' to convert broadcast packets to unicast packets - the ip helper command. The 'helper' receives broadcast packets and convert them to unicast packets and send them to the DHCP server which inserts the GIADDR field in DHCP packet with the ip address of the sub interface where it received the DHCP request (broadcast). This field is used by the DHCP server to lookup the appropriate scope from which the ip address should be leased to the client.

More on the ip helper command is available here: http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_command_reference_chapter09186a0080087387.html#wp1018606 and more on DHCP operation here http://www.dhcp.org.

Cheers,

Josef.

[carl_townshend]

So where do I need to put the helper address, Do I need to put this on the core switch ?

[Kevin Dorrell]

You need to put it on the switch/router that is handling your inter-vlan routing, on the virtual interface representing the client's VLAN, and pointing to the address to the DHCP server.

Kevin Dorrell

Luxembourg

[carl_townshend]

I have got a big problem, I cant enable routing because vlan 1 pc's are on a 10, class A network, the phones are on a 10.81.123 class c network, when I try to put the 10.81.123 ip address on vlan 200 it comes up with "overlaps with vlan 1"

is there anything I can do ?

please help asap

[Ryan_Grant]

You must put the vlan in a separate subnet. eg. 10.81.124.0 255.255.255.0.

Then enable ip routing on the switch. If you don't want to enable dynamic routing protocols use static routes.

Also, you have to enable the switchport voice vlan # on the interface connected to the phone. In the phone you have to set the VLAN id that you assigned on the interface.

[carl_townshend]

the phones are to get there ip from a dhcp server thats plugged into vlan 200, the vlan is in a seperate subnet, it is 10.81.123.x 255.255.255.0 . my concern is that pc's can still pick up an address from the dhcp server even though is on a seperate vlan ?

please help

[thisisshanky]

Do you have any superscopes defined in your server ?

[Ryan_Grant]

x.x.x.x is the ip address of the DHCP server.

ip forward-protocol udp bootps

ip forward-protocol udp bootpc

Interface Vlan 200

ip helper-address x.x.x.x

[Ryan_Grant]

Your pcs can not pickup a DHCP address from your server in a different subnet without the commands I did in my previous post.

VLANs are on different subnets, and by default they are not allowed to forward broadcast. DHCP clients use broadcast to get there ip address from a DHCP server. (DORA, Discover, Offer, Request, Ack) both Discover and Request are broadcast. The broadcast will not pass from a one VLAN to another unless you issue the ip directed-broadcast under interface config, which is a bad thing to do, because VLANs purpose is to isolate certain traffic from reaching other host.

[carl_townshend]

so can you tell me what I need to do ?

i am a little stuck.

thanks a million

[Ryan_Grant]

You can solve this by doing one of these.

1. You can add a DHCP server to the clients subnet.

2. You can add static ip address to the clients.

2. You can use the ip helper-address x.x.x.x and

setup up a DHCP scope for the clients pointing to the DCHP server on the VLAN 200 subnet.