DHCP Scope option - Metric Base

[craig.juhas]

Hi All,

I'm hoping someone can advise whether we are able to apply a DHCP scope option on our Cisco switches that is currently used on a Microsoft DHCP server.  Essentially we have a LAN and WLAN and we want to ensure that when a user has a network cable plugged in that route is always preferred.  With the DHCP scope for wireless managed in our HQ by a Microsoft DHCP server they have implemented Option 003 - Microsoft Default Router Metric Base with a hex value that make the WLAN metric higher and less desirable.  This is working very well.  Now I would like to implement this in our branches where there are Cisco switches handling the DHCP.  Is it possible to apply this special option that increases the metric for Windows machines?

When I do a 'route print' from my Windows laptop it shows a variation of this where blue is the wireless interface:

Active Routes:

Network      Destination      Netmask       Gateway       Interface Metric

0.0.0.0        0.0.0.0            10.0.0.254     10.0.0.177    20
0.0.0.0        0.0.0.0            10.0.1.254     10.0.1.142    25

Is this possible to achieve with Cisco DHCP?

Thanks!

Craig

[IAN WHITMORE]

Hi. If you've got each VLAN on a different subnet and you are using DHCP, why don't you just use two different pools?

A guide on how to configure can be found here:

http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfdhcp.html#wp1010265

Regards,

Ian

[craig.juhas]

We are using separate DHCP Pools in the Cisco switch - one for the Wired subnet, and one for the Wireless subnet.

What I want to know is can I add an IP Option to the Wireless DHCP Pool that will allow me to specify the 'route print' metric on Windows machines when they pick up an IP address.  This can be done if we were using a Microsoft DHCP server but it would be great if there was an equivalent command that I can use to achieve this in a Cisco switch.

Craig

[IAN WHITMORE · Accepted solution]

Don't think it's possible. You can configure option 150 but not option 003 which should be configured on a dhcp server as you pointed out. Remeber switches are well...switches. DHCP on a switch or router is a cheap and quick optin if no server is available but of course they are not real servers and lack some options.

[craig.juhas]

I thought that might be the case but asked just in case someone was able to rig Cisco DHCP to do it.  Nevermind, thanks anyway.

Craig

[Garry Cross]

This thread does not have the correct answer in my opinion. To do what is being requested you would configure option 43 Vendor Specific Information from RFC2132. See also Microsoft Default Router Metric Base Option.

The following config works.

ip dhcp pool test
 network 10.100.20.0 255.255.255.0
 default-router 10.100.20.1
 option 43 hex 0304.0000.0020

This adds 32 to the metric that the interface dynamically configures.

03 is the option

04 is the length

00000020 is the metric to add.