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DHCP Configuration on a MLS Switch

Hi All,

I have read in CCNP Switch book that we can use the MLS Switch to be used as a DHCP Server. However, we can define only one DHCP Pool on the switch. Now the question is that how hosts located on different VLANs get IP Addresses from the same subnet, since we normally use different subnets for different VLANs?

Is there any mechanism through which the switch will allocate IP Addresses from differnt subnets to different hosts on different VLANs???

Another option of using switch as DHCP Relay Agent is also mentioned, but can we use a single DHCP Server to allocate different IP Addresses to hosts as per different VLANs??? Or we need to use different DHCP Servers for each VLAN IP Addresses allocation??

Best Regards,

Daud Parvez

1 REPLY
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Re: DHCP Configuration on a MLS Switch

Hello Daud,

you should be able to define multiple DHCP pools on IOS based LAN switches as it happens on routers.

see for example

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_55_se/configuration/guide/swdhcp82.html#wp1098426

In any case if you use an external DHCP server, it can serve IP addresses for different pools on multiple IP subnets you don't need a dedicated DHCP server per client IP subnet,

DHCP relay agent requires the use of ip helper-address on the switch SVIs associated to the client Vlans.

Actually this means converting the non routable client originated DHCP request ( that has a broadcast 255.255.255.255 destnation that is not routable) to a routable packet with IP destination = remote DHCP server address.

Within the packet a field is filled with the IP address of the SVI that received the original request.

This info is used by the DHCP server as a search key to find a free address in the appropriate DHCP pool.

The DHCP offer reply is sent back to the DHCP relay agent that sends it out the SVI facing the client.

Hope to help

Giuseppe

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