DHCP Issue

Discussion in 'Cisco' started by ciscolio, Aug 7, 2008.

  1. ciscolio

    ciscolio

    Joined:
    May 28, 2008
    Messages:
    3
    I'm having a weird issue with DHCP on a 2621, IOS 12.2

    I have several clients who connect from different offices, each office is hardwire eth to a closet access switch, 1900. The access switches all connect to a 3500 command switch.
    This switch connects to an Eth int on the rtr. The rtr is set to pass DHCP to all the clients connected via 10.x network. 10.x.x.1-10.x.x.20 are set to be excluded. Problem is, clients are getting addresses in this range. Funny thing is, none of them show up when I do show ip dhcp binding. So apparently they aren't getting their IP via DHCP.

    How would they get these...Do they have their cpu's set to a static IP? My guess this is the case, but I just find it very odd so many would be doing this, as typically our clients don't know the first thing about how to even setup TCP/IP let alone know enough to give themselves a static 10.x.x.5 or something address...

    rtr#sho arp
    Internet 10.x.x.2 12 0004.xxxx.xxxx ARPA FastEthernet0/1
    Internet 10.x.x.1 - 0004.xxxx.xxxx ARPA FastEthernet0/1
    Internet 10.x.x.7 0 0014.xxxx.xxxx ARPA FastEthernet0/1
    Internet 10.x.x.4 0 001a.xxxx.xxxx ARPA FastEthernet0/1

    rtr#sho ip dhcp binding
    IP address Client-ID/ Lease expiration Type
    Hardware address
    10.x.x.22 0100.xxxx.xxxx.xx Aug 08 2008 10:22 AM Automatic
    10.x.x.24 0100.xxxx.xxxx.xx Aug 08 2008 10:53 AM Automatic

    rtr#sho run
    no ip domain-lookup
    ip domain-name xyz.com
    ip dhcp database tftp://x.x.x.x/log
    ip dhcp excluded-address 10.x.x.1 10.x.x.20
    !
    ip dhcp pool notworking_dhcp
    network 10.x.x.0 255.255.255.0
    default-router 10.x.x.1
    dns-server x.x.x.57 x.x.x.20
    Last edited: Aug 7, 2008
    ciscolio, Aug 7, 2008
    #1
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  2. ciscolio

    macdaddy

    Joined:
    Aug 9, 2008
    Messages:
    3
    Are these Windows clients by chance? Do you have the ability to run an ipconfig /all on one of the Windows DHCP clients to see what DHCP server assigned it the IP in question? That would at least give you a DHCP server to point the figure out.

    I saw a similar problem once where a tech had been messing with both bridging functions and Internet connection sharing on his Windows laptop. He had his laptop answering DHCP queries with a dynamically-created DHCP pool based on the network settings his own NIC received via DHCP. Obviously that caused a few conflicts.

    Assuming that the router is handing out excluded IPs for some reason you can confirm it with some debugs, assuming that the problem is reasonably reproducible.

    debug ip dhcp server events
    debug ip dhcp server packets

    With that you should be able to see both the DISCOVER and OFFER and the contents of each. Then look at a newer IOS release and bug scrub it for DHCP related bugs. That would also be a good one to hand to TAC.

    J
    macdaddy, Aug 9, 2008
    #2
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