DHCP-relay via unnumbered interfaces

Discussion in 'Cisco' started by Arjan, Jul 20, 2004.

  1. Arjan

    Arjan Guest

    Hi all,

    I've got a setup with two 1721 routers, connected via DSL. Both routers
    have a /32 on lo0 and a /29 on fas0. One of the routers has a DHCP
    attached to fas0, one of of them is a relay-agent and has "ip
    helper-address" (pointing to dhcp server) configured on fas0, that has a
    laptop attached to it, trying to obtain a DHCP lease from the non-attached
    router. (using build-in ios dhcp for lab purposes, the real-life setup
    uses a seperate server). Both routers are running the latest IOS

    The problem: there seems to be an issue with IOS and relay via unnumbered
    interfaces. I've noticed bug CSCds21050 and I've also noticed this cco


    This indicates that "ip helper-address" should also be set on the incoming
    unnumbered interface. I've tried both with and without this extra helper

    However: the dhcp request that arrives on the server is broken. In the IP
    header there is source address is, in the DHCP packet the
    relay-agent field shows the same.

    In a different setup with a numbered link between two routers (actually
    two 806 boxes) this works fine and the DHCP traffic will pass the relay

    Any comments/tips?
    Arjan, Jul 20, 2004
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