Arp or Proxy Arp

Discussion in 'Cisco' started by Darren Green, Feb 20, 2009.

  1. Darren Green

    Darren Green Guest

    I read with interest the recent thread entitled 'Aging Implementation'
    and my understanding of Arp is much better than it was but I am
    scratching my head on this.

    Scenario is 2 x 877 (ADSL) routers (primary & secondary) on a remote
    site connected to different carriers. On the LAN side 2 x VLAN's (1 &
    2). Both routers connect to the same switch (trunk ports), only port 24
    on the switch is in VLAN 2, all other ports are on VLAN 1 (NB HSRP on
    the VLAN's)

    When checking the ARP cache on router 1 see that many of the ARP entries
    (VLAN 1) map to the MAC address of RTR-02's VLAN 1. The IP's seem to be
    for hosts that no are no longer active, when I ping them nothing!. The
    active hosts all have valid MAC addresses and I can ping these.

    The routers have IP helper statements on to relay requests to a remote
    DHCP server. Is it possible that either:

    1) When a packet comes in from the WAN on RTR-01 it ARP's for the
    packet, gets a response from the real host and then RTR-02 answers the
    ARP a few milliseconds later effectively overwriting the MAC entry sent
    by the real host


    2) RTR-01 is periodically checking to see what hosts are alive. As the
    DHCP relay agent I note a debug DHCP server events turns up a message
    every 2 x mins telling me the router is checking for valid addresses.
    Assuming the host is not live, could the response be somehow coming from

    Out of interest the RTR-02 ARP cache has very few arp entries in,
    certainly no duplicates.


    Darren Green, Feb 20, 2009
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