Routers forwarding DHCPOFFERS incorrectly if ip is in arp cache

Unanswered Question
Mar 16th, 2010

We have verified that 2811 routers running 12.4 versions of code are behaving this way:

1) An IP address is released by a client

2) The arp cache for the router still has the client's mac address

3) A new client boots up and sends a DHCPDISCOVER

4) The DHCP server (on another network, so I have ip helper-addresses on my 2811 routers) replies with a DHCPOFFER

5) The DHCPOFFER is the IP address released by the 1st client

6) The 2811 router forwards the DHCPOFFER to the mac address in the arp cache, not the DHCPDISCOVER client (this was verified with a sniffer).

Anybody else seen this?  Is this a bug or a "security feature" to try to prevent duplicate IP addresses?

I have this problem too.
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Giuseppe Larosa Tue, 03/16/2010 - 14:42

Hello Christopher,

this should be a recent bug I remember a recent thread about this strange behaviour

in that case there was a message in the log that helped in finding the bug.


I've found the bug

CSCsu67864            Bug Details

DHCP Offer is unicasted to the old MAC on the ARP  cache

After the end user replaced his HGW  ( DHCP client ) physically, the new HGW never received DHCP
Offer  from ASR1000. It resulted in the failure of IPv4 address allocation of  the newly replaced


it should  be fixed in 12.4(24)T2

Hope to help



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