Cat3750 - DHCP VRP; Alternative to exclude-address

Unanswered Question
Mar 9th, 2009

hey all, looks like the exclude-address feature is not available for vrf-enabled dhcp pools.

http://www.cisco.com/en/US/docs/ios/12_2t/12_2t8/feature/guide/ftondhcp.html

as an alternative, we are trying to hard-code certain addresses in the dhcp pool - static dhcp ip to mac-address binding. but, whenever the machine with that mac-address tries to get dhcp it gets another address in the available range ... instead of the one that we set for it.

for example, we are trying to force the machine with mac 0016.41a9.7f9d to get dhcp ip 10.0.100.10. instead, it is getting dhcp 10.0.100.2.

ip dhcp pool enovatel

vrf testing

network 10.0.100.0 255.255.252.0

default-router 10.0.100.1

dns-server 66.135.224.90 4.2.2.2

lease 0 1

address 10.0.100.10 hardware-address 0016.41a9.7f9d

!

interface Loopback1

ip vrf forwarding testing

ip address 10.0.100.1 255.255.252.0

!

interface Vlan3802

ip vrf forwarding testing

ip unnumbered Loopback1

!

thanks all for the help.

I have this problem too.
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Laurent Aubert Mon, 03/09/2009 - 18:21

Hi,

Did you try to use the client-identifier instead of the hardware-address ?

run debug ip dhcp server when the host is making a request to learn its client-id.

HTH

Laurent.

Robert Ho Mon, 03/09/2009 - 18:41

hey laurent, thanks for the help. using the client-id as opposed to he hw-addr worked. it may not fit in our environment. what we are trying to do is reserve the first 50 addresses in the pool. they will be assigned to managed devices on customer premise over a period of time. the client id will be unique and we cannot run debug each time we need to install one of those managed devices. is the hw-addr option a bug? thanks again, robert.

from the debug,

DHCPD: DHCPDISCOVER received from client 0100.1641.a97f.9d

Bindings from VRF pool testing:

IP address Client-ID/ Lease expiration Type

Hardware address/

User name

10.0.100.10 0100.1641.a97f.9d Infinite Manual

Switch#

Laurent Aubert Tue, 03/10/2009 - 07:39

Usually, the default client-identifier format is 01+mac address. Cisco devices configured as DHCP client will use another format as there is no restriction from the RFC

The router acting as DHCP server will use the client-identifier for the binding if present in the DHCP Discover packets and the Hw address if there is no client-identifier.

HTH

Laurent.

Robert Ho Tue, 03/10/2009 - 11:42

Indeed, it seems as though the router/dhcp-server uses 01 as the client ID for all the hosts. we thought it was random and very hard to predict without enabling debug. hey, thanks again for your help!

Laurent Aubert Tue, 03/10/2009 - 14:09

The client-Id is set by the host and added to the DHCP discover packets using DHCP option 61.

As it must be unique per host (used by the DHCP server to index its database), it makes sense to see the interface mac-address but it could be anything else. The 01 prefix refer to ethernet type interface

Laurent.

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