Your DHCP servers should be looking at the "giaddr" field in the
DHCP packet being relayed by your router. It's this field that
the DHCP server uses to determine which scopes are applicable.
I looked into this before and I believe cisco has changed the default
value it uses for the intserted giaddr value. A long time ago, it used
to
be that the giaddr value inserted was the interface's primary IP
address. They then changed to a mode where the router would monitor
the DHCP packets and if an OFFER was not observed after 2-3 client
attempts would then use the secondary IP address as the giaddr. If
you had N secondaries, it would eventually cycle through all N
secondaries. (Note, this was done on a per client basis).
Cisco now has introduced a way to specify the giaddr behavior
desired via the "dhcp-giaddr" command. There's a cable version
of this (for CMTSs) and looks like one for radio interfaces as
well:
cable dhcp-giaddr
radio dhcp-giaddr
The "primary" value tells the router to always insert the interface's
primary IP address as the giaddr value. The "policy" value tells
the router to do the DHCP OFFER monitoring and to cycle through
all the interface IP's (again on a per client basis).
I don't know if this is in the general interface code train.
I guess there are some DHCP servers which don't allow one to define
relationships between scopes, but the DHCP servers I use do and so
I tend to set the giaddr to primary and then define primary-secondary
scope relationships on the DHCP server. This will allow clients to
obtain IP addresses much quicker and allows one to drop multiple DHCP
secondaries on an interface. This configuration is common on CMTSs.
"D'souza, Henry (MED, TCS)" wrote:
>
> Hi ,
>
> We have a single VLAN, VLAN92, that we use for wireless on the mfg shop
> floor. 3.57.92.0/24 was the primary address, used for bar-coding with
> STATIC IP's only, and 3.57.93.0/24 was the secondary address, for DHCP
> for wireless PC usage.
>
> On the previous version of IOS 12.1(2)E, the DHCP packets were able to
> be sourced off the secondary address (3.57.93.x), and as such, the
> clients would get a DHCP address. (I don't know for sure, but I am
> speculating that it sent out DHCP packets with Source addresses from
> both the primary IP and the secondary IP).
>
> Anyway, with the new version of code,IOS 12.1(12c)E1 it evidentially
> ONLY sources the DHCP requests from the primary address. DHCP server
> looks at the incoming packet, checks the SA, sees that it now does NOT
> match any of it's scopes, and then drops the packet.
>
> swapped the primary and the secondary addresses, and everything is now
> working fine.
>
>
> Henry D'souza
> Network Engineer
> General Electrical Medical Systems
> Email # [EMAIL PROTECTED]
> 8200 West Tower Avenue Milwaukee, WI USA 53223
> Work (414) 362-2431 Fax: (414) 362-2352.
> Home (262) 547-8163.
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=53855&t=53440
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