[c-nsp] Forcing dhcp lease renewal

[Tassos Chatzithomaoglou]

In latest IOS you can use "release dhcp intX" in order to force a immediate release of the 
DHCP lease.

You can also use "renew dhcp intX" if you want to renew the dhcp lease, like in your case.

-- 
Tassos


Luan Nguyen wrote on 16/01/2009 21:12:
> Things point to Cradlepoint don't they?  I've used Digi ConnectPort with
> lots of success.
> Or go with the 3G-Wireless HWIC card or ask VzW for a static IP address.
> The last thing would be to use object tracking in conjunction with EEM to
> solve your problem.
> 
> Regards,
> 
> Luan Nguyen
> Chesapeake NetCraftsmen, LLC.
> [W] http://www.netcraftsmen.net
> [M] luan at netcraftsmen.net
> [Blog] http://cnc-networksecurity.blogspot.com/
> 
> 
> 
> 
> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net
> [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Bob Tinkelman
> Sent: Friday, January 16, 2009 11:35 AM
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] Forcing dhcp lease renewal
> 
> For a cisco router with an interface like this:
>    interface FastEthernet0/1
>     description Verizon EVDO via Cradlepoint CBA250
>     ip address dhcp
> 
> I'm looking for a way to force the router to issue a dhcp
> lease renewal request.
> 
> I can do this manually, for example via
>    config term
>     int fa0/1
>     shut
>     no shut
>     exit
> but I'm looking for a way to trigger this automatically.
> 
> 
> (Or possibly I'm trying to solve a problem in the wrong way.)
> 
> 
> 
> Background:
> 
> We have a good many customers with T1 or multi-T1 service,
> and with fall-back routing configured over a "cheap path",
> typically a dynamic-ip cable-modem service or dsl.  Our
> configs use a combination of gre-tunnels (to preserve
> customer-site address ranges) and sometimes object tracking
> and policy routing (often to direct web requests to a
> higher-speed cable-modem service in cases where NATing is
> acceptable).  We've been doing this for a good while and
> have a set of configs that provide pretty solid service.
> 
> I have been testing, in a lab environment, a configuration
> to do the same thing with Verizon's EVDO service using a
> Cradlepoint CBA250 (Cellular Broadband Adapter).  It's not a
> router; just a "pass-through device".
> 
> The same configuration that we use with dynamic-ip cable-
> modems works.  However, several times/day, things "break".
> 
> Output of "show interface", "show dhcp lease", etc., show
> that the cisco router doesn't think anything's changed.  The
> interface has the same dhcp-assigned ip address and default
> gateway.  But the default gateway is no longer pingable.
> 
> Doing a "clear int Fa0/1" doesn't help.  A "shut" and "no
> shut" will cause the router to issue a new dhcp request, get
> a new (and different) ip address and gateway, and start
> working again.
> 
> 
> My current working hypothesis is that the EVDO link between
> the CBA250 and Verizon was interrupted, possibly very
> briefly, that Verizon noticed and invalidated the dhcp
> lease, but that no indication of this reached the router.
> 
> It's a weak hypothesis.  I'm bothered by the fact we've
> never seen this problem with similar cable-modem setups,
> e.g., with Time Warner and with Cablevision.  I've sent
> email to support at cradlepoint.com even though I really don't
> see how their equipment could be involved.
> 
> 
> 
> I could use object tracking to discover when the link over
> EVDO stops working.  But I'm not sure what do to with that
> info.  Is there a way to force a new dhcp request to go out
> based on object tracking?  (To date, I've used object
> tracking mostly to enable/disable specific ip route
> commands.)
> 
> 
> I have the strong feeling that I'm trying to solve this in
> the wrong way, and that if I really understood what was
> going wrong, I'd be working in a different direction.
> 
> So, any hints would be much appreciated
> 
> - Bob
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