Re: Cisco ASR 9006 – IOS XR 5.1.3 with DHCP Proxy = address flopping

John Wobus jw354 at
Fri Feb 27 16:31:38 UTC 2015

Re my impression/recollection of how dhcpd works:

-You say the lease is still active, but in your scenario, you show a  
release.  Wouldn't that terminate the lease?
-On the other hand, if a lease is still active, isn't a discover a  
request for another IP?  A MAC address is allowed to have multiple IPs  
by doing additional discovers.  dhcpd can be configured to refuse to  
give more than one IP address to any specific MAC address but I don't  
recall what it does to an additional discover in that case.
-Does dhcpd take some time to bring a lease back to the "offerable"  
state when a lease terminates?

The symptoms suggest the lease isn't eligible to be offered at the  
desired moment so a different free lease is offered, perhaps because  
the lease is still active and the server interprets the discover as a  
request for another address.  I assume these are dynamic, e.g. in a  
pool.  For dynamic addresses, the lease file is essentially a short- 
term change log for leases and you may be able to locate the  
successive changes to the lease state for each IP to get a more  
detailed view of what is happening.

John Wobus
Cornell IT

On Feb 26, 2015, at 12:52 PM, perl-list wrote:

> Folks,
> A customer of mine has a problem where if a user device discovers  
> due to reboot or whatever, they will not get the same IP they had  
> previously (even tho current lease is still active).
> Scenario:
> 1) Reboot device - release / discover / offer / request / ack - get  
> ip x.x.x.12
> 2) renews happen no problem - lease still active.
> 3) reboot device - release / discover / offer / request / ack - get  
> ip x.x.x.42
> 4) renews happen no problem - lease still active.
> 5) reboot device - release / discover / offer / request / ack - get  
> ip x.x.x.12 (note that it went back to the original IP).
> 6) renews happen no problem - lease still active.
> 7) reboot device - release / discover / offer / request / ack - get  
> ip x.x.x.42 (note that it went back to IP obtained in step 3 above).
> Specifics:
> This configuration is with a Cisco 9k router with DHCP Proxy as  
> noted by their network admin:
> "Cisco ASR 9006 – IOS XR 5.1.3
> We are using sub-interfaces configured with IP unnumbered pointing  
> to loopback which contains all dynamic pools.
> DHCP Proxy is a profile type created within the DHCP configuration  
> and is configured to point to the DHCP servers. This profile is  
> applied to each sub-interface. Proxy is also responsible for host  
> route management."
> What we have observed is the the Cisco with DHCP Proxy is sending a  
> Release before sending the discover (please note that the client DID  
> NOT send a release).  I don't know if that has anything to do with  
> it or not.
> Also - there are two DHCP servers in a failover pair.  Each running  
> 4.2.5-P1.  According to documentation that we have found (and what  
> i've always understood), they should get the same address again.   
> The customer does not want the address to flop like this as it is  
> causing other problems.  I am at a loss as to why this is happening.
> It should also be noted that we have a packet capture from both  
> sides of the router and that there doesn't really seem to be any  
> difference in the packet content (aside from the added release  
> packet that was never sent by the client device).  Actually, there   
> was one thing that I was unsure about from the packet capture.. the  
> release packet had the same transaction ID as the subsequent  
> discover / offer / request / ack packets according to Wireshark.  I  
> don't know if that is a problem or not, however.
> Also - the client device (a modem / router) is not sending the  
> Client Identifier option (nor is the Cisco inserting it).
> Cisco tells them that the release sent before the discover is a  
> feature of DHCP Proxy and cannot be changed.
> They cannot use normal DHCP relay (ip helper address x.x.x.x;) due  
> to their network configuration (or so Cisco said).
> This whole thing was not previously a problem when they had a Cisco  
> 10k router using normal DHCP relay.
> Thoughts as to this address flopping?  Anyway to stop it?
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