Failover + PXE

Todd Snyder tsnyder at rim.com
Tue Nov 27 15:40:55 UTC 2007


Thanks - I assumed so, but wanted to make sure.

Cheers!

Todd. 

-----Original Message-----
From: dhcp-users-bounce at isc.org [mailto:dhcp-users-bounce at isc.org] On
Behalf Of Glenn Satchell
Sent: Tuesday, November 27, 2007 10:39 AM
To: dhcp-users at isc.org
Subject: RE: Failover + PXE

When a failed server returns it automatically sorts out the leases,
before it starts responding to requests, so no conflicting ones are ever
assigned.

regards,
-glenn

>Subject: RE: Failover + PXE
>Date: Tue, 27 Nov 2007 09:53:55 -0500
>From: "Todd Snyder" <tsnyder at rim.com>
>To: <dhcp-users at isc.org>
>
>Thank you - that's quite helpful.
>
>One question out of it - if one server goes down, and the other server 
>starts handing out leases in its part of the pool (partnerdown state) 
>and the dead server comes back, will it hand out conflicting leases, or

>does the failover server send a list of all its leases to the (was 
>dead) server?
>
>Again, thanks for the help.  I've gotta get this up and running this 
>week!
>
>Cheers,
>
>Todd.
>
>-----Original Message-----
>From: dhcp-users-bounce at isc.org [mailto:dhcp-users-bounce at isc.org] On 
>Behalf Of Glenn Satchell
>Sent: Saturday, November 24, 2007 8:15 AM
>To: dhcp-users at isc.org
>Subject: RE: Failover + PXE
>
>
>>
>>Thanks for the information!  Much appreciated, esp the tip on TFTP, I 
>>hadn't thought of mirroring that source yet.
>>
>>A quick question regarding the failover methodology.  From what I've 
>>read on the subject, it would appear that this set is more about load 
>>balancing than HA (in my understanding).  Since it splits the pool in 
>>2
>
>>and each server hands out a limited range, this seems to be less about

>>HA.  Does each server keep track of what the other server has handed 
>>out, so when the other fails it keeps things organized?  Is it 
>>possible
>
>>to run one as a 'cold' standby, that keeps a copy of the leases but 
>>doesn't actively respond to anything unless the other one is down for 
>>XX seconds?
>>
>>Just curious - we're setting this up in an HA environment, and I would

>>like to understand the behaviour a little more before I write the 
>>document on how it behaves and what our options are.
>>
>>(wow, that appears to be a lot less quick than I'd hoped for, but the 
>>answer should be easy, I hope)
>>
>>Cheers,
>>
>>Todd. 
>
>The two servers divide each pool in half (actually depends on the split
>values) and both offer addresses from their portion of the total pool 
>to clients. If one server stops, then the other continues to offer 
>addresses from its share of the pool.
>
>The dhcp servers communicate with each other and so keep the dhcp lease

>information synchronised between the two servers.
>
>When a client accepts an assdress from a dhcp server, it remembers the 
>server's address and periodically (usually half way through the lease) 
>requests a renewal of that address from the server. When a dhcp server 
>failure occurs, clients that were requesting address renewal stop 
>receiving replies. Remewmber we're only half weay through the ;lease, 
>so the client keeps using the existing IP address. The client continues

>attempting renewal until near to the end of the lease. The client then 
>broadcasts for a new dhcp server and receives a reply (hopefully) from 
>the partner dhcp server. The new dhcp server assigns an address from 
>within it's pool.
>
>There is a mode, the so called 'partner down' mode, where you tell the 
>dhcp server that it's partner has failed. The surviving dhcp server 
>will now allocate IP addresses from the whole pool of IP addresses. Any

>clients that broadcast for a new DHCP server will be offered their old 
>IP address. This is usually only required for extended outages, eg the 
>server crashes on Friday arvo and you won't be able to fix it until 
>Monday.
>
>Once upon a time I wrote a simple script that polled the other dhcp 
>server and switched to partner down if it failed to contact it for 30 
>minutes. Search the archives, I'm sure it has been posted. It uses 
>OMAPI to communicate with the dhcp server.
>
>Remember the client is only impacted if one dhcp server is down for 
>more than approximately 50% of the lease time. I typically use at least

>one day as the lease time, so any downtime needs to be >12 hours to 
>really bother the dhcp clients. If you use longer leases then the 
>corresponding failure period can be greater. If you have a stable 
>source of clients or plenty of spare IP addresses there is no reason to

>not have much longer leases. For IP phones, which don't move around I
use a week.
>
>No special action needs to be taken when a failure occurs or when the 
>failed server returns to service. The server returning to service 
>communicates with the running dhcp server and they synchronise lease 
>databases and return to the normal mode of operation.
>
>I've been running DHCP failover since 3.0.1rc6 and on the whole find 
>that it works really well. The load is shared between the two servers, 
>which can be in completely different locations. There is no downtime 
>during failover, the partner server just keeps on running. For extended

>outages the remaining server can be switched to partner-down mode where

>it will hand out addresses from the full range.
>
>There is a section in the dhcpd.conf man page titled DHCP FAILOVER that

>covers this area. There is also a document distributed with dhcp-3.0.5 
>and earlier titled "draft-ietf-dhc-failover-07.txt" in the doc 
>directory of the source distribution which covers the failover mode in 
>RFC-like detail. :) In later versions there is a reference to the IETF 
>document draft-ietf-dhc-failover-12.txt. Use google to locate a copy.
>
>If any of the above is not clear please post questions to the list.
>
>regards,
>-glenn
>--
>Glenn Satchell     mailto:glenn.satchell at uniq.com.au | It's a dog  eat
>dog
>Uniq Advances Pty Ltd         http://www.uniq.com.au | world, and by
>golly,
>PO Box 70 Paddington NSW Australia 2021              | we better make
>sure
>tel:0409-458-580  tel:02-9380-6360  fax:02-9380-6416 | we're the dog.
>
>
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