How to implement DHCP server with fail-over high availablity
Simon Hobson
dhcp1 at thehobsons.co.uk
Wed Feb 16 08:09:03 UTC 2011
Joe Shen wrote:
>we want to set up DHCP service with 99.999% availiablity.
>In order to reach this target, we want to set up dhcp cluster on
>different physical sites. Even when one site is totally down, our
>customer's connection should be kept, and new connection request
>should be serviced by other sites.
>On the other hand, load should be balanced between different clusters.
>
>Could anybody do me a favor on giving some advice?
Put simply there are two options for you.
1) Run Failover between a **pair** of servers per pool.
2) Run multiple servers with independent non-overlapping pools.
Because of the way DHCP must work, each server must have an accurate
record of what leases it has given out. That means you cannot have
independent servers handing out overlapping addresses.
Failover will allow two (and only two) servers to co-operate in
managing a pool. If one fails, the other can continue managing the
pool - but this is not fully automatic by default.
You can have different server pairs for different pools, and you can
have multiple (non-overlapping) pools in a subnet/network.
Without failover you could simply have multiple servers, each with
it's own pool. Clients will naturally tends to spread their leases
across the servers - but this will be biased towards the ones that
answer quickest. If a server fails then it will not be around to
renew any leases - and so clients will start to run out after a
while. In practice, clients normally start trying to renew their
leases at half the lease time, so if you set your leases to (say) 14
days, then clients would have a full week before they notice. If
their lease does run out, they would then get a **different** address
from another server.
If you have clients with static addresses, then you can have as many
servers as you like handing out those addresses since there is no
co-ordination required.
--
Simon Hobson
Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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