How to implement DHCP server with fail-over high availablity

[Simon Hobson]

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

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