Help me make DHCP's documentation better.
David W. Hankins
David_Hankins at isc.org
Thu Aug 17 21:45:18 UTC 2006
Amidst the failover changes in 3.1.0, two new options have been added.
Below, I've pasted the relevant RELNOTES lines and the new manual page
entries for these configuration options (scoped in the failover {}
section).
If folks could give a read and tell me if this makes sense or not, I'd
appreciate it.
- Failover pairs now implement 'MAC Affinity' on leases moving from the
active to free states. Leases that belonged to the failover secondary
are moved to BACKUP state rather than FREE upon exiting EXPIRED state.
If lease rebalancing must move leases, it tries first to move leases
that belong to the peer in need.
- The server no longer sends POOLREQ messages unless the pool is severely
misbalanced in the peer's favor (see 'man dhcpd.conf' for more details).
- Pool rebalance events no longer happen upon successfully allocating a
lease. Instead, they happen on a schedule. See 'man dhcpd.conf' for the
min-balance and max-balance statements for more information.
The max‐lease‐misbalance statement
max‐lease‐misbalance percentage;
The max‐lease‐misbalance statement tells the DHCP server what per‐
centage of total free leases (as defined as the total number of
leases in either the FREE or BACKUP states) a peer is allowed to own
before a rebalance check is made. Configuring higher values causes
the server to rebalance less frequently, but permits a larger mis‐
balance between the FREE and BACKUP lease pools. Configuring a
lower value causes the server to rebalance more frequently, but
keeps the pools more balanced. ISC DHCP servers no longer send
POOLREQ messages unless the misbalance is at least twice this per‐
centage in the peer’s favor. Valid values are between 0 and 100.
The default is 15.
The max‐lease‐ownership statement
max‐lease‐ownership percentage;
The max‐lease‐ownership statement tells the DHCP server what per‐
centage of total free leases either it or its peer are normally
allowed to own in excess of balance for the purpose of MAC Address
Affinity. When a server undergoes a lease rebalancing operation, it
first tries to move as many leases as it can to the peer whose pre‐
vious client was Load‐Balanced to that peer (as governed by the Load
Balance Algorithm, see the split configuration value). The max‐
lease‐ownership value determines the maximum percentage of leases
either server will hold before giving its peer the oldest leases
(regardless of the previous client’s place in the Load Balance algo‐
rithm). Valid values are between 0 and 100, and should probably be
less than the max‐lease‐misbalance value. Larger values will allow
servers to retain leases to reallocate to returning clients, smaller
values promote pool balance. The default is 10.
The min‐balance and max‐balance statements
min‐balance seconds; max‐balance seconds;
The DHCP Server schedules pool rebalance events at a time between
these two values, estimated to be when the the max‐lease‐misbalance
percent of leases have been allocated by its peer. This estimate is
reached from however many seconds have elapsed since the oldest
lease in the failover peer’s pool has been expired.
The min‐balance value defaults to 60, one minute, and the max‐bal‐
ance value defaults to 3600, one hour.
Lease rebalancing events can be CPU intensive, particular on instal‐
lations where failover peers may have large numbers of pools and
addresses to examine, so these parameters should be used to keep the
estimation of the need for pool rebalance sane...not so long that
you are in danger of exhausting your pool, not so short that your
server is constantly rebalancing.
--
ISC Training! October 16-20, 2006, in the San Francisco Bay Area,
covering topics from DNS to DDNS & DHCP. Email training at isc.org.
--
David W. Hankins "If you don't do it right the first time,
Software Engineer you'll just have to do it again."
Internet Systems Consortium, Inc. -- Jack T. Hankins
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