Commit e898172a authored by Marcin Siodelski's avatar Marcin Siodelski
Browse files

[5478] Added a section about transitioning between the servers.

parent b2ff311b
......@@ -292,6 +292,38 @@
(see below).</para>
<section xml:id="ha-scope-transition">
<title>Scope Transition in Partner Down Case</title>
<para>When one of the servers finds that its partner is unavailble,
it will start serving clients from its own scope and the scope of the
partner which is considered unavailable. This is straight forward
for the new clients, i.e. sending DHCPDISCOVER (DHCPv4) or Solicit
(DHCPv6), because those requests are not sent to any particular server.
The available server will respond to all such queries when it is
in the <command>partner-down</command> state.</para>
<para>When the client is renewing a lease, it will send its
DHCPREQUEST (DHCPv4) or Renew (DHCPv6) message directly to the
server which has allocated the lease being renewed. Because this
server is unavailable, the client will not get any response. In
that case, the client continues to use its lease and re-tries to
renew until the rebind timer (T2) elapses. The client will now enter
the rebinding phase, in which it will send DHCPREQUEST (DHCPv4) or
Rebind (DHCPv6) message to any available server. The surviving
server will receive the rebinding request and will (typically)
extend the lifetime of the lease. The client will continue to
contact that new server to renew its lease as appropriate.</para>
<para>When the other server becomes available, both active servers
will eventually transition to the <command>load-balancing</command>
or <command>hot-standby</command> state, in which they will be
responsible for their own scopes. Some clients belonging to the
scope of the started server will be trying to renew their leases
via the surviving server. This server will not respond to them
anymore and the client will eventually transition back to the
right server via rebinding mechanism again.</para>
<section xml:id="ha-load-balancing-config">
<title>Load Balancing Configuration</title>
<para>The following is the configuration snippet which enables
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