Intro:
------
This is an SNMP-based fencing agent for RHCS.  It was designed with the use-case
of disabling ethernet ports on an iSCSI SAN, but could be used to disable any
port on any SNMP v2c device that implementes the IF-MIB.

The script requires PySNMP version 2 to be installed and working on all nodes
in the cluster.  There are no requirements for any MIBs to be setup --- all of
the required OIDs are hard-coded into the script.  Since the IF-MIB is an IETF
standard, these identifiers are very widely supported and will not change.


Typical usage:
--------------
To use this agen with the switch used on the iSCSI network, you'll require:
   1) A managed switch running SNMP.
   2) An SNMP community with write privileges.
   3) Permission to send SNMP through any ACLs or firewalls from the nodes.
   4) The ifIndex associated with the ports being used by the cluster nodes.

Consider a three-node cluster composed of A, B, and C.  Each node has two
network interfaces - one used for network and cluster communication, the second
used for iSCSI traffic.  If A needs to be fenced, B and C will run this script
to administratively disable the switchport for A's connection to the iSCSI
storage.

If you are using a single interface for cluster and iSCSI traffic, this will
still work, but you will lose network connectivity to the fenced host.


cluster.conf:
-------------
There is no GUI support for this fence agent at this time.  To use it, you will
need something like this cluster.conf

<fencedevice agent="fence_ifmib" name="myswitch" comm="fencing" ipaddr="sw1"/>

In a node's fencing methods, you'll include a line like this:

<device name="myswitch" ifindex="43" option="off"/>

This node will be fenced by disabling the port with ifIndex 43 on the host sw1.
In SNMP speak, we set IF-MIB::ifAdminStatus.43 = down(2).


