Re: SNMP net-structure algorithm

[Fred Baker <[e-mail]>]

>> I'm looking for some kind of algorithm, which allows to analyze
>> the structure of a segmented LAN. The algorithm should only use the
>> addressbook information of the dot1bridge-mib ( MIB-2 ).

Well, here's the simplest approach I know. It has the weakness that
if a MAC address ages out of its local bridge's FdbTables, the result
can be indeterminate.

Definition:
	A MAC Address (System Interface) is BETWEEN two bridges if and
	only if it is in the FDB of both bridges and the associated
	port indicates the same LAN.

	A useful special case of this is where one of the two bridges
	is the STP designated bridge on the LAN and its MAC address is
	the value of dot1dStpPortDesignatedBridge in the other bridge.

Definition:
	A LEAF BRIDGE (with respect to a given port) is the designated
	bridge on the port in question, and no other bridge shows its MAC
	address on that port as dot1dStpPortDesignatedBridge
	
Definition:
	A MAC Address (System Interface) is BEYOND a leaf bridge port
	if its MAC address is associated with that port.

OK, so dividing your network up is a matter of identifying the systems
that are on the LAN segments, which is to say, the ones BETWEEN certain
bridges or BEYOND certain ports.

The Algorithm:

First, find all the bridges in the network. If they are all running RFC
1286, this SHOULD be a matter of sending out a single SNMP GET as a
(one) broadcast.

Second, dump all of their dot1dTpFdbTable,
dot1dStpPortDesignatedBridge, ifPhysAddr, and
dot1dStpPortDesignatedRoot values.

Put all of the MAC Addresses in a file or table in memeory.

Recursively starting with the root, and in that case setting INPORT to
an unused port number:

	sort the MAC Address Table by dot1dTpFdbPort on this bridge
	FOR each value of dot1dTpFdbPort DO

		IF dot1dTpFdbPort == INPORT THEN

			these MAC addresses are BETWEEN the designated
			bridge and this bridge

		ELSE IF there is a bridge with dot1dStpPortDesignatedBridge ==
			the address of this system THEN

			FOR each such bridge, 

			    call this procedure with
				that bridge
				the MAC address subset for this port
				INPORT = the port number on that bridge for
					which this is true

			END FOR
		ELSE
			these MAC addresses are BEYOND this port and
			it is a Leaf Bridge Port.
		END IF
	END FOR

I hope that this helps.