Sunday, August 23, 2009

STP Features

I felt a little weak on certain STP features, and the BCMSN lab manual unfortunately did not touch on any of them. In particular they were:

Root Guard: Protects ports from receiving superior bpdu's. I.E. Root Bridges can not be detected on these ports once it is enabled. If superior bpdu's are detected the port will be placed into a root-inconsistent state. This can be verified with the sh span inconsistent-ports command.

BPDU Guard: Use this if you want not bpdu's to be accepted on a port. If bpdu's are found, the port will be placed into err-disabled. Use it where portfast is enabled!

LoopGuard: If you have a port in blocking, root, or maybe any other non-designated role that experiences a sudden loss of bpdu's, you could be in trouble. Once that bpdu max-age timer expires the switch will role that blocking or whatever port into the forwarding stage because bpdu's are suddenly gone. This prevents a switching loop. Loopguard prevents this...check it out. I had two switches connected, and used bpdu filtering on the port connected to this switch...thus cutting off the bpdu's..heres what happened:

*Mar 1 00:27:04.387: %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port Fast Ethernet0/2 on VLAN0001.
3550-48#sh spanning

Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000d.65eb.5e80
Cost 19
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0014.692f.7f00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 19 128.1 P2p
Fa0/2 Desg BKN*19 128.2 P2p *LOOP_Inc
Fa0/16 Altn BLK 19 128.16 P2p

3550-48#sh spanning-tree inconsistentports

Name Interface Inconsistency
-------------------- ------------------------ ------------------
VLAN0001 FastEthernet0/2 Loop Inconsistent

Number of inconsistent ports (segments) in the system : 1

Dont forget about udld, and bpdu filtering (which essentially tells the switch to not accept or send bpdu's on certain interfaces).

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