Hello.
I don't really have a problem so much as I am trying to get a better
understanding of how Spanning Tree works. I am currently studying for my
switching exam and am reviewing Spanning Tree. I have the following
statement from the Cisco course manual and in the case of redundant paths to
the root bridge, it states "In order to choose which port will be forwarding
data and which ports will be blocking data, the switch looks at two
components in the BPDU, as follows:
1. Path Cost
2. Port ID
The switch looks at the path cost first to determine which port is receiving
the lowest cost path. If the path cost is equal, as in the case of parallel
links, the bridge goes to the port ID as a "tie-breaker". The port with the
lowest port ID forwards and all other ports block."
I decided to verify this on one of my 4006's which connects to my 6509
(which is the root) and here is what I found:
pet4006_8 (enable) sh spantree statistics 1/2 1
Port 1/2 VLAN 1
SpanningTree enabled for vlanNo = 1
BPDU-related parameters
port spanning tree enabled
state forwarding
port_id 0x8002
port number 0x2
path cost 4
message age (port/VLAN) 0(20)
designated_root 00-d0-01-98-5c-00
designated_cost 0
designated_bridge 00-d0-01-98-5c-00
designated_port 0x8087
top_change_ack FALSE
config_pending FALSE
port_inconsistency none
PORT based information & statistics
config bpdu's xmitted (port/VLAN) 0(8702816)
config bpdu's received (port/VLAN) 831061(1662121)
tcn bpdu's xmitted (port/VLAN) 1(1)
pet4006_8 (enable) sh spantree statistics 1/1 1
Port 1/1 VLAN 1
SpanningTree enabled for vlanNo = 1
BPDU-related parameters
port spanning tree enabled
state blocking
port_id 0x8001
port number 0x1
path cost 4
message age (port/VLAN) 1(20)
designated_root 00-d0-01-98-5c-00
designated_cost 0
designated_bridge 00-d0-01-98-5c-00
designated_port 0x80c8
top_change_ack FALSE
config_pending FALSE
port_inconsistency none
However, as you can see from the above output, Port 1/2 is the port chosen
to forward and it appears to have a higher port ID number. Can someone
please explain what I am missing here?
Thanks!
Heather Buri
CSC Technology Services - Houston
Phone: (713)-961-8592
Fax: (713)-961-8249
Mobile:
Alpha Page:
Mailing: 1360 Post Oak Blvd
Suite 500
Houston, TX 77056
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