Hi Heather,
In fact the STA, regarding which ports are in forwarding or blocking state,
works as follow:
1. Lowest Path Cost
2. Lowest Designated Bridge ID (MAC address + Priority)
3. Lowest port ID
That is, imagine that bridge A is running STA to decide wich ports should
forward and wich ports should block traffic. If bridge A is receiveing BPDUs
on two ports, it will choose the port that is receiving BPDUs announcing the
lowest Path Cost. In case of a tie, the port that will be in forwarding
state will be the one that is reiceiveing BPDUs in which the Bridge ID is
the lowest. If there is a tie at this stage (which happens when the
designated bridge is the same bridge in both ports) the lowest port ID from
the designated bridge acts as the tie breaker. This is what is happening in
your lab.
Your 4006 is bridge A and the 6509 is the designated bridge which is
connected to the 4006 by two links. Because the 6509's port that is
connected to the 4006's port 1/2 has a lower port ID (0x8087)than the 6509's
port that connects to the 4006's port 1/1, the 4006 switch will put port 1/2
in forwarding state.
I hope this helps.
I appologize for my (portuguese)english.
Regards,
Nuno Morais
From: Buri, Heather H
Reply-To: Buri, Heather H
To: [EMAIL PROTECTED]
Subject: question regarding spanning tree [7:6485]
Date: Wed, 30 May 2001 13:20:07 -0400
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
stateforwarding
port_id 0x8002
port number 0x2
path cost4
message age (port/VLAN) 0(20)
designated_root 00-d0-01-98-5c-00
designated_cost 0
designated_bridge00-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
stateblocking
port_id 0x8001
port number 0x1
path cost4
message age (port/VLAN) 1(20)
designated_root 00-d0-01-98-5c-00
designated_cost 0
designated_bridge00-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
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Houston, TX 77056
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