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
>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
>
>
>
>EOM
>
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