BPDUs and spanning-tree

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Oct 22nd, 2008
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I have the following:


Port 27 (GigabitEthernet1/0/27) of VLAN0001 is forwarding

Port path cost 4, Port priority 128, Port Identifier 128.27.

Designated root has priority 24577, address 0013.1922.ea00

Designated bridge has priority 24577, address 0013.1922.ea00

Designated port id is 128.27, designated path cost 0

Timers: message age 0, forward delay 0, hold 0

Number of transitions to forwarding state: 1

Link type is point-to-point by default

BPDU: sent 2646535, received 5355



The last line reads BPDU: sent 2646535, received 5355.


I also have:


Port 22 (GigabitEthernet1/0/22) of VLAN0001 is forwarding

Port path cost 4, Port priority 128, Port Identifier 128.22.

Designated root has priority 24577, address 0013.1922.ea00

Designated bridge has priority 24577, address 0013.1922.ea00

Designated port id is 128.22, designated path cost 0

Timers: message age 0, forward delay 0, hold 0

Number of transitions to forwarding state: 1

Link type is point-to-point by default

BPDU: sent 2641388, received 0


This last line reads BPDU: sent 2641388, received 0.


My question is:


Can I ASSUME that because I received BPDUs back, that this is a switch on that port? The ports that have 0 on BDPUs received is more than likely a host, or is this a VERY INCORRECT ASSUMPTION?


Thanks!


--John

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milan.kulik Wed, 10/22/2008 - 11:35
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Hi,


generally yes, but you are not 100% sure.

If there were a switch with a BPDU filter configured, e.g., it would not send any BPDUs to you.

And received 5355 BPDUs might be only historical data and the swich could have been replaced by a host recently.

If all switches in your LAN are Cisco ones, CDP would show you a neighbor switches more reliably.


BR,

Milan

francisco_1 Wed, 10/22/2008 - 11:38
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guessing here. looks like port 27 is a taking part in spanning-tree properbly connected to another switch since it receiving and sending BPDU's and also in forwarding state forwarding traffic for vlan 1. Port 22 looks like a normal switch port in vlan 1. maybe you have a PC connected to this port only receiving BPDU messages but not sending. this is normal for standard switch port.


to be more accurate, send us the output "sh spanning-tree vlan 1"



Francisco.


John Blakley Wed, 10/22/2008 - 11:42
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Well, my ultimate goal is to figure out what ports I can enable portfast on and I noticed that.


Thanks!

John

Giuseppe Larosa Wed, 10/22/2008 - 11:53
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Hello John,

the local switch is currently the root bridge:


Port path cost 4, Port priority 128, Port Identifier 128.27.

Designated root has priority 24577, address 0013.1922.ea00

Designated bridge has priority 24577, address 0013.1922.ea00

Designated port id is 128.27, designated path cost 0

you can see this because Root BridgeID = Designated Port ID and designated path cost=0.


the few BPDUs received can be the result of STP topology changes BPDU sent back to the root bridge or the result of a change in STP Root Bridge ID after some time.:


Number of transitions to forwarding state: 1


But also the port can be an access port now and was connected to a switch in the past.


You can check with sh cdp n and also with


show mac-address-table interface g1/0/x:


if you see a single MAC there should be a PC or server, if you see multiple MAC addresses here you have a switch or an unauthorized hub /switch.



Hope to help

Giuseppe


kelvintommy Sun, 10/26/2008 - 19:06
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Hi,

 

Maybe, the attached document can help

 

Thanks

 

 

 

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