The difference between BPDU & Bridge ID

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Oct 9th, 2008
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Although the BPDU & Bridge ID are both used in ROOT SWITCH SELECTION(BPDU) & ROOT BRIDGE SELECTION(BRIDGE ID).


Are they the same and if not why not i.e whats the similarity & differences between bpdu & bridge ID and root/switch selection?


Also BPDU only transmits on STP but not on Portfast thats why portfast converges faster; is that correct? GREAT THANKS guys if you can help me on this!

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Marwan ALshawi Thu, 10/09/2008 - 01:39
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ok bpdu is the carrier of the control traffic of STP root selction while the Bridge ID is a number ID that is represented by lowes MAC address in the switch

so bridge ID one of ht eoptions that determine if the switch will be root or not while the bpdu is the carrier of that communication between switches


with portfast the port skip the learning and lisstening stages and go directly to forwarding state which is useful only with access ports connected to hosts and this port will not participate in root selction and if u make it trunk or connect switch to portfast it will make loop


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Giuseppe Larosa Thu, 10/09/2008 - 03:27
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Hello Baljit,

BPDU and Bridge-id are not the same thing


BPDU = Bridge Protocol Data Unit is a frame that contains STP information

inside a BPDU you can find two fields that each is a Bridge-ID

Root Bridge-ID

Sender Bridge-ID


Bridge-ID: 8 bytes made of 2 bytes priority followed by 6 bytes of a MAC address.

Lowest priority is preferred

Every switch has its own BID when taking part in STP.

BID varies for different vlans in two ways:

a different MAC for each vlan (old way)

a different priority (extended -id)


Portfast DOESN't imply that BPDU are not sent out or received just that listening and learning states are bypassed


Hope to help

Giuseppe


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