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New Member

Bridege ID for STP

Is the bridge ID simply the MAC of bridge or MAC+Priority calculation for STP calculation.

  • LAN Switching and Routing
6 REPLIES

Re: Bridege ID for STP

The bridges look for the lowest Root Bridge Identifier (BID), an eight-byte field composed of a Bridge Priority and a Media Access Control (MAC) address. This allows the entire bridged network to elect a single Root Bridge.

HTH and please rate if it does.

New Member

Re: Bridege ID for STP

Is the case same for port ID i.e. MAC+Priority? If a tie occurs between 2 ports on a segment in a STP process how is the designated port selected between these two ports is it by lowest MAC+Priority of the port?

Re: Bridege ID for STP

Port ID (a unique index value for every port in a bridge or switch) is evaluated if all three of the previous criteria tie. I can not find what exactly makes up the index value, but I would assume the MAC has something to do with it.

Re: Bridege ID for STP

The port ID is a 16 bit integer made using the same scheme as the bridge ID, but using a port number instead of a mac address. The port number is (currently) a 12 bit integer, identifying with no ambiguity the port on a given bridge. The 4 bits with the highest weight in the port ID are the port priority, settable by the user.

There is no need for a mac address in the port ID because the mac address of the bridge on which the port is located was already included in the sender bridge ID (one of the 3 fields that are taken into account before using the port ID as a tie breaker).

Regards,

Francois

Re: Bridege ID for STP

Is it then converted? A port ID looks something like 0x8002.

Re: Bridege ID for STP

Sorry, I don't understand the question. What do you mean by converted?

0x8002 is a 16 bit integer. The 4 highest bits are the priority and the 12 lowest ones are the port number

So priority 0x8, port number is 0x002

The trick is that the priority is generally expressed as a byte (it used to be 8 bits priority and 8 bits of port number before 802.1t changed all this to 4 and 12 bits respectively). So in Cisco's CLI, the priority that you see on this port will be 0x80 = 128 decimal. That's the default priority.

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