Selecting The Optimum Configuration Bpdu; Designating The Root Port - 3Com 7700 Configuration Manual

Selecting the Optimum
Configuration BPDU
Designating the Root
Port
Configuration BPDU of Ethernet 1/0/7: {1, 0, 1, e1/0/7}
Configuration BPDU of Ethernet 1/0/4: {1, 0, 1, e1/0/4}
Switch C
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Configuration BPDU of Ethernet 1/0/1: {2, 0, 2, e1/0/1}
Configuration BPDU of Ethernet 1/0/5: {2, 0, 2, e1/0/5}
Every switch transmits its configuration BPDU to others. When a port receives a
configuration BPDU with a lower priority than that of its own, it will discard the
message and keep the local BPDU unchanged. When a higher-priority
configuration BPDU is received, the local one will be updated with it. And the
optimum configuration BPDU will be elected through comparing the configuration
BPDUs of all the ports.
The comparison rules are:
The configuration BPDU with a smaller root ID has a higher priority
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If the root IDs are the same, perform the comparison based on root path costs.
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The cost comparison is as follows: the path cost to the root recorded in the
configuration BPDU plus the corresponding path cost of the local port is set as
S, the configuration BPDU with a smaller S has a higher priority.
If the costs of a path to the root are the same, compare, in sequence, the
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designated switch ID, designated port ID, and the ID of the port through which
the configuration BPDU was received.
In simpler language, we assume that the optimum BPDU can be selected through
root ID comparison in the example.
The port receiving the optimum configuration BPDU is designated to be the root
port, whose configuration BPDU remains the same. Any other port, whose
configuration BPDU has been updated, as explained in "Selecting the Optimum
Configuration BPDU", will be blocked and will not forward any data. In addition, it
only receives but does not transmit a BPDU and its BPDU remains the same. The
port, whose BPDU has not been updated, is the designated port. Its configuration
BPDU is modified as follows: substituting the root ID with the root ID in the
configuration BPDU of the root port, the cost of path to root with the value made
by the root path cost plus the path cost corresponding to the root port, the
designated switch ID with the local switch ID and the designated port ID with the
local port ID.
The comparison process of each switch is:
Switch A
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Ethernet 1/0/1 receives the configuration BPDU from Switch B and finds out
that the local configuration BPDU priority is higher than that of the received
one, so it discards the received configuration BPDU.
The configuration BPDU is processed on the Ethernet 1/0/2 in a similar way.
Thus, Switch A finds itself the root and designated switch in the configuration
BPDU of every port; it regards itself as the root, retains the configuration BPDU
of each port and transmits configuration BPDU to others regularly thereafter.
By now, the configuration BPDUs of the two ports are as follows:
STP Overview 207