CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

- Below describes the STP convergence process:
i) Elect the root bridge. There is only one root bridge per network. All root bridge’s ports
(which are also designated ports) are placed in forwarding state.
ii) Selects the root port on all non-root bridges. Root port is the port that is closest to the
root bridge, which means it is the port that receiving the lowest-cost BPDU from the root.
Every non-root bridge must have a root port. All root ports are placed in forwarding state.
Note: It is not always the shortest path but the fastest path to reach the root bridge.
iii) Selects the designated port from the designated bridge for each LAN segment.
Designated bridge is the lowest-cost bridge on each segment that forwards the lowestcost
BPDUs into the segment. If the costs are the same, the port of the lower BID bridge
will be selected as the designated port. If there is a tie in the BID, the port with the lowest
port ID will be used as the final tiebreaker (this happens when a switch connects back to
itself with a cross cable). All designated ports are placed in forwarding state.
iv) All other ports – non-designated ports, are placed in blocking state.
- STP root bridge election process started with each bridge claiming itself as the root bridge by
sending out multicast frames called Bridge Protocol Data Units (BPDUs), which are used to
exchange STP information between bridges. A BPDU contains the following important values:
The Bridge IDs of the
Root Bridge and the
Transmitting Bridge
The cost to reach the
root from this bridge
(Root Path Cost)
BID is an 8-byte field that is composed of the bridge priority value
(0-65535, 2 bytes) and the bridge MAC address (6 bytes). The root
bridge is the bridge with the lowest bridge ID. A bridge can become
the root bridge by lowering the priority value (eventually the BID),
which is normally being practiced in large switched networks.
This value is set to 0 at the beginning of STP root bridge election
process since all bridges claim to be the root. The lower the cost, the
better chance to become a designated port. The range is 0-65535.
Port ID The port ID of the originating switch port of the BPDU frame.
- The bridge with the lowest priority becomes the root. 32768 is the default bridge priority value.
If there is a tie occurred on priority, the bridge with the lowest MAC address becomes the root.
Root BID: 32768:1111.1111.1111
Cost: 0
Local BID: 32768:1111.1111.1111
SW1
Fa0/1
Fa0/2
Root BID: 32768:1111.1111.1111
Cost: 0
Local BID: 32768:1111.1111.1111
Root BID: 32768:3333.3333.3333
Cost: 0
Local BID: 32768:3333.3333.3333
Figure 5-2: Root Bridge Election
- Figure 5-2 shows the STP root bridge election process. Each switch sends out BPDUs to claim
itself as the root. Assume that SW2 receives the BPDU from SW1 faster than SW3, SW2 agrees
that SW1 is the root and start forwards out SW1 BPDUs to SW3 with the accumulated cost.
After a while, SW3 will also agree that SW1 is the root bridge after received BPDU from SW1.
40
Fa0/1
Fa0/1
Fa0/2
Fa0/2
SW2
Root BID: 32768:1111.1111.1111
Cost: 100
Local BID: 32768:2222.2222.2222
Root BID: 32768:3333.3333.3333
Cost: 0
Local BID: 32768:3333.3333.3333
SW3
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