STP

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STP:802.1d

1) Prevent L2 loops

2) Bridge ID: Priority + MAC address. default priority:32768

In BPDU, following parameters:

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|Root Bridge ID + Sending Bridge ID + cost to root + Port ID

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Selection process:

1) Select root bridge:

a) All switches starts sending BPDUs.switch with lowest bridge ID becomes the root bridge. Once root bridge is selected, only root bridge sends BPDUs.

b) Hello timer of BPDUs is 2 sec

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port state ( for normal user data) --------------------------------------port role ( In spt,what is my role)

a)Forwarding----------- -----------------------------------------------------------> a) root port: on this switch, which port has least cost to root bridge

-------------------------------------------------------------------------------------->-b) designated port: on this collision domain, this provides least cost path to the root bridge

b)Blocking --------------------------- ---------------------------------------------->c)non-designated port

d) cost

bandwidth-- cost

10mbps----100

100mbps--19

1 Gig -- 4

10 Gig -- 2

2) Selection of Root port

all non-root bridge will now select one port as root port which has lowest cost to root bridge.

total cost = sending bridge id cost + incoming interface local cost

3) Select Designated port per collision domain: DP forwards BPDU

on this collision domain, this provides least cost path to the root bridge

if cost is same, tie breaker:

a) lower sending bridge id

b) lower port id: priority + port identifier

Timers - only timers of root port matters:

Hello timer : 2 sec

Max Age : 20 sec

Forward delay : 30 sec

STP port states:

1)Disabled: port is down

2) blocking: port is up

3) listening: listening.sending for BPDU. cannot receive/send data. can actively participate in STP. by default 15 sec

4) learning state: can learn mac address.cannot receive/send data.

5) forwarding state:

PVST: Cisco default.

TCN

criteria:

1) change from learning to forwarding (and at-least one designated port)

2) learning to blocking

3) port is disabled

4) if portfast is configured, not consider for topology change

- TCN is transmitted to root port until ack...

- TCN doesn't tell what has changed

- once root bridge gets TCN BPDU. it will start sending BPDU is TCN bit set

- root bridge will reduce its CAM table timer from 5 minutes to forward delay time

- all other bridges once get BPDU will TCN bit set - will reduce the CAM table timer to forward delay

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