STP Basics

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2021-12-19 Network Engineering Foundation (12) STP Spanning Tree Foundation_Goose One Blog-CSDN Blog

Supplemental

How to view the root bridge

[SW1]display stp-------[CIST Global Info][Mode STP]-------CIST Bridge: 32768.4c1f-ccf5-70e8Config Times :Hello 2s MaxAge 20s FwDly 15s MaxHop 20Active Times :Hello 2s MaxAge 20s FwDly 15s MaxHop 20CIST Root/ERPC :32768.4c1f-cc51-2794/20000CIST RegRoot/IRPC:32768.4c1f-ccf5-70e8/0CIST RootPortId: 128.1BPDU-Protection :DisabledTC or TCN received :49TC count per hello :0STP Converge Mode :NormalTime since last TC :0 days 0h:4m:10sNumber of TC:5Last TC occurred :GigabitEthernet0/0/1----[Port1(GigabitEthernet0/0/1)][FORWARDING]----Port Protocol :EnabledPort Role :Root PortPort Priority :128Port Cost(Dot1T ) :Config=auto / Active=20000Designated Bridge/Port: 32768.4c1f-cc51-2794 / 128.1Port Edged :Config=default / Active=disabledPoint-to-point :Config=auto / Active=trueTransit Limit: 147 packets/hello-timeProtection Type:None---- More ----

The fourth line, CIST Root in CIST Root/ERPC :32768.4c1f-cc51-2794 / 20000 is the root bridge.

Modify interface cost command

[SW1-GigabitEthernet0/0/1]stp cost ?INTEGER<1-200000000> Port path cost

Modify interface priority

[SW1-GigabitEthernet0/0/1]stp port priority ?INTEGER<0-240> Port priority, in steps of 16

The role of the RP root port

BPDUs used to receive the root bridge

The role of DP designated port

BPDUs used to send and forward the root bridge

The role of AP blocking ports

Used as a backup for the RP root port

About forwarding delay

In STP, both listening and learning have a 15 second delay.

Configure BPDU capture and observation

1 Protocol ID: Total value 0.stands for STP protocol

2 Version ID: Total value 0.stands for STP protocol

3 BPDU type: 0x00 represents configuration BPDU.If it is 0x80, it represents a TCN BPDU, which is sent when the network topology changes.

4 flags

5 Topology Change Confirmation Bit: No

6 Topology Change Bit: No

7 Bridge ID of root bridge: 32768 4C1FCC512794

8 Cost to reach the root bridge (RPC): 20000

9 Sender's bridge ID: 32768 4C1FCCDB6A7B

10 The sender's port ID: 0x8003 represents priority 128 and the interface number is 3

11 Message lifetime: 1 means it is 1 hop to reach the root bridge

12 Maximum lifetime: 20 seconds.

13 BPDU sending interval: 2 seconds.

14 Forwarding Delay: 15 seconds for listening and learning.

TCN BPDU packet

When the topology changes, the switch will send TCN BPDUs to the root bridge to announce the change, and the sender's MAC aging time is forcibly changed to 15 seconds.

When the root bridge receives the TCP BPDU, it will send a message with both Topology Change Confirmation and Topology Change being Yes to inform the non-root switch to confirm receipt.Then it will continue to flood the packets with the topology change bit as Yes to all non-root switches (about 16-17 times), and the received non-root switches will force the MAC aging time to 15 seconds, and then recompute the spanning tree.

STP Timer

stp has 3 timers

forward-delay: The forward delay for listening and learning, the default value is 15 seconds.
hello: BPDU sending interval, the default value is 2 seconds.
max-age: The maximum aging time of BPDUs, the default value is 20 seconds.

Related commands

Modify stp timer (only valid on root bridge)

[SW1]stp timer ?forward-delay Specify forward delayhello Specify hello time intervalmax-age Specify max age

STP deficiencies

1 The convergence speed is slow, and there is a forwarding delay of 15 seconds for listening and learning. If the topology changes, there is a forwarding delay of at least 30 seconds, plus the link aging time of 20 seconds, and a maximum waiting time of 50 seconds.

2 Port division and state division are unclear.

3 Configure BPDUs that only the root bridge can send, and non-root switches can only forward them.

Because of various shortcomings of STP, RSTP has improved STP.Next we start to learn RSTP.