[hcie discussion] STP-A

subject 1：SW2 Of 3、4 What port role is an interface ? Please write down the analysis process .（3 branch ）
answer ：SW2 Of g 0/0/3 and g 0/0/4 Interfaces are designated ports DP（ Let's say 3、4 Port reference g 0/0/3、g0/0/4 Interface ）.
According to the spanning tree algorithm ,SW1、SW2 and SW3 Through interaction BPDU message , Compare BPDU Priority vector , Finalize the root bridge and port roles . Here's the thing to watch out for , Deploy Eth-Trunk after , The member port is no longer participating in the spanning tree calculation , But by the Eth-Trunk The interface participates in the calculation as a whole . Designed during the following calculation RPC The value is assumed to be 802.1t standard .
From the spanning tree algorithm itself BPDU The analysis process of interaction is as follows ：
○1 First, the election of the root bridge ： All devices begin to think of themselves as root bridges , And send the BPDU. The election rules of genbashi are ： Cut in BID The smallest device becomes the root bridge ,BID By bridge priority and MAC Address composition , Compare in turn BID Bridge priority and MAC Address , The smaller the value, the better . because SW1、SW2 and SW3 Of BID Respectively 4096.xxxx.xxxx.1111,
4096.xxxx.xxxx.2222 and 8192.xxxx.xxxx.3333, namely SW1 and SW2 The priority of the bridge is 4096, Less than SW3 Of 8192,SW1 Of MAC Address less than SW2 Of MAC Address , therefore SW1 Elected as root bridge .
○2SW2 Of Eth-Trunk1 Port received BPDU The optimal , So become RP port （SW2 Of Eth-trunk1 Port received BPDU in RPC by 0, and 3、4 Port received BPDU in RPC by 10000）, here SW2 Will be based on the root port 3、4 Port calculation configuration BPDU, And the calculated configuration BPDU And 3、4 Received at port BPDU compare , Result calculated configuration BPDU better （ Calculated BPDU in sender BID by 4096.xxxx.xxxx.2222, better , and 3、4 Received at port BPDU in sender BID by 8192.xxxx.xxxx.3333, Suboptimal ）, therefore 3、4 Port number becomes DP port .

subject 2： problem 2: hypothesis SW2 Of 1 No. interface exits ETH-trunk1,SW2 What is the role of each port of ? Please write down the analysis process （7 branch ）
answer ： Since link aggregation is in manual load mode , When SW2 Of G 0/0/1 After exiting the aggregation group SW1 Can't perceive , therefore SW1 Still think Eth-Trunk1 For a complete interface . Link aggregation defaults to flow by flow load sharing , When there are multiple active links in the aggregation group , The same stream switch will only select one link for forwarding . Namely root bridge SW1 Of BPDU Only one link will be selected to forward the message to SW2. Therefore, it is analyzed in two cases .
Situation 1 ： hypothesis SW2 from g0/0/1 I received it BPDU message .
SW2 The port roles of are as follows ：（max-age within ）
G 0/0/1 AP
Eth-Trunk1 RP
G 0/0/3 AP
G 0/0/4 AP
SW2 The port roles of are as follows ：（max-age in the future ）
G0/0/1 RP
Eth-Trunk DP
G 0/0/3 AP
G 0/0/4 AP
The analysis process is as follows ：
○1 Electoral root bridge
In the whole broadcast domain , because SW1 Of BID Minimum , therefore SW1 Elected as root bridge ,SW1 The interface on is selected as the specified port DP.
○2 analysis SW2 Of G0/0/1 and Eth-Trunk1 Port role .
SW2 According to the information received by each interface BPDU Calculate the configuration of each interface BPDU. Then select the optimal interface location according to the priority vector RP mouth . The priority vector is ：{BID、RPC、Sender BID、Sender PID、 Receive port PID}.
because SW2 Of G0/0/1 Arrive at SW1 Of RPC=2000, and G/0/0/3 and G0/0/4 Arrive at SW1 Of RPC=30000.
therefore SW2 Of G 0/0/1 Elected as RP mouth .
Eth-Trunnk1 and G0/0/1 The changing process of ports ：
1）SW2 Of G0/0/1 After exiting the aggregation group max-age Within time .
Because of the original RP port （Eth-trunk1） There is still （ Non root switches RP There is only one mouth ）,SW2 According to the root port is G0/0/1 Calculated configuration BPDU No from SW1 Of BPDU optimal , So now SW2 Of G0/0/1 The role of the port is AP Mouth and will continue max-age Time .
2）SW2 Of G 0/0/1 After the interface exits the aggregation group max-age After time .
because RP The port needs to receive... From the upstream device BPDU To maintain the port role , stay max-gae After the time, no... From upstream equipment has been received BPDU It is considered that the negotiation with upstream equipment has failed , The port needs to be set DP The mouth converges again .
here SW2 Will reselect RP Lift the port , As a result of G 0/0/1 Interface received BPDU The optimal , therefore SW2 take G0/0/1 Select the new port RP port , And calculate the configuration for other interfaces BPDU. The RP Port and port SW1 Conduct P/A Enter the forwarding status immediately after negotiation .
○3 Based on the analysis of SW2 Of G0/0/3 and G0/0/4 The interface role
because SW2 According to the root port is G0/0/3 and G0/0/4 Calculated configuration BPDU（RPC=20000） Not from SW3 received BPDU optimal （RPC=10000）, therefore SW2 Will 3、4 Port set AP mouth .
Situation two ： hypothesis SW2 from Eth-Trunk 1 I received it BPDU.
SW2 The port roles of are as follows ：
G0/0/1 DP
Eth-Trunk1 RP
G 0/0/3 AP
G 0/0/4 AP
because SW2 Of RP mouth Eth-Trunk1 It can continuously receive messages from upstream equipment BPDU（ The BPDU Is the best of all ports ）, therefore RP The port role remains unchanged .
SW2 Of G0/0/1 After exiting the aggregation group, the interface becomes DP The mouth converges again , Because I haven't received the message from the opposite end BPDU, Therefore, it becomes the optimal link in calculation BPDU, The port role is the specified port DP.
because SW2 According to the root port is G0/0/3 and G0/0/4 Configuration calculated by the interface BPDU（RPC=20000） Not from SW3 received BPDU optimal （RPC=10000）, therefore SW2 Will 3、4 Port set AP mouth .