CST public Spanning Tree

This experiment only discusses CST ( common spanning tree ) .

First, before the experiment, understand the 4 steps of Spanning tree decision:

Second, the experimental topology

1. Experiment Description:

Due to the requirements of the business, to have a reliable link to the link to achieve redundancy, but the link redundancy may bring the network broadcast storm , duplicate frames , MAC Address Table instability problem, Spanning Tree protocol The above three problems can be solved while redundancy can be achieved.

2. Lab 1: Validating CST (public spanning tree)

CST is a unique way for IEEE to process spanning trees on a virtual LAN, a VLAN solution called a single or public spanning tree. The spanning tree protocol runs on VLAN1, which is the default VLAN. All switches cite the same root bridge and establish a relationship with the root bridge

The CISCO switch enables the spanning tree spanning tree protocol by default, which enables the link redundancy capability without the need for configuration.

Experimental topology:

Experiment

Experimental topology Description:

In the above switched network topology only the VLAN1 that the switch exists by default, the switch by default all ports belong to VLAN1.

Based on the above topology, where only VLAN1 exists, we validate CST (public spanning tree)

It is recommended to manually configure the trunk mode of the SW1,SW2 F0/23,F0/24 interface

Configuration SW1:

SW1 (config) #int range fastethernet 0/23–24 ----- Note: Enter an interface range and configure multiple ports at the same time

SW1 (config-if-range) #switchport mode trunk

SW1 (Config-if-range) #no shutdown

Configuration SW2:

SW2 (config) #int range fastethernet 0/23-24

SW2 (config-if-range) #switchport mode trunk

SW2 (Config-if-range) #no shutdown

Verify the Spanning tree with show Spanning-tree on SW1 :

Sw1#show Spanning-tree

VLAN0001

Spanning Tree Enabled Protocol IEEE

Root ID Priority 32769

Address 0008.20ff.6400

This bridge is the root ------------------------ Note: SW1 is the root bridge

Hello Time 2 sec Max Age sec Forward Delay sec

------------- Note: The bridge ID of the root switch

Bridge ID Priority 32769 (priority 32768 Sys-id-ext 1)

Address 0008.20ff.6400

Hello Time 2 sec Max Age sec Forward Delay sec

Aging time------NOTE: SW1 Bridge ID

Interface Role Sts Cost PRIO.NBR Type

---------------- ---- --- --------- -------- --------------------------------

FA0/23 desg FWD 128.23 Peer

-------------------------------Note: DESG: Indicates F0/23 is the specified port

FWD indicates: The port's raw tree status

Represents F0/23 to the root bridge for forwarding status

The cost is 19 128.23: here 128 table

Port priority, 23 for port number,

This is not the first place to think about it.

FA0/24 desg FWD 128.24 Peer

<DESG Indicates the specified port >

Note: CST is a unique way for IEEE to process spanning trees on a virtual LAN, a VLAN solution called a single

Or a public spanning tree. The spanning tree protocol runs on VLAN1, which is the default VLAN.

Verify the Spanning tree with show Spanning-tree on SW2 :

sw2#Show Spanning-tree

VLAN0001

Spanning Tree Enabled Protocol IEEE

Root ID priority 32769-----------------------------Note: Bridge ID of the root switch

Address 0008.20ff.6400

Cost 19

Port (FASTETHERNET0/23)

Hello Time 2 sec Max Age sec Forward Delay sec

Bridge ID Priority 32769 (priority 32768 Sys-id-ext 1)

Address 000d.bce7.5940

Hello Time 2 sec Max Age sec Forward Delay sec

Aging Time-------------------SW2 's Bridge ID

Interface Role Sts Cost PRIO.NBR Type

---------------- ---- --- --------- -------- --------------------------------

FA0/23 Root FWD 128.23 Peer

FA0/24 altn BLK 128.24 Peer

Note: The F0/24 is ALTN (backup port), the spanning tree status of the port is Blk (blocking) state, root represents the root port, Altn represents

Backup port.

4 Steps to review spanning tree decisions:

The following results are obtained by show Spanning-tree on SW1,SW2.

SW1 (root switch)

F0/23-------------------specified port

F0/24-------------------specified port

SW2 (non-root switch)

F0/23-------------------Root Port

F0/24-------------------Blocking Port

Spanning Tree decision process :

• Select the root switch:

One root bridge per network (each with a single root bridge)

Election of the root bridge: Lowest bid (minimum bid)

SW1 's campaign root switch parameters:

Bridge id:priority 32768

MAC Address 0008.20ff.6400

SW2 's campaign root switch parameters:

Bridge id:priority 32768

MAC Address 000d.bce7.5940

Select the root switch first condition to see priority first and then look at the MAC address, the default SW1 and SW2 priority is 32768, pass

The root switch cannot be selected over the priority level, and the MAC address of the switch can be viewed through show version only with MAC address selection.

SW1 's MAC address 0008.20ff.6400 is smaller than the SW2 MAC address 000d.bce7.5940, the smaller the priority, so SW1 is the root

Switch

• Select the root port:

One root port per nonroot bridge (each of the non-root bridges has a root port)

Root port (RP): Lowest path cost to root bridge each of the non-root bridges has only one root port and the non-root bridge is required to reach the root bridge

The port that is the least expensive is the root port. (Can forward traffic)

Method of electing RP/DP: (rp-root port, dp-specified port)

1.Lowest RID (minimum rid) here is the bid for (Root bridge)

2.Lowest path cost to root bridge (minimum path overhead for reaching the root)

3.lowest Sender Bid (minimum send bid)

4.Lowest Sender Port ID This option is used when there are two direct connections between the two switches (e.g. in this experiment

The selection of the approved port will use this one)

SW1 is the root bridge and there is no root port because only the non-root bridge has a root port:

SW2 is a non-root bridge, the root port is definitely f0/23, one of the f0/24:

F0/23, f0/24 This two port to elect the root port condition: (The port that the non-root bridge needs the least cost to reach the root bridge is the root port)

F0/23 the overhead of reaching the root bridge (cost) is 19

F0/24 the overhead of reaching the root bridge (cost) is 19

Minimum overhead required to reach the root bridge via a non-root bridge This condition is not able to select the root port, only the 3rd condition of the spanning tree decision

Lowest sender bid (minimum send bid)

By lowest sender bid (minimum send bid) election, but f0/23,f0/24 are all two ports on SW2 (non-root bridge)

Lowest sender BID is:

Bridge id:priority 32768

MAC Address 000d.bce7.5940

So there is no way to select the root port by lowest sender, only to see the 4th condition of the spanning tree decision lowest sender Port

ID, which is used when there are two direct connections between the two switches (the smaller the port number, the higher the priority)

F0/23 is smaller than f0/24, so F0/23 is preferred. Finally, you can select the root port f0/23

The f0/24 on SW2 is blocking port.

Show Spanning-tree verification on SW2:

Sw2#show Spanning-tree

VLAN0001

Spanning Tree Enabled Protocol IEEE

Root ID Priority 32769

Address 0008.20ff.6400

Cost 19

Port (FASTETHERNET0/23)

Hello Time 2 sec Max Age sec Forward Delay sec

Bridge ID Priority 32769 (priority 32768 Sys-id-ext 1)

Address 000d.bce7.5940

Hello Time 2 sec Max Age sec Forward Delay sec

Aging Time 300

Interface Role Sts Cost PRIO.NBR Type

---------------- ---- --- --------- -------- --------------------------------

FA0/23 Root FWD 128.23 Peer--------------Note: F0/23 is the root port

FA0/24 altn BLK 128.24 Peer------Note: SW2 on F0/24 is the blocking port

Summarize:

See, all data traffic from SW2 to SW1 finally arrives through line point 1, SW2 f0/24 is in the backup state, and on SW2 show Spanning-tree can see F0/24 's port role as ALTN, which is line 2 as the backup link on line 1.

CST Disadvantage: Eventually there is a link is always in the state of backup, as in this experiment, line 2 in the state of backup, we imagine that if line 1 will never problem, if so, it seems that the existence of line 2 is redundant, so we have an idea is not able to two links are used up, For example, part of the traffic Line 1, another part of the traffic Line 2, in fact, it is possible, Cisco's PVST+ generation is the origin of this, we will introduce pvst+ in the next experiment.

CST public Spanning Tree