STACK SWITCHES: BASICS & SCENARIOS


TOPICS

BASICS

HARDWARE DETAILS

CONNECTION TYPES : THREE TYPES

COMMAND LINE WITH OUTPUT DETAILS

STACK MASTER ELECTION

CREATING SWITCH STACKS : TWO CASES

ADD / REMOVE SWITCH IN STACK : THREE CASES


 

BASICS

What is Switch Stack?

A switch stack is a set of Cisco Ether Switch service modules or Catalyst 3750 switches connected through their Cisco Stack Wise ports.

IDENTIFY MASTER

The master LED in the front panel of the 3750 switch turns green when the switch becomes master in the stack.

ROLE OF BRIDGE ID

A switch stack is identified in the network by its bridge ID and, if the switch stack operates as a Layer 3 device, then by its router MAC address.

Bridge ID & router MAC address is determined by the MAC address of the stack master.

All stack members are eligible stack masters.

The default stack member number of a 3750 switch is 1.

ADDING NEW SWITCH TO STACK

The default stack member number of a 3750 switch is 1. When it joins a switch stack, its default stack member number changes to the lowest available member number in the stack. Stack members in the same switch stack cannot have the same stack member number. Every stack member, which includes a standalone switch, retains its member number until you manually change the number or unless the number is already used by another member in the stack.

HOW TO CHANGE STACK MEMBER NUMBER

  1. Go to global configuration mode.
  2. Issue the command switch current-stack-member-number renumber new-stack-member-number .
  3. Return to the privileged EXEC mode and reload the member with the command reload slot current-stack-member-number .
  4. After the member boots up, issue the command show switch to verify the stack member number. If the number is used by another member in the stack, the switch selects the lowest available number in the stack.

MOVING SWITCH FROM ONE STACK TO ANOTHER STACK

If you move a stack member to a different switch stack, the stack member retains its number only if the number is not used by another member in the stack. If it is used by another member in the stack, the switch selects the lowest available number in the stack.

SETTING PRIORITY VALUE TO SWITCH

From the global configuration mode, issue the command switch stack-member-number priority new-priority-value .

From the bootloader switch: prompt, issue the command set SWITCH_PRIORITY new-priority-value

The priority value can be 1 to 15

If priority value is changed, it will take only when next stack-master election occurs.


 

HARDWARE DETAILS

cat3750-1.gif

No. Description
1 StackWise Ports
2 RJ-45 Console Port
3 Fan Exhaust
4 AC Power Connector
5 RPS Connector

 

CONNECTION TYPES

FULL BANDWIDTH

cat3750-2.gif

HALF BANDWIDTH

When a cable fails at a single place in a Full Bandwidth connection, the stack will run with the half bandwidth connection.

cat3750-3.gif

STACK PARTITION

When a cable fails at a single place in a Half Bandwidth connection, the stack will be partitioned into two stacks.

cat3750-4.gif


 

COMMAND LINE

3750-Stk#show switch stack-ports  

Switch #    Port 1       Port 2

——–    ——       ——

1          Ok           Ok

2          Ok           Ok

3          Ok          Ok

Comments: Port 1 & 2 represents the stack wise ports of switch. Ok shows ports are working fine.

 

3750-Stk# show switch neighbors  

Switch #    Port 1       Port 2

——–    ——       ——

1         2            3

2        1            3

3         2            1

Comments:

Above output explains how switches are connected in the switch or simply above output helps in understanding topology of switches connected in stack.

We will explain first line of output. Output shows port details of switch 1. On switch 1, port 1 is connected to switch 2, port 2 is connected to switch 3.

Similar to this, complete the stack topology from above output.

 

3750-Stk>show switch

Current

Switch# Role     Mac Address     Priority     State


1       Slave     0016.4748.dc80     1         Ready

*2       Master    0016.9d59.db00     5         Ready

Comments:

Output shows priority & MAC address of switches in stack.

Switch 2 is acting as Master. Highest priority switch becomes Master switch.

The default stack member number of a 3750 switch is 1


 

STACK MASTER ELECTION

The switch that is currently the stack master

Note: When stacks merge, the elected stack master would have been the master of one of the merged stacks.

Note: When stacks partition, the stack master of the original stack will be the master of its partition.

 

The switch with the highest stack member priority value

Note: Cisco recommends that you assign the highest priority value to the switch that you prefer to be the stack master. This ensures that the switch is re-elected as stack master if a re-election occurs.

 

The switch that uses the non-default interface-level configuration

 

The switch with the higher Hardware/Software priority. These switch software versions are listed from highest to lowest priority:

Cryptographic IP services image software

Non cryptographic IP services image software

Cryptographic IP base image software

Non cryptographic IP base image software

Note: Switches that run Cryptographic or IP services image will take a longer time to load than Non-Cryptographic or IP base image. When you power on or reset an entire switch stack, some stack members will not participate in the stack master election. This is because stack members that are powered on within the same 20-second time frame participate in the stack master election and have a chance to become the stack master. Stack members that are powered on after the 20-second time frame do not participate in this initial election and only become stack members. At times, switches with lower software priority can become the stack master, but all stack members will participate in the stack master re-election.

 

The switch with the longest system up-time

 

The switch with the lowest MAC address

 

When is the stack master elected?

  • When the whole switch stack is reset1
  • When the stack master is reset or powered off

Note: If you reset the stack master, it would reset the whole stack.

  • When the stack master is removed from the stack
  • When the stack master switch has failed
  • The switch stack membership is increased by if you add powered-on standalone switches or switch stacks

 

CREATING SWITCH STACKS

CASE 1 : Create a Switch Stack from Two Stand-alone Stacks

This example shows how a switch stack is created from two stand-alone stacks.

  1. Switch-A and Switch-B are merged together to form the stack.cat3750-5.gif

    and

    cat3750-9.gif

  2. A stack-master election occurs between Switch-A and Switch-B; let us consider that Switch-B wins.
  3. Switch-A will reload itself and join the switch stack.
  4. The stack member number of Switch-A will change since it conflicts with Switch-B. Switch-A will choose the lowest available stack number, which in this case, is number “2”.cat3750-10.gif

 

CASE 2: Create a Switch Stack from Two Two-member Stacks

This example shows how two switch stacks merge to form a stack.

  1. The first switch stack consists of two members: Switch-A and Switch-B.cat3750-10.gif

    The second switch stack consists of members Switch-C and Switch-D with Switch-C as the stack master.

    cat3750-6.gif

  2. When these two switch stacks are merged together, the election for stack master occurs; let us consider that Switch-B wins the election.
  3. Switch-A will retain its stack member number.
  4. Switch-C and Switch-D will reload themselves and join the stack with new stack member numbers of “3” and “4” respectively.cat3750-7.gif

ADD / REMOVE SWITCH IN STACK

CASE 1: Tips to Add a Switch as a Slave to the Stack

To add a switch, as a slave, to a stack, complete these steps:

Note: Make sure the switch that you add into the stack has the same IOS version as the switches in the stack. Refer to Catalyst 3750 Software Upgrade in a Stack Configuration with Use of the Command-Line Interface to upgrade the IOS in a catalyst 3750 switch.

  1. Change the switch priority of the switch to be added to “1”.switch stack-member-number priority new-priority-value Note: This step is optional, but it will make sure that the switch has fewer chances to become a stack master in the future.
  2. Power off the switch that is to be added.
  3. Make sure that the stack is fully connected so that, when you connect the new switch, the stack will be at least in half connectivity and does not partition.
  4. Connect the new switch to the stack with the Stack Wise ports.
  5. Power on the newly added switch.
  6. After the new switch comes up, issue the command show switch to verify stack membership.

 

CASE 2: Tips to Add a Switch as a Master to the Stack

To add a switch, as a master, to a stack, complete these steps:

Note: Make sure the switch you add into the stack has the same IOS version as the switches in the stack. Refer to Catalyst 3750 Software Upgrade in a Stack Configuration with Use of the Command-Line Interface to upgrade the IOS in a catalyst 3750 switch.

  1. Issue the show switch command.to get the priority value of the members of the stack.
    3750-Stk>show switch
                                                   Current
    Switch#  Role      Mac Address     Priority     State
    --------------------------------------------------------
     1       Slave     0016.4748.dc80     1         Ready
    *2       Master    0016.9d59.db00     5         Ready
  2. Change the priority value of the switch to be added to a value greater than the highest priority of the stack. In the illustration, the priority value has to be a value of more than “5.”.switch stack-member-number priority new-priority-value
  3. Make sure that the stack is fully connected so that, when you connect the new switch, the stack will be at least in half connectivity, and do not partition.
  4. With the new switch powered on, connect the StackWise ports of the switch to the stack.
  5. The election for the stack master occurs, and the new switch will be elected as the master since it has the highest priority value.
  6. The members of the previous stack will reboot themselves to join the new stack. After all the members come up, issue the command show switch to verify stack membership.

 

CASE 3: Remove a Member from the Stack

Complete these steps to remove a member from the stack:

  1. Make sure that the stack is fully connected so that, when you remove the member, the stack will be at least in half connectivity, and do not partition.
  2. Power off the member to be removed.
  3. If the member was the stack master, the stack master election will occur, otherwise no election will occur.
  4. Remove the Stack Wise cables from the member and close the stack ring.
  5. Issue the command show switch to verify the stack membership.

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3 comments

  1. Hi Sumit, your posts are very much informative, I usually don’t document any tasks I do, but this post you have given everything step by step. Thanks a lot and really thanks a ton for all other posts and suggest you to keep posting new scenarios. Hope you also post some firewall, load balancers posts as well.

    Like

    1. Hi Shivaprasad,
      Thanks a tonn for the complement.
      Two things always leads to Fruitful results.
      ” HARDWORK + KNOWLEDGE ACQUISITION ”
      So, study the concepts & save the knowledge in appropriate manner.
      Definitely, i will put more concepts & scenarios in very well categorization.
      Keep Networking .. !!

      Like

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