Cisco Stackwise

Lessons Discussion
41

Abhishek,
The short answer is that a switch stack will not change spanning tree as opposed to just a single switch.

From the perspective of the rest of the network, a stack of switches appears to be the same as a single switch. This is because a stack uses a single bridge ID, and appears as just one STP node.

42

Hi rene

With respect to stacking ,

Active and Master both are same terms ?

Thanks
Abhishek

43

“Master” is the preferred Cisco terminology, because both switches are Active in the sense they are processing traffic. “Master” would probably be less confusing than “Active.”

44

Hello ,

I have a doubt from config point of view of stack

  1. lets say we have 2 3850 switch that we are going to connect in a stack - do we need to configure both the switches with identical config OR just config one switch and other will automatically take the config?
  2. other than the stack cable between these 2 switch - what all we need to connect between them ? Ether-channel OR trunk ?
  3. whenever we config anything on master switch, same config will replicate on member as well ?

Thanks
Abhishek

45

Hello Abhishek.

For question 1:
When you stack switches, one of the switches is chosen as a Master. (Click this link to see how a Master is chosen and how you can affect which one will be chosen. The process is the same for the 3750 and 3850 switches: the http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/71925-cat3750-create-switch-stks.html#anc8). The configuration of the Master including its VLAN configuration will be replicated onto the other switches, making the stack function as one switch. So, any configuration that the other switches may have will be overwritten automatically.

For question 2:
Other than the stack cable you do not require any other connections between the switches. All data communication between switches occurs over the stack cables. Keep in mind that the stack cable goes from switch 1 to switch 2 to switch 3 to… switch N and one more cable goes from switch N to switch 1. (see the link above for related diagrams).

For question 3:
In a sense yes. It is more accurate to say that once the stack is created, you configure the stack as a whole. You no longer worry about the actual location of the config file(s). You just configure THE WHOLE STACK and it does the replication of configurations to all switches for you.

I hope this has been helpful.

Laz

1 Like
46

Yes that’s clears my doubt. Thanks a lot.

47

Hello Abhishek.

Glad I could be of help!

Laz

48

Hi Rene,

Are there multiple flavors of Stack. i.e Ring or chain topology ?

can you please tell what is the difference and which one is used mostly in today’s networks ?

Thanks
Abhishek

49

Hi team,

I am working on a stack design and need some help here. We have 4 switches that need to be part of stack . Hence we will configure first switch that we want to be master with high priority (default 15) and then will boot other switches as shown in this article.

My Q is :

  1. since other 3 switches also have their default priority as 15 how will they automatically become members ?
  2. If we want one of the switch to become hot standby what needs to be done?
  3. once master goes down then hot standby takes over , but if master comes back will it become master again ?

please suggest

Thanks
Abhishek

50

Hello Abhishek

Concerning your question about ring or chain topologies for stacks, Cisco Stackwise technology requires that all switches be connected in the following manner:

 --> S1 <--> S2 <--> S3 <--> ... --> SN <--
|                                          |
 -----------------------------------------

Where the last switch connects to the first… You can see this clearly in the attached image. This in essence is a ring. This is the ideal situation.

Now, if one of these links does go down, or if you choose not to connect one link (which is not recommended) then you would have a chain topology. According to Cisco, in this case, “A break in any one of the cables will result in the stack bandwidth being reduced to half of its full capacity.”

So yes, you can have a chain topology, however, this is not recommended.

I hope this has been helpful!

Laz

51

Hello again Abhishek.

Concerning your questions about the stack design, I will attempt you answer here below:

1. since other 3 switches also have their default priority as 15 how will they automatically become members ?

First of all, the default priority value for switches in a stack is 1. If all of the switches have a priority of 1, then the election takes place using the following criteria. In case of a tie, the next criterion is used:

  1. The switch with the highest priority value
  2. The switch that uses a non-default interface-level configuration.
  3. The switch with the higher Hardware/Software priority. This priority is based on the software version of IOS that is being run by the Switch. These switch software versions are listed from highest to lowest priority:
    a)Cryptographic IP services image software
    b)Noncryptographic IP services image software
    c)Cryptographic IP base image software
    d)Noncryptographic IP base image software
  4. The switch with the longest system up-time.
  5. The switch with the lowest MAC address."

To find out more, take a look at: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/71925-cat3750-create-switch-stks.html?referring_site=RE&pos=1&page=http://www.cisco.com/c/en/us/products/collateral/switches/catalyst-3750-series-switches/prod_white_paper09186a00801b096a.html#anc8

2. If we want one of the switch to become hot standby what needs to be done?

Stacking does not allow for the logic of a hot standby. If a master goes down, the very next switch in line becomes master based on the above criteria. If you want a specific switch to be “second in line” then just make it the second highest priority.

3. once master goes down then hot standby takes over , but if master comes back will it become master again ?

If the original master comes back up, it will not become the master again unless a new election takes place. An election is triggered only under the following circumstances:

- When the whole switch stack is reset
- When the stack master is reset or powered off
- When the stack master is removed from the stack
- When the current stack master switch has failed
- When the switch stack membership is increased by adding an additional switch

So, for question 3, it is not possible to have the original master come back up unless a new election is triggered.

I hope this has been helpful!

Laz

1 Like
52

19 posts were merged into an existing topic: Cisco Stackwise

53

Rene,

great video as always. my question is about fail over, and configuration push.

If you configure the master will the command copy over to the other switches in the stack; like vlan assignments and default routes?

also if you lose one switch due to power, those ports will still function as if they had power to that device?

how many switches can go offline before you lose the stack?

54

Hello Christopher.

When you have a stack configured, you must look at the stack as a single unit. You are not configuring the master, but you are configuring the stack as a whole. Each stack has only one configuration file, which is distributed to each member in the stack. This allows each switch in the stack to share the same network topology, MAC address, routing information, VLAN configurations… everything. In addition, this allows for any member to become the master, if the master ever fails.

If one switch is lost due to power, the rest of the switches will continue to function. However, the end devices served by the switch that lost power will no longer function. However, starting from the 3750 series onwards, the stacking technology includes what’s called StackPower, which provides power through the stack from one switch to another if a power supply fails, or if a switch is accidently unplugged.

As for how many switches must go down before you lose the stack, that depends on what you mean by “lose the stack”. If power goes off on some switches, the end devices on those switches will no longer be connected. Also, if the switches with the uplinks to the distribution layer go down, connectivity to the distribution layer will be interrupted. Also, if two switches lose power and they are not adjacent to each other on the stack, then the stack will split into two autonomous stacks. All of the above create some level of malfunction on the network, however, individual switches which still have power will continue to function even if all other switches have gone offline.

I hope this has been helpful!

Laz

1 Like
55

great thank you for your help and great explaination.

1 Like
56

Hello Rene,

How many switches we can connect in stack??
Also if we want to connect 3 switches in stack how to do so??

Regards,
Swapnil

57

Hi Swapnil,

This really depends on the Switch model. For example, I believe the 2960s can be stacked up to four switches, the 3750 up to nine.

Here’s a good example how to connect 3850 switches in a stack:

58

Hi Rene
I have cisco switch 4503 , I need to shut down power supply to check the redundancy performance, I have did access switch 3750x by command ( #power supply 1 slot a on/off) my question I need to do same idea in switch 4503 ,

Regards,

59

Hi Emad,

The power supply command is not available on the 4503, it’s one of the new commands for IOS XE.

On your 4503, I would make sure the power supplies are set to redundant (power redundancy-mode redundant) and see with the show power command if one power supply has enough power to provide everything. Then test it during a maintenance window.

Rene

60

Hi Rene,

let’s say I have a stack comprised of 5 switches, the top switch has uplink to distribution layer, and the bottom switch also has uplink to distribution layer and both of the links bundled to form etherchannel to the distribution layer, but when it comes to stack priority, top switch has the highest stack priority and its the master, while the 3rd switch in the middle of the stack has the second highest priority, what will happen it the top switch link got disconnected and the master switch went down? will the stack lose its connectivity to the distribution switches since the priority made the 3rd switch to become the master instead of the bottom switch?