3750 stackwise redundancy?

[Oliver42]

Hi,

I made a simulation of the final network in PT and I would like to use two 3750 switches in stackwise configuration instead of just one like in my PT network.

The 3750s I'll be using are the ones at the top of the pyramid in my network. I would like to provide redundancy for all users except for the ones at the very top of the network, but I've never configured stack switches.

The single 3750 switch in my PT network has been configured like I usually do, but I'm wondering how I'm going to configure it if there are two switches in stack instead of one.

Thank you for your help! 

[Giuseppe Larosa · Accepted solution]

Hello @Oliver42 ,

>> but I'm wondering how I'm going to configure it if there are two switches in stack instead of one.

From outer world the two C3750 stack behaves like a single switch electing a master switch that will handle all L2, L3 control traffic like STP or a routing protocol like OSPF.

In real world what are really needed are the stack cables to create the stack. They form a ring each switch has two stack ports.

The rest of configuration tuning deals with selecting one switch as the master using switch priority.

And ensuring MAC address persistency when a new master is selected (to avoid MAC address changes on SVIs and so on)

The two switches will become switch member 1 and switch member 2 and their ports will be addressed as

gi1/0/x or gi2/0/y.

Also the association of a specific switch with a number can be specified using provisioning.

See the following documents

! creation and management of C3750 stacks

https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/71925-cat3750-create-switch-stks.html?dtid=osscdc000283

! configuration example of cross stack etherchannel , a bundle using links from different stack member switches

https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/69979-cross-stack-etherchannel.html?dtid=osscdc000283

Hope to help

Giuseppe

[Oliver42]

Thanks a lot for your help Giuseppe, I appreciate it. 

I used one stack cable so far just to see how the switches would react and it seems to work, but from what I've seen so far it looks like I have to use two cables and both stack ports? Port 1 of switch 1 to the port 1 of switch 2 and port 2 of switch 1 to port 2 of switch 2?

Also, is giving priority to the master switch absolutely necessary? The switch I wanted to be the master automatically became the master so I left everything intact. 

I'm not sure how to ensure MAC address persistency either. It's my first time stacking switches together.

I want to make redundancy between my etherchannels a priority too so I did a little research and I think I've found a solution, but I'm not sure it would work. Before I go ahead and try it, I'd love to get your input. If you're willing to help me then that would be amazing. 

Thanks again!

[Giuseppe Larosa]

Hello @Oliver42 ,

>> both stack ports? Port 1 of switch 1 to the port 1 of switch 2 and port 2 of switch 1 to port 2 of switch 2?

if you look at the pictures in the first link I have provided you will see that the best practice is to cross port 1 switch 1 to port 2 switch 2 and viceversa.

Using both cables provides double bandwidth between the two members and fault tolerance in case one stack cable fails so it is necessary for having a stable stack.

>> Also, is giving priority to the master switch absolutely necessary? 

No , but it is highly recommended and easy and it does not cause any issues

conf t

switch stack-member-number priority 15

This will provide consistency over time on master identity.

>> I'm not sure how to ensure MAC address persistency either. It's my first time stacking switches together.

There is a specific command for this .

see

If you enter 0 , the stack MAC address of the previous stack master is used until you enter the no stack-mac persistent timer command, which immediately changes the stack MAC address to that of the current stack master. If you do not enter the no stack-mac persistent timer command, the stack MAC address never changes.

stack-mac persistent  0

from

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/15-2_2_e/configuration/guide/b_1522e_consolidated_3750x_3560X_cg/b_1522e_consolidated_3750x_3560X_cg_chapter_0101000.html#task_1233369

Hope to help

Giuseppe

[Martin L]

I am sure You know that it is not possible to do stacking in PT or any other virtual lab; you just need real gear.

You do not have to configure 2nd switch; any new members will copy config from Master sw during bootup. you could set up new member switch with number and priority before adding it.  The most important thing is that current Master sw has priority 15 so that any new sw do not override current conifgs. You can check and set up number/priority in "switch mode", or from privileged mode.  By default every new sw is number 1 with priority 1 and has default sw config. New sw will change his number from 1 to next available when you add sw to current stack (all existing sw are up).

When you rack new sw, connect stack cables, power on. New sw will copy configs from Master. In case you are replacing old with new, you must boot Master 1st, wait 2-3 minutes, power on 2nd one, and so on. After Master is done booting and up, connect, check settings and paste your backed up config.

If you just moving switch stack (no new sw, no new config), you could power all of them on at the same time.

We had a few accidents during sw refresh when new switch did override current config and network was down (default sw config was applied).

You can find videos about process on youtube,

Regards, ML
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[Martin L]

Note: All switches in stack behave like 1 switch;  Stacking just adds more ports to your switch.  i.e. you have office expansion or adding more devices. you could replace 24-port with 48 port sw. but what if that is not enough?  you could add another switch and connect 2 with etherchannels.  but then you 2 independent switches, 2 configs, STP gets involved, more potential issues may arise. Traffic flow, bandwidth and through-output is affected.  i.e Traffic from PC in sw 2 goes via etherchannels to sw 1 to go to Main router or uplink switch to get to HQ or Internet. However, if they are stack-capable switches, you do add new sw to increase density of ports.  2 switches behave like 1 without STP or other control plane protocols between those 2. Speed of stacking cables is super fast; something like 300-450 Gbps depending on models

Regards, ML
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[Martin L]

Stack redundancy between etherchannels is not what you think (if I think what you think : ) 

1. Stack sw redundancy among switches is via stack cables going to stack ports (in back of sw).  2 stack cables create a ring, 1 stack cable gives you half-ring which does not provide redundancy.  if each switch in stack has 2 stack cables connectivity, you have redundancy; if just 1, you do not have redundancy

2. example: If you have stack of 2 switches in main comm room, MDF,  and another stack of 2 in IDF.  Because of Ethernet length limits for CAT5/6, it is not possible for PCs to reach Main comm room but it is possible with fiber links. You add another switch and connect PCs to it. this becomes IDF comm room.

You can connect those stacks with Fiber etherchannels. For redundancy, you must connect Sw #1 in MDF to Sw #1 in IDF; and Sw #2 in MDF connects to Sw 2 in IDF.  In case of failure of sw 1 in IDF Or failure of 1 of 2 fibers, your fiber etherchannels is still good, running on 1 fiber instead of 2. 

iF you connect 2 fibers from sw #1 to sw #1 in IDF only, if that sw #1 fails, you connectivity is gone.

In case of your PT topology shown above, redundancy between top 2 L3 switches is done with etherchannels. 

Regards, ML

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[Oliver42]

Thank you for your response.

If I understand correctly, I have to connect my two 3750s switches together with two stack cables in a cross pattern (sw 1 port to sw 2 port 2 and sw 1 port 2 to sw 2 port 1)? Or could I just connect the ports 1 to 1 and 2 to 2?

Also, if I want to use etherchannels between the switches, could I split one end of the etherchannels between the two 3750 switches? I would connect one ethernet cable to the master switch in the stack, the second cable to the other switch and connect both ends of those two cables to my 2950 switch. Would that work? 

[Martin L]

Yes, Stack cables will make a cross pattern (forming X) going from left port to right port. 

Yes, you can split Etherchannel connections between two 3750 switches as long as those 2 are in the same stack.

Regards, ML
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