stack behaviour of 3750 switch

[mahesh18]

Hi Everyone,

I configured 3750 A  switch with vlan 20 and its IP address 192.168.20.41

Its default gateway was 192.168.20.3

Then i configured 3750 B  switch with same default gateway and vlan 20 IP   192.168.20.43

After doing this i stack them together.

sh switch

Switch/Stack Mac Address : 0011.bb5e.a080

                                           H/W   Current

Switch#  Role   Mac Address     Priority Version  State

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

*1       Master 0011.bb5e.a080     4      0       Ready

2       Member 0011.bb5e.e200     2      0       Ready

My question is now when we stack it becomes single switch and now vlan 20 ip address is 192.168.20.43 thats only IP i can see.

So how does stack switch choose vlan 20 IP?

Does it choose highest IP address between two switches if they have same vlan 20 as in my case?

Also when i go to switch 3750 b by session command and do sh ip route  it does not show ip default gateway .

Also it shows vlan 20 as admin down .

So is above 2 things is default behaviour?

Thanks

mahesh

[Abzal · Accepted solution]

Hi,

How is the stack master elected?

These rules have been defined to determine which unit within a stack  is chosen as the master. When switches are added or stacks are merged,  the master will be chosen based on these rules, in the order specified:

1. 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.

2. 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.

http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_example09186a00807811ad.shtml#election

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, and routing  information. In  addition, it allows for any member to become the master,  if the master  ever fails.

The configuration manager is a Cisco IOS control program that runs on  the stack master. The configuration manager performs these functions:

Maintains  the running and saved configuration files and distributes them from the  stack master to the member switches. The complete stack entity has one  saved and one running configuration. All the switches in the stack use  the stack-master running configuration.

I think vlan 20 ip address 192.168.20.43 belongs to master switch because it has higher priority than second one.

About sh ip route and Vlab20 down state, I think because they were configured like this on master before you stacked it. And master distributed it to other stack members. But you easliy change it by setting higher priority.

From the global configuration mode, issue the command

switch stack-member-number priority new-priority-value

Hope it will help.

[Reza Sharifi · Accepted solution]

Hi Mahesh,

When you give switches individual IP address and than stack them, the master switch will take over the entire stack and will override the config on the members  So, that is why you only see one management IP address.  As you know the entire stack acts as one switch now.  As long as the vlan 20 in in up and up mode on the master, you are good to go.

HTH

Reza

[mahesh18]

Hi Reza,

When i removed switches from stack then other switch with vlan 20 IP 192.168.20.41 had same IP as master .43.

You are right master switch overwrites the config of memebers.

Now i can understand better as whats going on in stack config.

Many thanks for your great reply

regards

mahesh

[Reza Sharifi · Accepted solution]

Hi Mahesh,

You are very well come. Happy to provide the info you need.  Just remember, when switches are stacked, the entire stack acts as one switch and whatever config you apply to the master, it will get synced to all the members.  So, there is no need to touch the members. Of course, this is all provided the stack is functioning correctly.

Hope some of the snow in your area is melted by now.

Reza

[mahesh18]

Hi Reza,

Snow is melted and weather is good currently.

Regards

MAhesh