Dual 3750 Stack Configuration Question

[yemaya]

Hi Guys,

We need to implement 2 3750 stack switches for an emergency purposes, I've have never configured switches in stack before, i was reading the Cisco guide but i can't find what i need to know, those switches are new, never been configured, OK here is my question:

We will have one Master and one Slave, when i configure the switch do i have to do it in the Master switch only ??, meaning, i configure the Master and then the Slave will take that same configuration from the Master?, will the only difference be the Interface number EX: Master 1/0/1 - Slave 2/0/1, is this all correct?

Thanks in advance.

 

[ADB100]

When you connect the two switches together they negotiate who is the master and who is the slave. You can influence this by setting priorities assuming they are both powered on at the same time. However the master may change if they get power cycled out of sync.
With regards to the configuration when they are stacked they appear as a single entity with each switch having a number. The number stays the same whether it is master or slave. Ports are numbered switch/module/port - so for example the 1st Gigabit Ethernet port on the 1st switch would be GigabitEthernet 1/0/1 and the 1st on the 2nd switch would be GigabitEthernet 2/0/1. Configuration is replicated between the Master and the Slave(s).

HTH
Andy

 

[yemaya]

Hi Andy,

Thanks for your reply, really appreciate.

So it means that, i would have 2 routers but internally only one big switch with 96 ports and each port has nothing to do with the other, i can have GigabitEthernet 1/0/1 assigned to do one thing and GigabitEthernet 2/0/1 other right??.

Another question, i would have traffic in/out from different vlans, do i have to setup a trunk between then to let that traffic goes through??

Thanks.

 

[ADB100]

yep, its one logical device and does layer-2 and layer-3 forwarding - You could have a 96-port router if you wanted. Typically though you wouldn't have this many seperate IP interfaces and would maybe have only a few VLANs. Its up to you though you could have 96 IP interfaces?
You don't need any external cabling to route between the switches as the stackwise ports take care of this. Stackwise is effectively a 32GB backplane between the switches. If you are familiar with a chassis-based switch such as the 4500 or 6500 then a stack of 3750's is configured the same.

Andy

 

[yemaya]

Thanks Andy,

Got it, I'm familiar with the 6500 and 4500 switches but not with the stack switches tough.

Last question, with this configuration, both switches will share the configuration right?, if the master goes down, the slave will take the role and will transparently keep working without any downtime ??.

Thanks in advance.

 

[VinceWhirlwind]

The stack always has one master, so if one member of the stack goes down, and that was the master, a new master takes over, using the same config that is held on the whole stack.

As mentioned above, configuring a stack of 3750s is exactly the same as a 6500/4500, except you effectively have a supervisor module in each "blade" (ie stack member).

The only slightly different bit is when you have to update firmware - you can load it up to each member one by one, although depending on the firmware version change involved, this can be fully automated by the stack master.

Make sure you stack-cable them as per the install guide that is included in each switch's packing.

 

[ADB100]

One thing that has caught me out a couple of times is the MAC addresses used by the SVI interfaces. These are taken from the Master's pool of MAC addresses and by default if the master fails and one of the members takes the master role over all the MAC addresses change. Gratuitous ARPs are sent from the new master to attempt to get clients to update their ARP tables for the new MAC address of thier default-gateway (typically) but I have come across s few devices that this doesn't work on.
Cisco added a new feature 'Persistent MAC Address' that allows a stack to maintain the original masters MAC addresses (or delay the switching) should a new master take over.

http://www.cisco.com/en/US/docs/swi...se/configuration/guide/swstack.html#wp1206500

Andy

 

[ITGUY53]

Last question, with this configuration, both switches will share the configuration right?, if the master goes down, the slave will take the role and will transparently keep working without any downtime ??."

- you need to have the same IOS for all 3750 that you intend to stack. you DO NOT need to configure each 3750; just the master stack (aaa, vty, line con 0, etc, etc). if the master goes down such as where you only have one SFP installed and being used as an uplink (ex: Gig1/0/1), then all your stack will go down. unless you have more infrastructure as backup where you should put it on Gig2/0/1 for redundancy. we usually have 1 uplink for sometimes a 240 port 3750 that reduces over head with IP.

 

[VinceWhirlwind]

then all your stack will go down"

I know what you mean, but some might misunderstand that - the stack doesn't actually go down. If you only have one uplink from the stack, and the stack-member that holds that uplink goes down, then you lose your uplink.

Actually, one other thing I've seen with stacks is where the stack-master is the only member of the stack that has Advanced IP-services firmware. In that case the additional routing functions only work until that master goes down. People sometimes do this because they don't want to buy the licence for all the other stack members. It can be expensive, but it's a pretty silly decision on their part.