C3850 Stack Config and Questions

[P Y]

Firtsly, thanks for reading and putting up with my basic questions. 

I have two new Catalyst 3850 switches that i am installing in a stack as my new Core Switch. I have the switches wired up and stacked and the proper master picked and all switches labeled properly. My question regards how the SVIs are configured to work in the stack.It is my understanding and i see that once the switches are stacked they are a single logical unit. This means that if i add an interface for intervlan routing, it becomes active for all switches in the stack.

Question 1. So then i do not need to worry about HSRP?

I currently only have one available connection as my default route for the core switch's next hop. I am asking our provider to provide another so that i have a dual path.

Question 2. Would i then create a port channel on the new stack and assign it to two interfaces (one on each physical switch) so that i have two paths in case of failure. 

Question 3: Would the opposite end (Cisco 2811 router) also need to be configured in an ether-channel?

For my access switches, i usually have one connection back to the core.Now that i have two switches i would like to add connections here as well (one to each core switch.)

Question 4: would i create a port channel on the new stack and assign it to the two trunk interfaces, one on each physical switch

Question 5: would the access layer switches also need to be configured with a port channel, or are standard trunk ports OK?

Last section... I swear. In the event of failure, i know that the stack will elect a new master and  proceed to function. I can understand that if the physical switch is broken, the other switch will be the master and continue to work without issue. 

Question 6: However unlikely, what happens if both stacking cables break. Meaning that both switches are still functioning, but cannot contact the other. They will both "wake up" as the Master switch and could, in  my mind, start causing loops and problems. Is this correct?Am i totally incorrect?

Thank you!

[Bilal Nawaz · Accepted solution]

Q1) no need for HSRP, remember its single logical switch. If we were talking about two switch stacks, then you could have HSRP between the 2 stacks of 3850's.

Q2) Yes this is generally the case, to provide end hosts or network devices/security appliances with x2 links, one from each switch, which can aggregate bandwidth as well as provide chassis redundancy.

Q3) Yes indeed, most appropriate would be to have x2 interfaces on both sides, and use etherchanneling between the devices.

Q4) here is an example for you.

Switch

interface gi1/0/1

channel-group 1 mode active

interface gi2/0/1

channel-group 1 mode active

interface port-channel 1

####CONFIG FOR PORTS GOES HERE###

so basically put two physical NICs in to a port-channel, then configure the port-channel to be a trunk or access port etc...

Q5) Access switch's if you want to provide resilience it is best to port-channel them. If its non critical and you don't mind access switch's becoming offline due to some failure, then sure, single link is ok

Q6) if the stack cables break then yeah i guess they could end up active / active or split brain. But the cables are designed so that they do not fail, you must screw them in!

Hope it helps

Bilal

[Madhukrishnan Gopinathan Nair]

Hello Patrick,

Here are my answers.

1. No. 

2.When you say another from provider. are you saying you are going to get one more router or another connection teminating on your existing router. In case of only 1 router, then you can think of creating a port-channel for redundancy purpose if the router allows it. If you are getting another router, connect it to a port on second switch as a back up, so that incase of failure it takes over.

3. If you want to add redundant path over same network to router yes, and i am not sure the router supports the config, may be an ethernet card is needed for that, but i leave it for others to comment. If you can have 2 different subnets for 2 connections between router and switch, then use 1 port member switch 1 and for the other connection use ports switch 2. However i leave it for more routing experts also to comment.

4. yes

5. yes, they need to be port-channel as well.

6. Not tested it before, but i guess you may experience some problems. But it has very rare chance of failing both stack cables together :)

Hope this helps and as always kindly rate the post if it is useful for you.

Thanks,

Madhu.