ASR 1006 Connecting to a 3850 stack - Avoiding the Single point of failure that is the switch stack.

[johnl3]

We ran into a software bug recently that caused the entire stack to fail - and we are looking at ways of removing the stack as a single point of failure.

Currently we have the following setup: (Port channel from the ASR to SW1, ASR port-channel subinterface is the gateway for VLAN 2)

              ____ASR____

                         |             

                      SW #1 

                         |         

               BLADE 1 (VLAN 2)       

               BLADE 2 (VLAN 2)

What I would Like, is this:

As far as I can tell, It's not possible, given the ASR is a router, and doesn't support vlan interfaces, just subinterfaces with encapsulation. (Ignore the fact that the ASR is also a single point of failure in this diagram)

              ____ASR____

              |                    |

          SW #1 ----- SW #2

              |                    |

         BLADE 1        BLADE 2

Am I missing anything? I guess I could move the gateway down to the switches, and use HSRP to achieve redundancy of the gateway, but then I'm stuck on how I can get redundant links from the switches into the ASR (since the ASR can't do spanning tree?)

Or maybe I should use mLACP? (not something we've done before)

Thank you!

[Abzal]

Hi,

As you said yourself you could make switches standalone trunk them to each other and do routing for your LAN. Then you can connect them to ASR over L3 link to route packets upstream. That is for me a better option from all other options.

[johnl3]

That kind of dramatically changes how we are set up - I kind of wanted to keep the 3850's at layer 2, if possible. 

[Joseph W. Doherty]

https://www.cisco.com/c/en/us/td/docs/routers/asr1000/configuration/guide/chassis/bdi.html
https://www.cisco.com/c/en/us/td/docs/routers/asr1000/configuration/guide/chassis/xe-16-6/asr1000-software-config-guide-16-6/bdi-asr.html

[johnl3]

I've never heard of a BDI...could you explain how it applies here? 

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@Joseph W. Doherty wrote:
https://www.cisco.com/c/en/us/td/docs/routers/asr1000/configuration/guide/chassis/bdi.html
https://www.cisco.com/c/en/us/td/docs/routers/asr1000/configuration/guide/chassis/xe-16-6/asr1000-software-config-guide-16-6/bdi-asr.html

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[Joseph W. Doherty]

You're looking for a way to attach the router to your L2 switches, correct? BDI might be the solution you seek (additional information on the approach might also be found under IRB [from earlier routers].)

[johnl3]

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@Joseph W. Doherty wrote:
You're looking for a way to attach the router to your L2 switches, correct? BDI might be the solution you seek (additional information on the approach might also be found under IRB [from earlier routers].)

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We are already connected to the single switch stack - I'm trying to connect to two switch stacks, while not creating a loop - will this accomplish that? The gateway for vlan 2 is a port-channel subinterface on the ASR (since it can't do regular l3 vlans like a switch) so I can't connect it to more than one switch. 

[Joseph W. Doherty]

Yes, I understand. BDI (or IRB) creates bridge ports on a router.

Have you read the links, at least their overview sections? If so, if you can be specific on what you don't understand, perhaps I can further explain.

[johnl3]

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@Joseph W. Doherty wrote:
Yes, I understand. BDI (or IRB) creates bridge ports on a router.

Have you read the links, at least their overview sections? If so, if you can be specific on what you don't understand, perhaps I can further explain.

---

I apologize - I was reading some outdated articles, and have configured this up in a lab - So instead of putting the layer 3 interface on the port channel sub-interfaces, I've creating BDI interfaces, and setting up the port channel as a service interface for the BDI. 

Are there any gotcha's on this scenario? Would there be any reason to use port channel subinterfaces vs BDI?

[Joseph W. Doherty]

Gotcha's? Not that I recall, but it's been a long, long time since I've used this technology.

If you can get your redundancy via a BDI port to each switch, that should replace the need you had for a port-channel to a stack (different stack members).

[johnl3]

Thank you! 

We will still need the port channel as we will be using switch stacks, and if a member fails, one port of a port channel being down is much less of an impact than having to have STP reconverge.

May ask if this technology has been replaced by something else? Or what you would use to do the same thing? (Just curious why you haven't used this technology in a long while)

Thanks!

[Joseph W. Doherty]

With L3 switches, I would do as Abzal suggested (which is why it's not a technology I've used in a long, long time).