Redundant links between 3750x stacks at L3

[janit]

Hi, yeah, this is between switching or routing problem. Got two buildings connected with a very old fiber. We got a wireless bridge for failover redundancy. Basically availability of the link is critical and not the performance. Buildings very close to each other.

Two 3750x (soon 9300s) independent L3 stacks connected by a L3 link over fiber. How can I add Wireless Bridge to the mix to automatically fail over ? I could probably do STP between both redundancy links, but I think I would have to introduce L2 switches at each end in front of the stacks..

Both ends run VTP, which probably makes it a bit harder to configure. I've tried a lot to find a solution and there are lots of discussions but not what I'm looking for. My buddy suggested BFD but that makes the solution so convoluted. I'm thinking STP, just not sure how to implement it within a stack and add Layer 3 config on top. I'm not that good with these things and prefer simple even if not optimal solution.

Thank you guys

[balaji.bandi]

If your exiting Fibre link is Layer 3, i go with same method for the Wireless bridge with Layer 3 connection between site.

use any IGP peering like eigrp or OSPF  - play with cost to prefer fiber Link, and alternative path Wireless link.

Or use major traffic towards Fiber Link, some less important traffic subnet wireless link and fail over option.

[Joseph W. Doherty]

I'm thinking, I'm likely thinking, much like @balaji.bandi.

If you connect each side's wireless bridge to a 3750X stack member, would not the 3750Xs "see" a p2p L2 connection (i.e. much the same if you interconnected with copper/fiber Ethernet)?

If they do, than you can define the 3750 ports as non-switchports (i.e. routed ports) (like you do with the fiber link?).

If a Balaji also mentions, if you're routing over the fiber link, you should be able to also route over the wireless link, just give it less preference so it's only used for backup (unless its a pretty high speed wireless connection, i.e. about equal to your fiber connection [gig?]).

For redundancy, since you mention stacks, the wireless connection should NOT be made to the same stack member with the fiber connection.

If the wireless connection is much slower than the switch<>wireless-bridge connection, you may want to shape the 3750 ports to that speed.  This so, you can easily "see" congestion at the switch port; also provides the option to use QoS for congestion management.

One curious item, you mention running L3 between the stacks, but also VTP?  Is this correct?

If so, L2 trunk with SVI for L3?  However/whatever you're doing, good chance, if desired, you could do the same with the wireless link, but if you have redundant L2 paths, then yes, you'll need to consider something like a STP variant.

[janit]

Tx for replies gents. My goal is redundancy, not load balancing. Both stacks are totally separate with their own VTP domains. They are currently connected over L3 fiber link. We've added a wireless link purely for failover redundancy. The question is how do I add wireless bridge to stacks so it automatically fails over in case of fiber failure? I was thinking create L2 STP between stacks and leave current L3 config as is. Let's say fiber is one port-channel and wireless another interface. What's the way to combine them and control them with STP ? All I read and know about this shows multiple links to switches and routers but no point to point links. Seems simple and obvious but I just can't find any solutions. Really appreciate any ideas. 

[Joseph W. Doherty]

Unclear why you cannot use the wireless as a L3 path.

If your fiber is L3, using wireless as just L2 likely will make redundancy an issue.

[janit]

I was thinking I can use L2 for both fiber and wi-fi in order to implement STP between them. In short L3 would not be aware of underlying L2 config as L2 would take care of failover. But I am opened to any suggestions. OSPF the whole thing if possible on point to point multilink ? So yes I can use wireless as L3. I guess you have something in mind. Honestly I am yet to find any solution. And yes I am a beginner, and yes this is to be implemented on 24x7, close to 365 days infrastructure. Tx

[Joseph W. Doherty]

Good you mention being a beginner because "I was thinking I can use L2 for both fiber and wi-fi in order to implement STP between them.", why are you thinking that?

Generally when you can use wire-speed L3, which your 3750Xs support, you don't use infrastructure L2, if you can avoid doing so.

As you mention you have L3 between your two stacks, and you want redundancy between them, which can be provided by another L3 connection, again, unclear why you think you need L2 and STP.

BTW, L3 needs L2, but with p2p, you don't need to extend that particular L2 beyond the two L3 end-points.

Now you also mention, OSPF on multipoint.  Again, unclear where/why you see a need for that.

Yes, would could create a L2 segment, using both the wireless and fiber connections, have your two router's peer across that, use STP to break the loop (ideally on the slower path between the two stacks), but mostly all that accomplishes is save you two (/31) or four (/30) IPs for the wireless link, but then introduces the need for a L2 loop breaker, like STP, or a L2 loop preventer, like Etherchannel (which wants "like" bandwidth links [and if you were adding a second fiber link, would likely recommend that]).

What I have in mind, is just using the wireless "link" as another p2p L3 link, which in OSPF, would have a higher cost than your fiber link.  I.e. it would only carry traffic if the fiber link cannot.

The only other consideration, that might come up, between using L2 vs. L3, is how fast each would re-direct traffic if your fiber path cannot.  Using their default settings, unsure whether PVST or OSPF would be faster, as much depends on what the fault actually is, can impact how each deals with a particular kind of fault.  Of the two, normally one does not get involved in STP "tuning", beyond running a "rapid" STP variant.  With OSPF, there's lots of tuning options, including, possibly, using BFD, so sub-second failover can often be ALWAYs achieved.  (BTW, generally, STP, was not originally designed for really fast redundancy recovery time.  Unsure it can guarantee sub-second.  This is one reason why modern networks, generally recommend using L3.)

[janit]

Now this is a great info. Not sure why it's not easy to find it out there. The conversion speed is secondary in this case. Redundancy is paramount. So I can do this with OSPF only? Don't have the knowledge to play/test BFD etc. Again, this is practically 24x7 link. I like reliable and simple solutions. I will have to find an example of OSPF between two switches. Everything out there is about huge networks. Also have same hardware wireless link this time at L2. I guess I can kill that one with just STP. Still can't find example of point to point. It's always triangles or squares, meaning bunch of switches. Thx a bunch. 

[Joseph W. Doherty]

Again, thought you have a working, L3, OSPF, fiber connection between your switches, now.

If that the case?

If so, you should be able to bring on-line your second wireless path connection, just like it, including using OSPF.

If it's a L3 p2p, you'll need a /30 or possibly /31 address block.

If running OSPF, you'll need to insure, if you only want this path as backup, that it's OSPF link cost is higher than the fiber connection.  (Easy to miss this the need for doing this.)

That's all you should need to do.

Of course, I'm assuming you also know how to do the above.  If not, might be able to provide some guidance there, too.

[janit]

No OSPF yet. Have /29 subnet between stacks. Now will have to read up on OSPF and off we go. I hope I can setup IPs for Wi-Fi gear for management and monitoring. It looks like I have a solution. Will keep you posted if interested.

[balaji.bandi]

Make small diagram for reference and use any IGP (like Eigrp or OSPF)  play with cost that will be ok for you this will have dynamic path available quicker.

there are other methods IP SLA and tracking, but that will make more complicated.(in this case i will not recomended)

[Joseph W. Doherty]

For p2p you only need a /30 or with newer OSPF implementations, a /31.

Is OSPF running at all, or this is just a question how to join the two stacks?

IPv4 or v4 and v6?

Yes, it you want you can post on OSPF setup.  If you do, suggest new posting in routing.

If you're new to OSPF, lots of us can help you there, including with "how to" which, again, isn't always easy to understand or do until you've reached "critical mass".

BTW "Have /29 subnet between stacks." that was assuming multipoint, using both links between the two routers?  If so, also BTW, cannot have two routed interfaces in the same subnet, on same device.  Also, your /29 can be split into two /30s, which you'll likely need.

Another BTW, don't be shy mentioning your lack of networking knowledge or experience.  You might get an even better response rate, and most will take more time explaining how to do something.  Again, we've all been there too, at one time.

[Joseph W. Doherty]

Oh, for personal learning, you might sign up on Cisco's network academy, where you can obtain a copy of their Packet Tracer tool.  For basic stuff, you can do quite a bit with it.  For example, you might be able to pretty closely model what you want to do, and configure in the simulator first.  At least when you break networking in such a simulator, people don't come with tar and feathers and/or torches and pitchforks.  ; )