Networking VLAN traffic query

Banzai_Joe · 15 Nov 2022 at 08:13

Hi all,
so our network of 16 Aruba switches is mostly a physical ring topology (so bi-directional). Some switches do have uplinks to other switches within the ring, so kind of giving a mesh topology for those portions of the network.
I recently had an issue whereby a couple of devices on a specific VLAN weren't grabbing an IP address from the DHCP server on our UTM.
I eventually tracked it down to one particular switch not having the relevant ports tagged for that particular VLAN.
Here's my query; why would the traffic NOT go the other way to its destination? once it hit a brick wall in one direction, why not go the other way round to get to the UTM?

That's it really?

any advice would be appreciated, thanks.

Banzai_Joe · 15 Nov 2022 at 08:25

ChrisD. said:
Most likely spanning tree, you shouldn't be deliberately creating a loop with switches. Why are they configured in a ring?

It's a football stadium, so imagine looking from a birdseye view, there are switches dotted around the perimeter. Its a fibre backbone but physically its more of a ring topology. the mesh topology is present where the main comms room is, purely because there are more switches within a smaller area so its easier to mesh them up if you get my drift.
STP is on by default.

Banzai_Joe · 15 Nov 2022 at 08:28

can i post a pic from onedrive in here (not posted a pic for years). If so, i'll link a diagram to what we have.

Banzai_Joe · 15 Nov 2022 at 08:31

ChrisD. said:
I don't admit to know much about networks in stadiums but similar principles to regular buildings should apply. Core, distribution, and access is still prevalent here as it's not a datacenter topology.

Lol, i really ought to finish my CCNA, but i've lost half of the info i've learned from simply not having to apply it.....until now.

Banzai_Joe · 15 Nov 2022 at 08:48

Network map

I inherited this, but have made changes as budget allows.

@Caged - most of our switches are 8-port with 2 x 1gb SFP's, would you mind expanding on what you said about the 2nd fibre circuit going in the opposite direction please?
I guess i assumed that a single fibre ring would allow traffic to go in both directions.

Banzai_Joe · 15 Nov 2022 at 08:55

Sinbad2000 said:
Having a ring is fine. STP will ensure all the switches are always reachable. However, if a vlan isn't trunked across an active link you'll never get that traffic through.

yea, it was just this one vlan wasn't tagged on the uplink for this one switch.

Banzai_Joe · 15 Nov 2022 at 08:57

If you guys look at my network map, you'll see that prior to putting the fibre in the copper network was a physical bus. Cab J was the end of the run (until the fibre created the ring)

Banzai_Joe · 15 Nov 2022 at 09:05

Caged said:
The reason why you saw the specific behaviour that you had is likely because you are using STP instead of PVST, but I wouldn't bother enabling that. Rather than chaining switches through each other, ideally you'd have a core and then all your access switches connected back to this core - you can use fibre and have one link go clockwise around the stadium and the other go anticlockwise if you want. Then the two fibre uplinks on each access switch are in a link aggregate and your core is either a stack or you're using switches that can do MLAG.

Having to diagnose every possible switch on the path is making things more complicated than they need to be. You also massively increase the bandwidth available if each switch has 20Gbit (for example) back to the core, rather than everything sitting on a single 10Gb link as it works it way back to the comms room.

Budget just isnt there i'm afraid. That said, cabs B, F-J are at the tea bars and are only used at home matches, so traffic throughput will never really be an issue.

Banzai_Joe · 21 Nov 2022 at 09:41

Caged said:
The reason why you saw the specific behaviour that you had is likely because you are using STP instead of PVST, but I wouldn't bother enabling that. Rather than chaining switches through each other, ideally you'd have a core and then all your access switches connected back to this core - you can use fibre and have one link go clockwise around the stadium and the other go anticlockwise if you want. Then the two fibre uplinks on each access switch are in a link aggregate and your core is either a stack or you're using switches that can do MLAG.

Having to diagnose every possible switch on the path is making things more complicated than they need to be. You also massively increase the bandwidth available if each switch has 20Gbit (for example) back to the core, rather than everything sitting on a single 10Gb link as it works it way back to the comms room.

Hi mate, been thinking on this. So currently I have a single OM4 cable looping in and out of each switch (daisy-chained). Can i use the same cable but a separate fibre pair to achieve the bi-directional circuit as you suggested but without the need for a 2nd cable.
I guess we'd still have a SPoF as we only have one cable (and rats do live here) but would it alleviate the issues if one switch were down as the traffic would simply try going the other way?
Advice appreciated (as always).