Spanning-Tree BLK Route

[CobbyJ]

Hello.

We have 2 Layer 3 switches and multiple Layer 2 switches we want our layer 2 switches to connect to both for redundacny but only choose the primary route unless it goes down in which case it will take the secondary route. i have tried with spanning-tree on the vlans but didnt seem to work or blk the secondary route, i tried changing the costs and other things but didnt seem to work.

They are already set to rapid-pvst or pvst

Can anyone recommend what i should be using, i did look at port priority but didnt seem to work.

Kind Regards,

Chris

[pieterh]

>>> but didnt seem to work or blk the secondary route <<<

BLK is a normal status in spanning-tree it means spanning-tree works
and YES this secondary route shows status BLK on one side of the link

with traditional spanning-tree STP one side of the link is in BLK status
with rapid spanning tree RSTP+ it is Altn BLK status and takes less time to to switch to forwarding
with per vlan spanning-tree PVST+ this occurs per each vlan

if you have
- two switches (A and B) interconnected
- and each connected to the root bridge (C)
then
- C to A shows as Desg FWD
- C to B shows as Desg FWD
- A to C shows as Root FWD
- B to C shows as Root FWD

- A to B shows as Desg FWD
- B to A shows as Altn BLK (or A <-> B other way around)

[Martin L]

STP goal is to eliminate L2 loops by blocking redundant links. however, if current link fails, blocking link start forwarding traffic. To implement redundancy on L2 switch domain ,you can do etherchannels (2 links between a and b). if one of etherchannel links fails, the other one still forwards. 

Better solution is implement HRSP at L3 switches (look for term NHRP redundancy, which you might already have.   Do you have such set up at L3 switch already?  STP will still block redundant links but with such setup you can divide STP domain into groups in order to better utilize links (L3 sw is mater for Vlans a,b,c,d while standby for vlans e,f,g,h. L3 sw does opposite.

There are examples of this in old CCNP Switch book exam (CCNP R&S cert, not sure about new 2020 cert).

Regards, ML
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[Richard Burts]

I am a bit puzzled about some aspects of the original post. It seems that the main point in the original post is about clients on layer 2 switches using the primary route on the layer 3 switches and not using the non-primary route unless the primary route is down. Routing is a layer 3 function and choosing which route to use would be made by the layer 3 switches and not by the client. 

The original post talks about trying to use spanning tree to enforce the route selection. And I am really puzzled about that. How is spanning tree (which operates at layer 2) going to enforce use of the primary route (which operates at layer 3)?

We do not know how these layer 3 switches are configured and that makes it difficult to give good advice about the issue. Do these layer 3 switches use HSRP? Do these layer 3 switches use any dynamic routing protocol? Or do they use static routing? For the clients on the layer 2 switches what is their default gateway?

Without knowing the answers to those questions I am going to make some assumptions and make a guess at what the issue is. I am assuming that the switches do not use HSRP. I am guessing that each switch has a route to whatever destination the original poster is interested in (perhaps it is a default route or all destinations). And I am guessing that the "backup" switch does not know that the "primary" switch has a route which should be preferred. And I am guessing that at least some clients on layer 2 switches have a default gateway that is on the "backup" switch. In this case those clients would send their outbound traffic to the backup switch and that switch uses its route to forward traffic to the destination and does not use the preferred primary route.