Primary and backup link between different switches

[sebastien3]

Hello,

I would like to be able to use a primary link and a backup link on two switches (SW1 and SW3) that are not stacked.

Both switches have a LAG with a trunk between them. It has multiple VLANs.

If the primary link fails, traffic must be directed to the backup link.

What is the best way to do this ? The two switches (SW2 and SW4) are not stacked.

Thanks !

[Karsten Iwen]

This can be achieved with a Spanning-Tree configuration. By making SW2 the root (and probably SW4 the backup root), and also giving the SW1/SW3 link a better Cost as the link between SW3/SW4, the traffic will flow as expected.

[MHM Cisco World]

this case beyond the STP L2 why ?
if SW2 and SW4 have VLAN 10 and SW4 is root still 
the router here have two interface to VLAN 10 with same subnet and that not accept from router 
router must see each interface or subinterface with different subnet except case of Port-channel which router have two interface (same port channel) with same subnet. 
so story start from are SW2 and SW4 is L2 or L3?

[sebastien3]

I don't really like playing with STP root or secondary.

As there are many vlans, you have to remember to use the command each time spanning-tree vlan XXXX root primary

@MHM Cisco WorldYou are right !

Between switches SW2 and SW4 and the router there is another switch.

The switch (not shown) has a LAG with the router.

The switch is directly attached to SW2 and SW4.

I wish I could set up some sort of FlexLink on two remote devices

[Karsten Iwen]

You really should set the root in *every* network. And it can be done for all VLANs at once.

[MHM Cisco World]

so can draw last topology, it will help me.

[sebastien3]

@MHM Cisco World: here is the updated diagram with the whole network

[MHM Cisco World]

all SW are L2 not L3, and Inter-VLAN is done in Router ?

[sebastien3]

Yes, all switchs are L2.

Inter-VLAN is done in Router.

On SW1 and SW3 is possible to swap connections from one switch to another.

This is done if, for example, SW1 breaks down, you just have to connect the cables to SW3.

[MHM Cisco World]

Friend since we talk about L2 not L3 protocol, then I need to mention here, 
make not double check but 20 times check before apply any config. 
my comment is suggest to help you to decide the final solution You see it OK for You. 

now return to issue 
the two SW (2 & 4) must run stack or VSS or vPC, because Root SW connect via PO to two different SW (two different SW ID)
we need to make Root SW see both SW as ONE virtual SW and hence one virtual SW ID. 
other is simple we must make the backup link high cost than primary and make STP elect the port in SW4 as BLK 
and it will be change to forward in only one case which is primary is down and no BPDU from Root SW anymore.

[paul driver]

Hello
Most simplistic solution (if supported) -  append Flexlink on the sinlge switch that is connecting to the router and Sw2/Sw4 as the primary path with be SW2 uplink

Int x/x
Description link to Sw2
switchport backup interface yy (link to sw4) preemption mode forced
switchport backup interface yy  (link to sw4) preemption delay xx

or

int portchannel xx
switchport backup interface porthchannel yy  (link to sw4) preemption mode forced
switchport backup interface porthchannel yy (link to sw4) preemption delay xx

[MHM Cisco World]

yes but SW5 and SW6 connect to SW2 and SW4, so he need SW2-Root SW and SW4-Root SW be always FWD

[paul driver]

Hello

---

@MHM Cisco World wrote:

yes but SW5 and SW6 connect to SW2 and SW4, so he need SW2-Root SW and SW4-Root SW be always FWD

---

Makes no differance the path from sw4 to that single switch will NOT be active so the path has to go via sw2 uplink  even for sw5/6

[sebastien3]

---

@paul driver wrote:

Most simplistic solution (if supported) -  append Flexlink on the sinlge switch that is connecting to the router and Sw2/Sw4 as the primary path with be SW2 uplink

---

I had already thought of that but the problem is that it gives a SPOF... If the SW breaks down, there is no longer any way to carry traffic.