[c-nsp] Layer 2 redundancy

[Alban Dani]

---SW1----|---------------------------------|CORE1|
     |                Provider A               |
     |                                              | hsrp
     |                                              |
----SW2----|--------------------------------|CORE2|
                     ProviderB

In the above diagram CORE1 and CORE2 are configured as hsrp peers.
SW1 connects to CORE1 on a layer 2 connection via Provider A and SW2
connects layer2 to CORE2 via provider B.

There is no spanning tree running accross those links.
How can one can provide redundancy so that in case the link from Provider A
(considered primary) fails the traffic passes through the Provider B link.

Thank you,

Alban