[c-nsp] QoS between 2 routers & many switches

[Tassos Chatzithomaoglou]

Suppose we have the following network:


     100       100        10       100       100
R1 <===> SW1 <===> SW2 <===> SW3 <===> SW4 <===> R2


All switches are just L2 devices and all connections are just plain ethernet.
As you can see there is a 10 Mbps link between SW2 and SW3, while all other links are 100 Mbps.
I need to give priority (& maybe reserve bw) to voice traffic between R1 and R2.

Does it make sense to try to use QoS (LLQ/CBWFQ) on the routers only? If yes (that would be the best for me), how can the routers 
"understand" the actual congestion between SW2/SW3, when all intermediate switches use their own (default ?) buffering/queuing 
mechanisms?
Will there be any difference on the "congestion feedback" that routers get, if instead of 4 switches, i have 8 switches?

On the other hand:
Should i try to implement QoS on the SW2/SW3 switches only?
Or even better, do i need to implement QoS at both routers & switches?

If the SW2/SW3 connection is EoMPLS, does it make any difference? Do i need some kind of QoS on the underlying MPLS network too?

-- 
Tassos