The MPLS WG Archive

[Curtis Villamizar <curtis@workhorse.fictitious.org>]

In message <39469E08BD83D411A3D900204840EC55FB70F9@vie-msgusr-01.dc.fore.com>, 
"Naidu, Venkata" writes:
> Peter,
> 
> -> Forgive me for being simple-minded:
> 
>   Forgive me for such a general explanation.
> 
> -> My question is: if I use RSVP-TE to be guaranteed that 10 
> -> Mb/s of bandwidth is reserved for my aggregate traffic, 
> -> would I ever want to use ECMP splits in the end-to-end path?
> 
>   Not required. But if ECMP is enabled you never know.
> 
> -> I understand that that is theoretically possible within 
> -> agreed-upon limitations. But are these limitations narrow 
> -> enough to make this a realistic scenario. Or do I just have 
> -> to reserve 10 Mb/s along each of the multiple paths?
> 
>   No. No need to reserve 10 Mb/s along each of the multiple path.
> 
>   Some ECMP models are control plane directed with a prior
>   knowledge of far away hops and current load on bottle-neck 
>   links. If, for example, there are two different paths to
>   reach the destination. Each of those paths have different
>   bottle-neck links and the demand for some links are high
>   along a path then ECMP can split traffic proportionately.
> 
>   In such a case reserving 10 Mb/s is not advised. Reserving
>   10Mb/s on every multiple path is an overkill. Some vendors
>   can do so for resiliency. I never heard of such.
> 
> Venkata.
>

If you create on LSP and want 10 mb/s then reserve 10 mb/s.  If it
goes over a hierarchical link that splits in two that logical link is
credited with another 10 mb/s and the two logical links should get 5
mb/s each.  A realy smart implementation might even tweak the number
of hash buckets on either side of the split if the split was uneven
(if for example you tossed a few PW LSPs inside an LSP with PW and
IP).

If you put one PW LSP over a hierarchical link that splits in two (and
nothing else so you can't possibly load split) you can't split the
traffic so all the traffic goes one way.  Its your network, why did
you configure it this way?  Can you fix it - sure - just remove one
component of the hierarchical link.

Note that for this to happen (a split in the middle of an LSP) the
operator had to configure two LSPs between the same pair of nodes and
then define a hierarchical link (or run LDP over it).  An explicit
configuration was needed to make this possible.  If you don't
configure hierarchical links, RSVP/TE links will never split in the
middle of an LSP.  If you never configure two LSP from the same
ingress to the same egress and run LDP over the this pair of nodes and
you disable LDP ECMP, LDP (over RSVP/TE or not) will also never split
traffic (although reservations and LDP don't go together).

I'm sorry if this is degenerating into a tutorial by examples.

Curtis