The MPLS WG Archive

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

In message <200311172033.PAA18768@hank.bcentralhost.com>, mark seery writes:
> 
> 
> ---- Curtis Villamizar <curtis@workhorse.fictitious.org> wrote:
> > 
> > In message <3FB8F9A5.5010306@interflect.com>, mark seery writes:
> > > David Allan wrote:
> > > 
> > > >BTW, would be nice to get some other opinions, IMHO this is rather
> > > >fundamental....
> > > >
> > > One of the fundamental precepts of routing, is that each node 
> > > understands the routing decisions each other node makes, this is to 
> > > prevent black holing among other things. So for example, the SPF 
> > > algorithm is not a secret, precisely so that each router in a network 
> > > can make consistent decisions. As ECMP algorithms impact the forwarding 
> > > direction of packets, then I would suggest while not on the same level 
> > > of criticality as SPF, there is a similar logic that could be applied to 
> > > this case. I have similar thoughts about end-to-end QoS (but that's way 
> > > OT, just sited as another example).
> > 
> > That was never the case.
> > 
> > Each OSPF node did not have to know whether the downstream node did
> > ECMP and if it did whether it was a dumb per packet split or a hash
> > based, or something else.  It only had to know that the next node
> > downstream kept the packet going in the direction of lower OSPF cost.
>  
> Curtis, I respect your opinion and experience, but it seems to me you
> keep referring back to ECMP in a native IP environment where everyone
> is using the same hashing algorithm.

That is not true.  Early on Cisco used per packet load splitting and
did not use a hash based algorithm.  It is still not certain what hash
algorithm anyone is using for those that do a hash.  It also does not
matter whether the hash is based on src/dst only or also includes the
TCP port numbers for TCP.

Therefore it is not "one of the fundamental precepts of routing" that
each node understand the ECMP algorithms used by the other nodes.
Each router must behave within some constraints required to keep the
routing protocol loop free.

Curtis