Yufei,
This is just a small excerpt from your reply.
In message <E2D27064CD59574F88D05AEF5728396D26D846@PH01SRV02.photuris.com>, Yuf
ei Wang writes:
>
> For your specific example, the traffic split between B-D and B-C-D due
> to limited capacity of B-D is not a "loopy routing" at all. It is a
> reasonable thing to do, and it can well be reproduced as shortest path
> routing. As a matter of fact, if we simply set weight of 2 for link
> B-D and weight of 1 for both link B-C and link C-D, then both B-D and
> B-C-D are always on the shortest paths regardless of weights of other
> links and traffic. At point B the traffic destined to D,E,F will be
> split in proportion to the capacities of B-D and B-C-D. In other
> words, if B-D and B-C-D have equal capacity, then traffic will be
> split 50%-50%. If the capacity ratio is 1 to 3, the traffic will be
> split according to the ratio of 1 to 3 as well.
OSPF does not have any concept of "capacity ratio" and therefore no
means to split the traffic 1 to 3.
MPLS can put the A-E traffic on one leg and the A-F traffic on the
other leg but cannot accurately balance the load. The only technique
so far that can accurately balance the load is OMP. With lots of MPLS
LSP, the CR based MPLS/TE load balance is generally not as good but
"good enough". With OSPF and ECMP metrics can be set so that load
balance is not as good as MPLS/TE. Some consider the ability of the
IGP to load balance to be good enough. Others don't consider it good
enough and consider IGP load balance too hard to manage and that was a
principle motivation for MPLS/TE.
With IGP load balance there can also be extremely poor load balance
after a failure in the topology until all of the costs are changed.
Curtis
> > Many simple topologies cannot be optimized by setting IGP metrics but
> > can easily be optimized using MPLS. For example:
> >
> > C E
> > / \ /
> > A---B---D--F
> >
> > The largest traffic contributors are A-E and A-F. The bottlenecks are
> > B-C or B-D where the available capacity of B-C and B-D are unequal and
> > the flows A-E and A-F are unequal. It is easy to pick values that
> > where neither ECMP or putting all of the flows on one leg would do as
> > well as a traffic engineered solution.
This archive was generated by hypermail 2b29 : Mon Aug 04 2003 - 04:10:03 EDT