Subject: RE: questions

From: "Peter Ashwood-Smith" <[email-masked]>
Date: Sat, 18 Mar 2000 00:30:59 -0600

>   in traditional router-based ip network, load
	> balancing is still available through metrics.
	>  the following is excerpted from mpls drafts
	> "Some degree of load balancing can be obtained by
	> adjusting the metrics associated with network links.
	> However,  there is a limit to how much can be
	> accomplished in this way, ...."
	>  can you tell me what the limitation is ?
	>  and why mpls doesn't have the limitation?

	Since I use driving analogies to explain routing behavior, here is
one that may help. 

	Imagine you are driving along and a see a sign post that says
"Adelaide 10 miles" on a right turn off your current route. Now imagine
another sign that says "Adelaide 5 miles" on a left turn off your current
route. You will turn left because its faster ... problem is, everybody else
will make the same decision and the 10 mile route will never get used. Now,
if you as a traffic engineer look at this problem and try to get people onto
the right turn by changing the sign to say 4 miles... well everybody will
now take It and nobody will take the previous turn. 

	Traffic engineered MPLS (i.e. CR-LDP or RSVP-TE) does not have this
limitation because the path for a set of related packets is predetermined
and does not have to follow the routes picked by intermediate nodes to its
destination. 

	You may find our Tutorial on MPLS available on the www.nanog.org
<http://www.nanog.org>  web site of some help as it explains this in quite
some detail. 

	Cheers,

	Peter