Subject: OSPF ECMP

From: Santosh Esale <[email-masked]>
Date: Thu, 6 Oct 2005 18:53:52 +0530

Hello Guys,

I have a question related to section 16.8 of RFC 2328.

16.8. Equal-cost multipath

The OSPF protocol maintains multiple equal-cost routes to all
destinations. This can be seen in the steps used above to
calculate the routing table, and in the definition of the
routing table structure.

Each one of the multiple routes will be of the same type
(intra-area, inter-area, type 1 external or type 2 external),
cost, and will have the same associated area. However, each
route may specify a separate next hop and Advertising router.

There is no requirement that a router running OSPF keep track of
all possible equal-cost routes to a destination. An
implementation may choose to keep only a fixed number of routes
to any given destination. This does not affect any of the
algorithms presented in this specification.

it doesn't specify clearly which areas nexthop should be preferred over the
other(like highest area id etc). As per section 16.4.1, intra-area path
using the non backbone is most preferred for external routes, as backbone
paths will be heavly loaded because of inter-area traffic.

So should the highest area ids nexthops should be preferred over lower area
ids(backbone 0) while calculating ECMPs to intra-area and inter-area routes.

Thanks in advance

Santosh

---

From: Acee Lindem <[email-masked]>
Date: Thu, 6 Oct 2005 10:41:50 -0400

Hi Santosh,

>paths will be heavly loaded because of inter-area traffic.
The assumption of the RFC is that a prefix will only be accessible in a 
single area. However,
there are situations where a prefix or host route is accessible from 
multiple areas. In fact,
I recall one implementation that would look for the a /32 corresponding 
to the router-id and,
if it existed, advertise it as a stub link in all attached areas.

>ids(backbone 0) while calculating ECMPs to intra-area and inter-area routes.
I know of at least one implementation that prefers the lowest area ID in 
this situation.

Hope this helps,
Acee