From: Jian Gu (guxiaojian@gmail.com)
Date: Wed Jan 03 2007 - 14:20:51 ART
I guess it is easier to have a real scenario:
R1, R2, R3 and R4 are connected (clockwise) via Ethernet in a ring topology,
all links are in area 0 except the link between R1 and R4 which is in area
1, link cost between R1 and R2 is 100, link cost between R2 and R3 is 5,
link cost between R1-R4 and R3-R4 are both 10. R1 has a static route
1.1.1.0/24 pointing to null0, redistribute this static route to OSPF with
metric 10 and metric type E1.
Question: what is the next hop for 1.1.1.0/24 on R2 and R3?
According to RFC, on R2 the next hop will be R3, and on R3 the next hop will
be R4, because the link cost to ASBR(R1) is lower if the traffic is going
clockwise than conter clockwise. But Cisco routers choose the higher cost
path.
Is this a bug?
On 1/2/07, Jian Gu <guxiaojian@gmail.com> wrote:
>
>
> Actually it is not that straightforward. RFC says intra area route is
> preferred when intra-area path is using non-backbone area, what happens if
> the intra-area is backbone area?
>
> Quote
> o Intra-area paths using non-backbone areas are always the
> most preferred.
>
> o The other paths, intra-area backbone paths and inter-
> area paths, are of equal preference.
> End Quote
>
> Jian
>
> On 1/2/07, Gavin Lawson <GavinL@titan.net.au> wrote:
> >
> > OSPF will select routes in the following order.
> > . Intra area route.
> > . Inter area route.
> > . External route.
> >
> > The cost is look at after the area identifier.
> > So if an external route has a lower cost than an intra area route - the
> > intra area route will still be selected.
> >
> > GL
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com ] On Behalf Of
> > Jian Gu
> > Sent: Wednesday, 3 January 2007 11:33 AM
> > To: Cisco certification
> > Subject: OSPF path selection
> >
> > Hi, all,
> >
> > If a router receives an external route with type 1 metric coming from
> > intra-area (higher cost) and inter area lower cost from the same
> > originator, how OSPF selects the routes? I am having hard time to grasp
> > how RFC says in this scenario.
> >
> > ********* RFC2328 ***************
> >
> > 16.4.1. External path preferences
> >
> > When multiple intra-AS paths are available to
> > ASBRs/forwarding addresses, the following rules indicate
> > which paths are preferred. These rules apply when the same
> > ASBR is reachable through multiple areas, or when trying to
> > decide which of several AS-external-LSAs should be
> > preferred. In the former case the paths all terminate at the
> > same ASBR, while in the latter the paths terminate at
> > separate ASBRs/forwarding addresses. In either case, each
> > path is represented by a separate routing table entry as
> > defined in Section 11.
> >
> > This section only applies when RFC1583Compatibility is set
> > to "disabled".
> >
> > The path preference rules, stated from highest to lowest
> > preference, are as follows. Note that as a result of these
> > rules, there may still be multiple paths of the highest
> > preference. In this case, the path to use must be determined
> > based on cost, as described in Section 16.4.
> >
> > o Intra-area paths using non-backbone areas are always the
> >
> > most preferred.
> >
> > o The other paths, intra-area backbone paths and inter-
> > area paths, are of equal preference.
> >
> > ************************************************************************
> >
> > ********
> > I don't quite understand what second bullet means, does it sugguest that
> > OSPF will loadbalance between intra-area backbone paths and inter-area
> > paths?
> > Or it is totally implementation dependent?
> >
> > Thanks a lot,
> > Jian
> >
> > _______________________________________________________________________
> > Subscription information may be found at:
> > http://www.groupstudy.com/list/CCIELab.html
This archive was generated by hypermail 2.1.4 : Thu Feb 08 2007 - 23:46:55 ART