From: Mas Kato (tealp729@xxxxxxxx)
Date: Sun Jul 01 2001 - 18:02:54 GMT-3
<<so which one does have more priority ?
"Use the ABR closest to the destination of traffic" or
"total ospf cost to destination">>
It depends on what kind of route you're trying to determine. Intra-area
routes are most preferred and total path cost is used. Inter-area routes
are preferred after intra-area routes but before external routes and
simple hop count is used.
In the example given, traffic needs to leave area 2 and make its way to
area 0, so it needs to find the best inter-area route. Based on your
observations, the R3-R1 link must be perceived as fewer hops to area 0
than what the virtual-link to area 0 presents.
Where is the virtual-link terminated in area 2? This would determine
which router becomes the ABR virtually connected to area 0. If the
v-link terminates on R2, then R3 would be a closer area egress point
than R2 for traffic coming from R4. Once the traffic makes it into area
0, then it would need to follow the best intra-area route (lowest cost).
If you "stretched" area 0 over to R3 or R4, then I would suspect that
the intra-area decision would come sooner for the traffic coming from R4
and your cost influence would come into effect.
If the v-link is indeed terminated at R2, try moving it to R3 and then
to R4 (all part of area 2, right?) and see if and how the behavior
changes.
So much for the theory...please let us know the results of your testing
if you try it.
Regards,
Mas
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
Wei Man
Sent: Sunday, July 01, 2001 6:31 AM
To: Ravi; ccielab@groupstudy.com
Subject: Re: OSPF Concept Question
>From my Lab, ospf cost R3 back to back connection with
R1 is greater than from R3-R2-R1.
so in theory, it should use R3-R2-R1 . In my lab
it is still using R3-R1 back to back connection.
According to OSPF Network Design Solution
page 235 (Controlling Inter-Area Traffic)
One of the condition of OSPF Traffic selection is
"Use the ABR closest to the destination of traffic"
so which one does have more priority ?
"Use the ABR closest to the destination of traffic" or
"total ospf cost to destination"
--- Ravi <s_ravichandran@hotmail.com> wrote:
> It depends on the total cost from host A to R4. OSPF
> will choose the route
> with lowest cost. So in your case R4 has two route
> to host A. Now ospf will
> check the cost and decide the best route. If it has
> equal cost then it will
> do load balance. You can also manipulate the cost of
> a link and choose the
> route that you would prefer.
>
> Regards,
> Ravi
> ----- Original Message -----
> From: Wei Man <mandraw1@yahoo.com>
> To: <ccielab@groupstudy.com>
> Sent: Sunday, July 01, 2001 7:41 AM
> Subject: OSPF Concept Question
>
>
> > I have question regarding how OSPF behave
> > in selecting route
> >
> >
> > A--R0--R1----R2----R3---R4
> > \ /
> > ----------
> > Where R1 is connected back to back with R2
> > using Serial 0
> > and R1 is connected back to back with R3 using
> > Serial 1
> >
> > A is host connected to R0 and is in Area 0
> > Link R0 to R1 , OSPF Area 0
> > Link R1 to R2 , OSPF Area 1
> > Link R2 to R3, OSPF Area 2
> > Link R3 to R4, OSPF Area 2
> > Link R1 to R3, OSPF Area 3
> >
> > There is virtual link to between R0-R1 to R2-R3
> > using Area 1 as transit area
> >
> > When R4 try to reach A, which Route R4 will use
> > Is R4-R3-R2(Virtual Link)-R1-R0-A or
> > R4-R3-R1-A ?
> > Can somebody explain to me how route selection
> > in ospf works in this case ?
> >
> > Thanks in advance
> >
> >
> >
This archive was generated by hypermail 2.1.4 : Thu Jun 13 2002 - 10:31:28 GMT-3