From: Ravi (s_ravichandran@xxxxxxxxxxx)
Date: Sun Jul 01 2001 - 16:55:18 GMT-3
I did the same lab and found what I said was correct.
For Router R4 it has only one route to any network, that is thru its serial
0. So R4 is not deciding which route it should take to reach the host at R0.
R3 is the router which has two routes to go to same destination. It purely
calculates the best route depends on the cost. Example, host A is connected
to R0 thru Ethernet, the cost is 10. R0 to R1 cost is 64. R1 to R2 cost is
64. R2 to R3 cost is 64. So total cost to reach host A from R3 is
10+64+64+64=202 thru R2, R1, R0. Another route to reach the host A from R3
is 10+64+64= 138. (64 is the cost for the link R1 to R3). So 138 is the best
route to reach the host A from R3. You can change this by changing the cost
of the link R3 to R1 using ip ospf cost command. If yo set the cost of the
link R1 to R3 to 128, you will see that the router R3 has two routes to
reach host A. at this point it (R3) would do load balance.
That is it.
Regards,
Ravi
----- Original Message -----
From: Wei Man <mandraw1@yahoo.com>
To: Ravi <s_ravichandran@hotmail.com>; <ccielab@groupstudy.com>
Sent: Sunday, July 01, 2001 9:31 AM
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
> > >
> > >
> > >
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