From: Shaun Nicholson (Shaun.Nicholson@xxxxxx)
Date: Fri Jan 19 2001 - 17:39:54 GMT-3
The OSPF demand circuit is an all round solution to the dial up use of OSPF.
Try it yourself set up two routers over a BRI only running OSPF and make the BR
I a demand circuit there is an initail dial up and the route gets set as a DNA,
then leave it. The circuit will stay down until a need to send traffic across
it arises. I left mine for about a week one time and as there were no network c
hanges to report the circuit remained down the whole time.
The BRI will be brought up every time there is a change in the OSPF routing tab
le or when you need access to a network across the BRI.
I have seen the OSPF demand circuit used for dial backup solutions to backup a
T1.
This worked well, when the T1 failed as the BRI came up the change was noted in
the routing table the BRI would then go down until traffic was sent across the
link. In this situation if the T1 was not under constant use as in this case I
refer to, then the link would stay down for hours at a time.
Again though in the lab I would check the wording but the bri should only come
up when there is a change in the routing table.
Thats one of the issues you find when redistributing RIP or IGRP into OSPF the
updates keep bringing the circuit up.
The link would come up for every link added or removed the area it is in does n
ot matter it the AS that counts. So any change in the AS and bingo the BRI come
s up for the changes to be recalculated and the routing table gets updated. But
once it has stabalised it stays down. However if you redistribute an IP routin
g protocol in there it becomes part of your AS. But if your redistribution rou
ter is your router with the BRI then you have to watch for the mask your networ
k uses for example
Int s0 100.100.20.1
Int s1 100.100.30.1
Int e0 100.100.40.1
int bri0 100.100.100.1
Now if you do router rip the network statement is by default 100.100.0.0 and yo
u cannot change this.
So lets say s1 is your only rip interface and your redistributing int OSPF. You
r rip network statement makes all of your interfaces s0 s1 e0 and bri rip updat
e interfaces IE they send out rip updates yeah. To stop this happening you need
to make the 100.100.0.0 subnet interfaces passive to rip and only allow the re
al rip interface to work with rip int s1. to do this use the command passive-in
terface BRI0 for the BRI.
Changes in the rip network will still bring up your BRI as your redistributing
but you need to look at the LSA generation debug to see what they are. But I ha
ve found using demand circuit you should not need to block any traffic with an
access-list and your dialer list should be permit protocol ip any.
Sorry if this is to simple an explanation and you already knew this.
I still dont think you need to use the backup interface command. But again go w
ith what you think is correct for the scenario you are presented with. I found
the wording of the lab often gave you a big clue as to the correct solution but
for me OSPF and DDR the only solution is Demand circuit if you can't use float
ing statics anyway. I would have done OSPF demand if I had to run OSPF as the r
outing protocol on the BRI and would not have used the backup interface command
. If I had been marked down for it then I would have explained why I chose that
option in a logical manor covering the fact that knew about the DNA option and
how the link would go down when no traffic was sent and how the change was ime
diate unlike the backup interface which takes a timeout period. I would also ha
ve mentioned how there was a much quicker convergance time with the demand circ
uit as the routes are already there so no addition learning was required. If th
at had not worked I would have broken into !
tears like a little baby until he gave me the points back !!!!!! :-)
Shaun Nicholson CCIE 6705
Lead Network Engineer
Kaiser Permanente
Silver Spring Data Center
301 680 1462
dotsonjl@epb.net on 01/19/2001 10:07:00 AM
To: Shaun Nicholson, erickbe@yahoo.com@Internet
cc: ccielab@groupstudy.com@Internet
Subject: Re: OSPF Demand circuit on backup interface
Hello all. I'm new to the group. Been lurking for a while.
Shaun, I understand where you're coming from, but I've got a question. Isn't
OSPF on-demand only to keep the BRI quiet in stable environments? If you add
a route into the OSPF AS, then wouldn't the BRI link fire up to propagate the
LS change (or does on-demand only look at it's particular area)? This would
happen when the primary link is up OR down. How is this an acceptable
solution to ONLY backing up a serial interface (I just fear I might loose
this argument - with one particular proctor I'm almost sure I would)?
Jimmy
>>> Shaun Nicholson <Shaun.Nicholson@kp.org> 01/19/01 09:30AM >>>
Ok
If I were setting this up I would set up 2 dialer interfaces one for the
backup circuit and one running the demand circuit.
I have never tried running them both on the same interface but as the dialer
goes into standby I would imagine that the circuit was unusable until the
serial fails but like I say I've never tried it.
I would have to ask why do you need the backup command anyway, OSPF demand
circuit dialer is a solution on its own. The backup interface command only
puts the interface into active mode when the serial line fails so you still
need traffic to bring the circuit up. I would certainly not block all OSPF
traffic from making the link active because of this reason.
If your interface ever bounces using the ospf demand circuit make sure all
other IP routing protocols on that router have a passive interface for the
BRI. Check what LSA's are bringing the line by debugging LSA generation and
if thats still no good use the no peer neighbor-route command that always
seems to work for me but you need to check the LSA generation first.
I would just use the OSPF demand circuit and forget about the backup interface
you dont need it and if I got a similar situation in the lab I would still
take that approach and then justify why I did it that way to the proctor. If
I was specificly told I had to use the backup interface and was not allowed
to put OSPF on the bri interface then I would look at the backup interface as
the solution.
If I was told I HAD TO use both then I would set up 2 dialer interfaces. One
for backup and one running OSPF demand.
But again I dont see how you could use ospf demand circuit with the backup the
interface goes int standby so becomes unusable until the interface your
backing up fails. So why use demand circuit do you see what I'm saying??? I
have been wrong and will probably be wrong again so please if this does work
I'm sure someone will let me know.
One or the other in my opinion.
So check the wording on the lab if this was to appear and if you have to use
OSPF on the bri then OSPF demand is IMHO the only solution.
Thanks
Shaun Nicholson CCIE 6705
Lead Network Engineer
Kaiser Permanente
Silver Spring Data Center
301 680 1462
erickbe@yahoo.com on 01/18/2001 10:27:00 PM
To: ccielab@groupstudy.com@Internet
cc: (bcc: Shaun Nicholson/MD/KAIPERM)
Subject: OSPF Demand circuit on backup interface
Hello,
I have a question regarding using ospf demand circuit
on a ISDN dialer interface that is in standby mode.
Main serial interface has backup interface command.
The problem is that with the ip ospf demand circuit
command on the dialer interface the dialer interface
keeps going up and down when the primary circuit is up
and active. It is in standby mode when it does this.
With the ip ospf demand circuit command off the dialer
interface it stops going up & down and behaves like a
standby circuit should.
I have tried deny'ing OSPF in the dialer list, moving
the ip ospf demand circuit command to the remote end
router and it continues to go up & down while in
standby mode. Anyone have any ideas?
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