From: Tony Olzak (aolzak@xxxxxxxxxxxxxxxxxxx)
Date: Fri Oct 27 2000 - 19:29:08 GMT-3
Except that with "backup interface" the interface actually goes down and
doesn't come up for routing updates. You'd need a static route for the BRI
so when the primary goes down you can route across the backup.
With demand-circuit, the interface will come up when the primary link fails
and go back down (spoofing up) when the primary comes back. This is all
assuming that the metric for your primary is better than your ISDN line.
Make sure it is.
Tony
----- Original Message -----
From: "Jay Hennigan" <jay@west.net>
To: "Shaun Nicholson" <Shaun.Nicholson@kp.org>
Cc: "Bccielab" <ccielab@groupstudy.com>
Sent: Friday, October 27, 2000 12:38 PM
Subject: RE: Losing marks for extra commands
> On 27 Oct 2000, Shaun Nicholson wrote in one long line:
>
> > Now this was not on my exam so I anit breaking NDA but when the wording
> > says that a DDR circuit (using OSPF) must come up immediately on failure
> > of a frame link and drop as soon as the circuit is restored the only
> > option in my opinion is to use OSPF demand circuit.
> > What do you guys think? would this be acceptable ? I think you cannot
> > use the Backup Interface circuit (if your allowed too) but there is a
> > lot of delays involved in this. I know you can tweak it but would that
> > be acceptable?
>
> You need demand circuit to keep the OSPF hellos from nailing the link up.
> But couldn't you also use backup interface to trigger the ISDN
"immediately"?
>
> Words like "immediately", "as soon as", and "failure" can cause problems
> here.
>
> If the frame link fails with loss of carrier, the line protocol will
> drop within a second or two, and within a reasonable approximation of
> "immediately" dialing can begin. If the frame link dies at the remote
> router, then there will be (default) 30 seconds of lost LMI before the
> line protocol drops, and up to 60 seconds before it comes back. This
> will be lower than the dead timer to detect a drop regardless of the
> OSPF network type.
>
> So, unless there's something I'm missing, the "immediately" part depends
> on the nature of the "failure", and dropping the ISDN "as soon as" the
> link returns doesn't mean when the cable is reinserted into the router
> because of LMI keepalive and full status timers. So, this might be a
> case to ask the proctor for clarification of words like "immediately"?
>
> Something like, "By 'immediately', do you mean as soon as the router
> detects that the line protocol is down? Are default backup interface
> timers adequate to satisfy the words "immediately" and "as soon as"?
>
> I'm assuming that floating statics are not allowed, that would be another
> approach.
>
> According to Thomas' OSPF Network Design Solutions p. 208, the OSPF
process
> is aware of the interface state, and will flood LSA on loss of carrier or
> LMI, even before the dead timer expires. So, demand circuit might indeed
> be all you need. The LSA caused by the change would be "interesting", and
> should bring up the link. Haven't tried this yet.
>
> Would you lose points for both backup interface and demand circuit?
> I don't know. It wouldn't hurt anything, the scenario would still work,
> and in the real world it probably wouldn't be a bad idea.
>
> > By the way these are all metaphorical questions and I'm again only
> > looking for some feedback on this one as I have (and sure other users
> > have) found this discussion very useful.
>
> Bring them on! Please! It helps to make us think.
>
> > Again please think about NDA before answering I dont want to upset
> > anyone out there. My aim is to make the lab a little bit clearer for
> > all involved especially myself.
>
> Agreed, and in my opinion the NDA responsibility is on the person *asking*
> the question, so state as you did that it's a hypothetical or a matter of
> technique that you're curious about.
>
> If someone by chance happens to ask something that's real close to an
> actual lab scenario, a flurry of, "You bum, I'm not going to tell you
> because of the NDA" responses not only causes resentment and stifles
> honest discussion, but also gives away the fact that the inquiry is close
> to home.
>
> --
> Jay Hennigan - Network Administration - jay@west.net
> NetLojix Communications, Inc. NASDAQ: NETX - http://www.netlojix.com/
> WestNet: Connecting you to the planet. 805 884-6323
>
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