From: daneyon hansen (daneyonhansen@xxxxxxxxxxx)
Date: Tue Apr 17 2001 - 22:20:32 GMT-3
If the demand circuit is in area 0 of your OSPF domain, why would the
router produce external LSA's for this route?
Daneyon
>From: "Mask Of Zorro" >Reply-To: "Mask Of Zorro" >To:
chuck@cl.cncdsl.com, burts@mentortech.com, jay@west.net >CC:
cmott@home.com, ccielab@groupstudy.com >Subject: RE: External LSAs
keeping ISDN line up!!! >Date: Tue, 17 Apr 2001 11:45:13 -0400 > >Here we
go again with the OSPF Demand Circuit thing again... > >OK - you have an
issue on the router where you have configured an >OSPF >demand circuit
over your ISDN link, but it keeps flapping up and >down. The >issue is
that this router is an ASBR between your OSPF domain and >some other
>routing protocol domain, let's say IGRP. On this ASBR you are
>performing >mutual redistribution - routes from OSPF are passed into
IGRP, and >routes >from IGRP are passed into OSPF. > >On this particular
ASBR with an OSPF Demand Circuit you have defined >interesting traffic
which will bring up the ISDN link. Usually this >interesting traffic is
anything IP. You test the link with a ping. >The link >comes up, the
pings are successful, and after the desgnated time-out >period >the layer
2 link drops. The OSPF Demand Circuit configuration quiets >the >LSA's
that would normally keep the link up, so the link drops as >desired. >
>Suddenly, the link comes back up again! Then after the timeout, it
>drops >again, and almost immediately comes right back up! Why?!?!? >
>Here's why - External LSA's are bringing up the link. These LSA's >are
the >result of a topology change. What change? The change that occurs
>whenever >the layer 2 ISDN link drops. Hmmm... The OSPF Demand Circuit
is >supposed to >let that link drop, so why is my OSPF process flooding
type 5 LSA'a >over the >link? It is learning about the change from
another process! The IGRP >process >notes the ISDN link dropping and is
redistributing it's change into >OSPF. >Think about this for a minute. >
>OK - now we know what is happening, how do we stop it. We COULD >simply
>eliminate OSPF traffic from what is interesting. In this case, OSPF
>LSA's >would no longer bring up the link. The problem is that OSPF LSA's
>would >NEVER bring up the link, even if a topology change occurs
elsewhere >in the >network and we want that change to reach the neighbor
over the ISDN >link. >This is not good... > >What else can we do? If you
consider the problem it becomes evident >that we >really only want to
stop LSA's from bringing up the link in one >case: when >the ISDN link
goes down. The solution, then, is to prevent the ISDN >link >drop from
turning into an OSPF LSA flood. Filter your redistribution >such >that
the OSPF process does not receive an update from your IGRP >process
>regarding THE ISDN LINK ONLY. > >This will stop the flapping of the
demand circuit without preventing >other, >potentially important,
topology changes from bringing up the link. > >There are numerous
practice labs available with examples of this, >showing a >sample config
in the answer key. CCBootCamp labs and SolutionLabs >come to >mind. > >So
there you go - problem solved. Next! > >Z > > > > > > >>From: "Chuck
Larrieu" >>Reply-To: "Chuck Larrieu" >>To: "Rick Burts" , "Jay Hennigan"
>> >>CC: "Chris Mott" , "CCIE" >>Subject: RE: External LSAs keeping ISDN
line up!!! >>Date: Mon, 16 Apr 2001 17:20:36 -0700 >> >>Isn't this where
the IP OSPF demand circuit command comes into >>play? >> >>Chuck >>
>>-----Original Message----- >>From: nobody@groupstudy.com
[mailto:nobody@groupstudy.com] On >>Behalf Of >>Rick >>Burts >>Sent:
Monday, April 16, 2001 5:05 PM >>To: Jay Hennigan >>Cc: Chris Mott; CCIE
>>Subject: RE: External LSAs keeping ISDN line up!!! >> >>Jay >> >>There
is one aspect of your reasoning that I do not understand. >>Assuming your
scenario: the dialer list will deny ospf as >>interesting >>traffic, and
a ping brings up the line so adjacencies are formed >>and >>ospf routes
are added to the table. What happens when the line has >>gone >>inactive
and some link change generates an LSA which we need to >>transmit to the
neighbor on the demand circuit but ospf cannot >>bring >>up the link
because it is not interesting ? I think this may >>appear >>to work in a
limited lab scenario but ultimately I think this is a >>broken
implementation. I agree with Chris that the better >>implementation has
ospf as interesting traffic in the dialer list. >> >>Rick >> >>Rick
Burts, CCSI CCIE 4615 burts@mentortech.com >>Mentor Technologies
240-568-6500 ext 6652 >>133 National Business Parkway 240-568-6515 fax
>>Annapolis Junction, Md 20701 >> >>Chesapeake Network Solutions has now
become Mentor Technologies. >>Mentor Technologies is a certified Cisco
Training Partner and also >>a Cisco Professional Services partner. >>We
offer most of the Cisco training courses. >>We also offer training in
Checkpoint Firewall software and >>Fore Systems (now Marconi) and
MicroMuse. >>We also provide network consulting services including
>>design, management, and problem solving. >>We have 20 CCIEs on our
staff. >>We offer the breakthrough VLAB remote access technology for
>>access to practice configuration on real equipment. >> >>On Sun, 15 Apr
2001, Jay Hennigan wrote: >> >> > On Sun, 15 Apr 2001, Chris Mott wrote:
>> > >> > > I've never understood the reasoning behind a dialer-list that
>>has >>"deny >>ospf >> > > any any", as I thought the whole reason for
an OSPF >>demand-circuit was >>that >> > > OSPF could bring up the link
if a routing decision deemed it >>necessary >>... >> > > please correct
me if I'm wrong ... >> > >> > The dialer-list keeps the OSPF multicast
traffic from bringing >>up the >>link, >> > but doesn't deny it from
traversing the link. Once the link has >>been >> > established once (like
by a ping), then the OSPF routes will be >>learned >> > and retained. >>
> >> > Normal IP traffic destined for that link (if not denied by an
>>access >> > list) will then bring up the dialer. >> > >> > -- >> > 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 >> > **Please
read:http://www.groupstudy.com/list/posting.html >>**Please
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