From: Brian McGahan (brian@cyscoexpert.com)
Date: Sat Jul 05 2003 - 18:19:39 GMT-3
Jim,
Yes and no. The 'broadcast' keyword on a NBMA mapping statement
(frame-relay map, dialer-map, etc) tells the router to send any
broadcast or multicast traffic arriving on that interface as a layer 2
replicated unicast to that circuit. Since OSPF hellos are multicast by
default, the 'broadcast' keyword on the dialer-map will allow the router
to pass the OSPF traffic over the dial circuit.
Since OSPF uses periodic hello packets to maintain a stateful
adjacency, your DDR circuit will remain up if these packets are
considered interesting traffic. However, when running OSPF demand
circuit, the routers stop sending their periodic hello packets used to
maintain adjacency. Once adjacency is established over the line, OSPF
packets will only flow over the line if something in the topology
changes. As long as the topology is stable, the DDR line will never
come up due to OSPF.
By denying OSPF as interesting traffic, you deny the router the
capability to bring up the line in the case of a topology change.
As for the accuracy of documentation, keep in mind that the only
difference between in theory and in practice is that in theory, there is
no difference. Don't believe a documented behavior unless you see it
perform that way first hand. Cisco doesn't put something in the bug
list unless someone reports it first ;)
HTH,
Brian McGahan, CCIE #8593
Director of Design and Implementation
brian@cyscoexpert.com
CyscoExpert Corporation
Internetwork Consulting & Training
Toll Free: 866.CyscoXP
Fax: 847.674.2625
> -----Original Message-----
> From: ccie2be [mailto:ccie2be@nyc.rr.com]
> Sent: Saturday, July 05, 2003 7:33 AM
> To: Group Study; Brian McGahan
> Subject: Re: OSPF Demand Circuit
>
> Hi Brian,
>
> Thanks for gettting back to me.
>
> In another earlier post, someone wrote that the reason for denying
ospf
> traffic was because in the dialer map included the broadcast keyword
which
> would keep the circuit up unless the access-list denied ospf hello's.
Did
> you see that post? This sounded reasonable to me so it's a very good
> thing
> I got your email.
>
> Also, although this is a scary thought, it's good to know that you
can't
> believe everything you see on the cisco web site. I hope there aren't
too
> many incorrect examples there.
>
> Happy 4th of July :-)
>
> Jim
> ----- Original Message -----
> From: "Brian McGahan" <brian@cyscoexpert.com>
> To: "'ccie2be'" <ccie2be@nyc.rr.com>; "'Group Study'"
> <ccielab@groupstudy.com>
> Sent: Friday, July 04, 2003 1:41 PM
> Subject: RE: OSPF Demand Circuit
>
>
> > Jim,
> >
> > This TAC document is wrong. The purpose of running OSPF demand
> > circuit is to maintain an accurate view of the routing topology,
while
> > minimizing the amount of time that your DDR link is up solely due to
> > routing protocol traffic.
> >
> > By denying OSPF as interesting traffic, adjacency cannot be
> > maintained over the DDR link unless it is up for some other reason.
> > When the link goes down due to no interesting traffic passing over
the
> > link within the idle timeout, OSPF adjacency will be lost as soon as
the
> > dead interval expires.
> >
> > When running OSPF demand circuit, OSPF *should* be specified as
> > interesting traffic.
> >
> > HTH,
> >
> > Brian McGahan, CCIE #8593
> > Director of Design and Implementation
> > brian@cyscoexpert.com
> >
> > CyscoExpert Corporation
> > Internetwork Consulting & Training
> > Toll Free: 866.CyscoXP
> > Fax: 847.674.2625
> >
> >
> > > -----Original Message-----
> > > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
Behalf
> > Of
> > > ccie2be
> > > Sent: Friday, July 04, 2003 5:22 AM
> > > To: Group Study
> > > Subject: OSPF Demand Circuit
> > >
> > > Hi,
> > >
> > > After checking the archieves, I didn't find anything that
specifically
> > > addressed this question, so here goes.
> > >
> > > I thought that when a BRI interface is configured as an ip ospf
> > > demand-circuit, it will automatically suppress ospf hello's as
long as
> > the
> > > interface is configured as a p2p or p2m ospf network type.
> > >
> > > However, in the example at
> > > http://www.cisco.com/warp/public/129/config-bri-map.html , it
shows an
> > > access
> > > list being used to prevent ospf hello's in addition to the ip ospf
> > > demand-circuit command being configured.
> > >
> > > Is it really necessary (or just sometimes necessary) to use an
access
> > list
> > > to
> > > deny ospf hello's (packets addressed to 224.0.0.5) when one side
of
> > the
> > > isdn
> > > circuit is configured as an ip ospf demand circuit? If so, why is
> > that?
> > > Also, if the access-list in addtion to the ip ospf demand circuit
is
> > only
> > > needed in certain situations, what are those situations?
> > >
> > > Thanks, Jim
> > >
> > >
> > >
> >
This archive was generated by hypermail 2.1.4 : Wed Aug 06 2003 - 06:52:25 GMT-3