It appears that there might be some small misunderstanding about DR and BDR
relationships.
On a Multi-access net the DR is the "pseudonode" which represents the net.
It is similar to NLSP in this regard.
All OSPF routers refresh their LSAs occasionally. The DR sends LSA type 2 to
all OSPF routers on the net. This way all routers get a refresh of who is
who etc. Hellos are only used to establish and maintain adjacencies.
Prior to 11.3 LSA's were sent every 30 minutes and all routers refreshed
their databases. Now the default for all LSAs is 4 minutes(240 sec). This is
configurable by using the command "timers lsa-group-pacing".
A BDR, being an OSPF router would miss any other OSPF router after 10 times
4 = 40 secs using the default. This will cause it to change its routing
behaviour, however the BDR to DR functionality would not take effect until
it did not see the LSA type 2 packets from the DR after the set time(4 min
default). I am not sure if that has been changed in 12.x
the DR is the only router who originates LSA type 2 packets.
Winston
-----Original Message-----
From: Frank B. [mailto:[EMAIL PROTECTED]]
Sent: Saturday, November 11, 2000 1:24 AM
To: David Armstrong; [EMAIL PROTECTED]
Subject: Re: Some OSPF Questions
You couldn't be more right! I jumped the gun. My response to your
question 1) was incorrect. The BDR to Dr transition doesn't use the
Hello protocol. The BDR listens to the LSAs from other routers
(non-DR/BDRs) but doesn't reflood or ack them unless the DR fails to.
I still can't find the exact timer value (or what it's called-if
anything) that must go by before the BDR takes over. I've reviewed the
RFC2328 section on flooding procedure and references to the DR/BDR,
Doyle's, Thomas' and Caslow's book but no mention of the length of time.
One response said 0.5 seconds and quoted an Exam Cram...I don't own any
Exam Cram's but shouldn't there be a source reference somewhere? That
very well could be correct but they had to get it from some where? Any
ideas?
Also, regarding question 3) Mr Berkowitz did come up with scenarios
where he used multiple OSPF processes on one box. I'm sure there are
enough possible permutations of networks, as well as IOS nerd knobs, out
there that could support nearly anything.
Generally speaking though I'd go with Cisco's guidance on this
one...this practice is highly discouraged because it can take a toll on
resources of the box. Just my opinion :)
Thanks to all everyone out there willing to help.....Frank
David Armstrong wrote:
>
> Frank,
>
> Thanks for your response. It seems there is some confusion over the things
> I'm confused about! :-)
>
> I appreciate your response. I've been looking at some of this as I could.
I
> do have a question about the BDR to DR promotion. We had found 2 sources
> that said the BDR listens for LSA's from the DR rather than Hello packets.
> That's really where the confusion came from. It would make sense that and
> adjacency was formed between the DR and BDR and that the BDR would wait
for
> the Dead Interval before promoting to DR but both of these said that it
> listened for LSA's.
>
> Do you know if this is correct?
>
> Thanks,
>
> David Armstrong
>
> -----Original Message-----
> From: Frank B. [mailto:[EMAIL PROTECTED]]
> Sent: November 10, 2000 1:49 PM
> To: David Armstrong
> Cc: [EMAIL PROTECTED]
> Subject: Re: Some OSPF Questions
>
> David,
> It appears you have received conflicting guidance
on
> your
> question...I'll throw my 2 cents in but I hope I don't add
> to the
> confusion:
>
> 1) RouterDeadInterval is the legth of time a router waits
> for a Hello
> packet from a neighbor before declaring it down...the same
> timer is used
> by the BDR to determine whether the DR is down. The
> RouterDeadInterval
> is recommended by RFC 2328 to be "some multiple of the
> HelloInterval
> (say 4)" With respect to the HelloInterval RFC2328 states
> "Sample value
> for a local area network: 10 seconds." These are the
> numbers Cisco's
> implementation uses : 10 seconds for Hello and 40 seconds
> for
> RouterDead...for NBMA it's 30 and 120 respectively.
>
> 2) OSPF enabled routers maintain a data struture for each
> OSPF enabled
> interface. When you type "sh ip ospf int x" the router
will
> disply the
> contents of the data structure. Point-to-point interfaces
> will NOT
> display any DR or BDR--because there are none. With only
2
> routers on
> the segment you don't need 'em.
>
> 3) I've never had a need to use 2 OSPF process but Someone
> already
> stated it being used to transition/migrate and that seems
> reasonable...but keep in mind you'd have duplicate
> everything! I would
> imagine the strain on resources, say for the network to
> synchronize,
> would be enormous for larger networks.
>
> I hope this helps you out man...aloha, Frank
>
> David Armstrong wrote:
> >
> > Last night at our BSCN study group meeting in Dallas we
> had some questions
> > about OSPF that we weren't able to resolve. If someone
or
> ones could answer
> > these it would clarify some areas we're a little fuzzy
on.
> Also, if you're
> > iin the Dallas Ft. Worth area and would like to attend,
> we'd love to have
> > you join us..
> >
> > Thanks for any help,
> >
> > David Armstrong
> >
> > 1) What is the default time period that the BDR waits
when
> listening to
> > LSA's from the DR before it decides that the DR is down
> and promotes itself
> > to DR. All the literature we could find simply said that
> the BDR waits for
> > the specified time period but never said what that
period
> is.
> >
> > 2) In a Point-to-Point network in which the router in
Area
> 0 is connected to
> > FR, ISDN, X.25 or ATM branch offices (networks), how
does
> convergence and
> > updates take place? From what we've found a DR and BDR
is
> not elected in a
> > strictly Point-to-Point network.
> >
> > I think an example would explain this question better:
We
> have one 3620
> > router in our Ft. Worth office connected to an office in
> Houston (via FR),
> > and office in Kansas City (via FR), an office in the DFW
> area (via ISDN) and
> > the owner's home (via ISDN). The 3620 is behind a
firewall
> (Pix 520) and the
> > firewall is connected to a 1720 going to the Internet.
I'd
> like to implement
> > OSPF on our network simply for the experience. However,
I
> don't have 2
> > routers internally on our Ethernet LAN that can be
> configured for Area 0 and
> > elected to DR and BDR. All other routers connected to
that
> router are via
> > NBMA Point-to-Point connections. Since I only have one
> router on the
> > Broadcast Multiaccess network (the 3620) and routers
> connected via PtoP
> > don't participate in DR and BDR elections, how would
> updates occur? Can
> > their only be one DR (in this case the 3620)?
> >
> > 3) The books and tutorials all state that "router ospf
6"
> defines ospf on
> > the router with a process ID of 6. They then all say
that
> you shouldn't
> > define more than one process. Does that mean that you
can
> have a router with
> > the following:
> >
> > router ospf 6
> > network 10.100.0.0 0.0.255.255
> >
> > router ospf 7
> > network 10.200.0.0 0.0.255.255
> >
> > If this is an allowed configuration, what kind of
> instances would it be used
> > for? Also, exactly what is the process ID used for?
> >
> > _________________________________
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