alt.certification.cisco - Few OSPF questions

Author

Few OSPF questions

Jason Thomas

2002-06-24, 6:39 pm

I have been doing quite a bit of reading on OSPF. I am confused on a few
points. If some could clarify, I would really appreciate it.

What type of routers can exist in Area 0? I remember reading that an ABR,
ASBR and Backbone can. The Sybex book states only a backbone router can
exist.

Would it be fair to compare a point to point or a point to multi-point link
to a peer to peer network where there is no central database just a bunch of
router establishing peerings?

Thanks,
Jason

Nathan

2002-06-24, 6:39 pm

> What type of routers can exist in Area 0? I remember reading that an ABR,
> ASBR and Backbone can. The Sybex book states only a backbone router can
> exist.

In Area 0, naturually a backbone router belongs, but also (by definition) at
least (if not more) ABR must be present. An ABR is a router that sits in
between two areas; therefore, participates in both areas. If you were going
to configure an ABR that is in Area 0 and Area 2, this is how you might do
it.
Router(config)# router ospf 1
Router(config-router)# net 10.32.0.0 0.31.255.255 area 0
Router(config-router)# net 10.64.0.0 0.31.255.255 area 2

-where there are two interfaces on the preceeding subnets.

> Would it be fair to compare a point to point or a point to multi-point
link
> to a peer to peer network where there is no central database just a bunch
of
> router establishing peerings?

Mmmm... not really. A point-to-point link only has two ends, only occupies
two IP addresses, and isn't capable of connecting hosts. A
point-to-multipoint link is still basically a point-to-point link, but is
capable of having multiple routers over a point-to-point interface (such as
serial) occupy the same subnet.
/----\___________/----\
\___/ \___/ point-to-point
this configuration will use only two IP addresses and can be wasteful
without a tool such as VLSM.
_ _ _ _ _ _ _ _ _/----\
/----\____/ \___/
\___/ \_ _ _ _ _ _ _ _ _/----\
\___/ multipoint
in this config, all three of these routers can be on the same subnet over
this link. You will typically find this sort of configuration in an NBMA
topology such as Frame Relay.

Sorry, if I kinda missed the topic a little. I'm not too sure I understood
your question, though. Hope I was a little help.
Nathan

Janine Hill

2002-06-24, 6:39 pm

Umm.. A backbone router is any router that is in the backbone, including an
ABR. An Internal router is one that only exists in one area. Area 0 is the
backbone, and if you have an interface in area 0, you are a backbone
router....

"Craig Small" <csmall@SPAMeye-net.com.au> wrote in message
news

an.2002.06.22.13.40.21.409301.19355@SPAMeye-net.com.au...
> On Fri, 21 Jun 2002 22:47:06 +1000, Jason Thomas wrote:
>
> > I have been doing quite a bit of reading on OSPF. I am confused on a
> > few points. If some could clarify, I would really appreciate it.
> >
> > What type of routers can exist in Area 0? I remember reading that an
> > ABR, ASBR and Backbone can. The Sybex book states only a backbone
> > router can exist.
> It all comes down to exist. A backbone router has all its interfaces in
> Area 0 only. So only a backbone router can exist WHOLLY in Area 0.
>
> ABR and ASBR usually have interfaces in Area 0, but they also have
> interfaces elsewhere (non 0 Areas and other ASes respectively).
>
> > Would it be fair to compare a point to point or a point to multi-point
> > link to a peer to peer network where there is no central database just a
> > bunch of router establishing peerings?
>
> I assume you are comparing these to, say, an ethernet subnet with DRs.
It's a
> strange comparison used, I just think of it there is no main spokesperson
> on behalf of the subnet.
>
> - Craig

Craig Small

2002-06-24, 6:39 pm

On Fri, 21 Jun 2002 22:47:06 +1000, Jason Thomas wrote:

> I have been doing quite a bit of reading on OSPF. I am confused on a
> few points. If some could clarify, I would really appreciate it.
>
> What type of routers can exist in Area 0? I remember reading that an
> ABR, ASBR and Backbone can. The Sybex book states only a backbone
> router can exist.
It all comes down to exist. A backbone router has all its interfaces in
Area 0 only. So only a backbone router can exist WHOLLY in Area 0.

ABR and ASBR usually have interfaces in Area 0, but they also have
interfaces elsewhere (non 0 Areas and other ASes respectively).

> Would it be fair to compare a point to point or a point to multi-point
> link to a peer to peer network where there is no central database just a
> bunch of router establishing peerings?

I assume you are comparing these to, say, an ethernet subnet with DRs. It's a
strange comparison used, I just think of it there is no main spokesperson
on behalf of the subnet.

- Craig

shope

2002-06-24, 6:39 pm

Jason

I think the original Qs are based on a false assumption - the "roles" of an
OSPF router are not all exclusive - there are several options, and some can
be in the same OSPF device.

So, a backbone router has an 1 or more interfaces within the backbone. It
may also be an internal router (if it doesnt have any other areas or
external route injuection set up). alternatively it may be an ABR and / or
an ASBR.

If you think about it, since an ABR must be connected to the backbone area,
then some of your backbone routers have to support ABR.

There are some roles that cannot be mixed - a device cannot be an internal
router in a stub network and a backbone router for example...

If you want to know about the various roles of an OSPF router - go back to
the original source and read whichever rfc is the current one for OSPF v2.

It is quite readable - honest......

--
Good luck

Stephen Hope - remove xx from address.

"Janine Hill" <janine@NOSPAMpobox.com> wrote in message
news:af4ptv$nh4$1@helle.btinternet.com...
> Umm.. A backbone router is any router that is in the backbone, including
an
> ABR. An Internal router is one that only exists in one area. Area 0 is the
> backbone, and if you have an interface in area 0, you are a backbone
> router....
>
>
> "Craig Small" <csmall@SPAMeye-net.com.au> wrote in message
> news

an.2002.06.22.13.40.21.409301.19355@SPAMeye-net.com.au...
> > On Fri, 21 Jun 2002 22:47:06 +1000, Jason Thomas wrote:
> >
> > > I have been doing quite a bit of reading on OSPF. I am confused on a
> > > few points. If some could clarify, I would really appreciate it.
> > >
> > > What type of routers can exist in Area 0? I remember reading that an
> > > ABR, ASBR and Backbone can. The Sybex book states only a backbone
> > > router can exist.
> > It all comes down to exist. A backbone router has all its interfaces in
> > Area 0 only. So only a backbone router can exist WHOLLY in Area 0.
> >
> > ABR and ASBR usually have interfaces in Area 0, but they also have
> > interfaces elsewhere (non 0 Areas and other ASes respectively).
> >
> > > Would it be fair to compare a point to point or a point to multi-point
> > > link to a peer to peer network where there is no central database just
a[col
or=darkred]
> > > bunch of router establishing peerings?
> >
> > I assume you are comparing these to, say, an ethernet subnet with DRs.
> It's a
> > strange comparison used, I just think of it there is no main[/color]
spokesperson[color
=green]
> > on behalf of the subnet.
> >
> > - Craig
>
>[/color]

Janine Hill

2002-06-24, 6:39 pm

shope

2002-06-24, 6:40 pm

pomerol82

2002-06-24, 7:44 pm

Backbone routers means a router having at least one interface in area 0. If
all its interfaces are only included in area, it is also called internal
router.

Example 1: A router with
Eth 0 : area 0
Ser 0 : area 1
It is a backbone router as well as an ABR.

Example 2 : A router with
All interfaces : area 1
It is an internal router

Example 3 : A router with
Eth 0 : area 0
Ser 0 : area 1
Ser 1 : EIGRP with PID 50
It is a backbone router, an ABR as well as ASBR

--
pomerol82
Poor is not a sin, but the rich makes it be sin;
Rich is not supreme, but the poor wishes be supreme.

Craig Small <csmall@SPAMeye-net.com.au> wrote in message
news

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