From: Carlos G Mendioroz (tron@xxxxxxxxxxx)
Date: Fri May 10 2002 - 20:19:21 GMT-3
I think I did not express well what I intended to.
Links are 2 (or more in the case of networks) interfaces
over which ospf routers make adjacencies.
For that to be the case, all intervening interfaces have to be
in the same area, so always the two (or more) ends of a link
are in the same area.
As many routers (almost all usefull ones) have many interfaces,
it can be the case that one has interfaces in more than one
area, and that we call an ABR.
Now, as I see them, virtual links are more or less tunnels
over an established link, where you get a new pair of interfaces
so to speak. This new "virtual" link HAS to be in area 0.
(meaning that both ends now have area 0 and area X
interfaces, so they both are ABRs now!, area X being the
transit area)
One interesting thing with VLs is that even though the routing
info come and go over the VL (the area 0 is the only that distributes
inter area routing info) the actual routes do not necesarily follow
the same path, and OSPF can do "shortcuts" when needed.
I.E.
net3 -- (a3)R3(a1) -- link1 -- (a1)R2(a2) -- net2
(a1) (a1)
| |
link3 link2
|---- (a1)R1(a1) ----|
(a0)
|
...bbone
In this setup (adapted from Moy's book, pp 128) you need 2 VLs
to link R3 and R2 to R1, but traffic from net3 to net2
can go via link1 (even though routing info goes via link2 and
link3).
"Howard C. Berkowitz" wrote:
>
> At 6:33 PM -0300 5/10/02, Carlos G Mendioroz wrote:
> >Howard,
> >isn't it "virtual links ARE always area 0" ?
> >
> >Areas define links aggregation, not routers, right ?
> >So always both ends of a link are in the same area.
> >For virtual links, this must be area 0.
>
> No, virtual links are an exception (or maybe a blurry area is a
> better term), since the original purpose was to have one end in a
> non-backbone area that did not have physical connectivity to area
> 0.0.0.0. Logical connectivity is achieved by running the virtual
> link through a second nonzero area, which must have transit
> capability.
>
> While this is the original intention -- that's from the mouth of John
> Moy -- the more common use these days indeed has both ends in area
> 0.0.0.0, but the link passes through a nonzero transit area with two
> connections to area 0.0.0.0.
>
> >
> >Carlos G Mendioroz <tron@huapi.ba.ar> LW7 EQI Argentina
>
> --
> "What Problem are you trying to solve?"
> ***send Cisco questions to the list, so all can benefit -- not
> directly to me***
> *****************************************************************************
***
> Howard C. Berkowitz hcb@gettcomm.com
> Chief Technology Officer, GettLab/Gett Communications http://www.gettlabs.com
> Technical Director, CertificationZone.com http://www.certificationzone.com
> "retired" Certified Cisco Systems Instructor (CID) #93005
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