From: Chuck Larrieu (chuck@xxxxxxxxxxxxx)
Date: Tue May 29 2001 - 01:48:59 GMT-3
According to RFC2328, the current standard, within the router LSA there is
something called the V bit, which defines the originating router as one end
of a virtual link.
"bit V When set, the router is an endpoint of one or more fully adjacent
virtual links having the described area as Transit area (V is for virtual
link endpoint). "
further, from the RFC
"Virtual links are part of the backbone, and behave as if they were
unnumbered point-to-point networks between the two routers. A virtual link
uses the intra-area routing of its Transit area to forward packets. "
note the specific wording "behave as if" as opposed to "are"
virtual links are established when the two endpoints agree that they are the
endpoints, having matched the V-bit and the RID's as described in the LSA.
Is the setting of a particular bit in the LSA is "software tunneling" ? I'm
hard pressed to see this as any kind of tunnel, no matter how you stretch
the definition.
Regards,
Chuck
Who doesn't believe in paper CCIE's either
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
eugeneonline
Sent: Sunday, May 27, 2001 3:58 PM
To: Curtis Call
Cc: ccielab@groupstudy.com
Subject: Re: Virtual Links
Hi Curtis,
My reference is to software tunneling, not IOS tunnel interfaces. Andy, I
appreciate your diplomacy, but myself and Curtis cannot both be right. I
believe there is a misunderstanding here which is now hopefully explained:
Software tunneling describes the provisioning of functionality normally
provided through a native means; ie physical/datalink connection in this
case. Since a physical connection is not present and is required, we provide
the connection through software, thus 'software tunnelling' principle is
implemented by IOS. This same kind of virtual connection over non-native
means is also used by IOS in DLSW to connect NetBIOS/SNA segments, IPX
segments over IP cloud etc.
You must understand that IOS 'Tunnel Interface' is something different
(versatile ready-made IOS tool used to implement the tunneling principle).
Vlink allows area 2 to simulate a direct connection to area 0 through
tunneling principle. 'Tunnel Interface' is just an IOS feature.With Vlink,
IOS provides layer 1/2 functionality at layer 3.
I rest my case.
Best regards,
Eugene Akhanoba
Curtis Call <curtiscall@home.com> wrote: I'm afraid I have to differ with
you on this one, but I'm doing so from
theory, I'll have to test this out in the lab on Monday. A virtual link
just transports routing information. When you setup a virtual link on your
router and so a show interfaces, do you see a "tunnel" interface for your
virtual link? I am assuming the answer is no, again when you debug ip
packets do you see them being sent on the virtual link? Once again, I
believe the answer is no. (But I have no routers to test this on at the
moment).
If a virtual link was a tunnel then there would be no requirement for a
transit area to not be a stub area. Since all traffic going over the link
would be encapsulated the routers in that particular area wouldn't need to
know how to reach any routers other than the two ABRs that made up the
virtual link and that are present in the area. However, given that a
virtual link is not a tunnel and is solely a transport of routing
information all packets will be sent hop by hop through the transit area,
that's why that area must have full routing information of the OSPF domain.
At 11:38 AM 5/27/01, you wrote:
>Hi Curtis,
>
>RFC states that all OSPF areas must be connected to area 0. A virtual link
>provides a transparent connection through another area (which must itself
>be connected to area 0) to area 0. This kind of 'virtual connection'
>concept is known as tunnelling; same as DLSW, IPX, IP over IP, VOIP
>tunnels.
>
>Regards,
>
>Eugene
>
>Curtis Call wrote:
>Traffic from R2 to R1 will go directly from R2 to R1. Remember that in
>order to have R2 be a virtual link it will have an interface in Area 1,
>therefore to reach any destination in Area 1 it will always use the
>intra-area route. Besides, Virtual Links are not tunnels, you can't
>transport traffic over them, they just carry routing information, the
>traffic still goes hop by hop throughout the OSPF domain.
>
>At 12:54 AM 5/25/01, you wrote:
> >Guys,
> >
> >I wonder if their is anybody who remembers the discussion on Virtual
> >Links in OSPF. It was posted some time ago but I can't seem to find it.
> >
> >The scenario was something like this:
> >________ _______ _______
> >|Area 0| |Area1| |Area2|
> >| R0 |--| R1 |--| R2 |
> >|______| |_____| |_____|
> >
> >There is a virtual link from area 2 to Area 0 via Area1. Traffic needs to
> >get to R1 in Area 1 from R2 in Area 2. Assume that the virtual link has
to
> >use R1 (To create the V.Link). Does the traffic flow passed R1 (in Area
1)
> >to Area 0 and then back to area 1, or does the actuall flow just to R1
> from R2.
> >
> >I cant remember the conclusion, and I cant seem to find it on the
> >archives. Quite interesting issues.
> >
> >Regards,
> >Andy
> >
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