From: John.K.Feuerherd@xxxxxxxxxxxxxx
Date: Fri Aug 17 2001 - 02:32:53 GMT-3
OSPF ONLY defines 5 network types:
Point-to-point networks
Broadcast networks
Non-broadcast networks
point-to-multipoint networks
and Virtual links
Virtual links are interpreted by the router as unnumbered point-to-point
networks. What this means is that the virtual link will transition to the
fully functional point-to-point interface state when a route to the
neighboring router is found in the routing table. (Doyle, Volume 1 page 466)
This is completely different then the demand circuit. Demand circuits are
usage-sensitive connections such as X.25 SVCs, ISDN, and dialup lines. OSPF
treats virtual links as a "usage-sensitive" link and that is why it is
treated as a demand circuit. OSPF will bring up the demand link to perform
the initial database synchronization and then it will bring up the link only
to flood LSAs when changes have occurred. No Hellos or LSA are being
exchanged on a regular bases.(Do a debug Ip ospf adj)
You can see that even more when you look at the next line below "Run as
Demand circuit" on the output of "Show Ip OSPF Virtual-Link". The "DoNotAge
LSA allowed" means that the LSAs will not reach MaxAge...
r3#sh ip ospf virtual-links
>
> Virtual Link OSPF_VL0 to router 170.100.4.1 is up
> Run as demand circuit
> DoNotAge LSA allowed.
> Transit area 2, via interface Serial2/0:1, Cost of using 74
> Transmit Delay is 1 sec, State POINT_TO_POINT,
> Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
> Hello due in 00:00:07
> Adjacency State FULL (Hello suppressed)
Hope that helps,
JF
-----Original Message-----
From: jonatale@earthlink.net [mailto:jonatale@earthlink.net]
Sent: Friday, August 17, 2001 12:01 AM
To: John.K.Feuerherd@WellsFargo.COM
Cc: chuck@cl.cncdsl.com; ccienxtyear@hotmail.com; ccielab@groupstudy.com
Subject: Re: OSPF Virtual Link question
i think demand circuit is a diff net type than p2p and than virtLink, demand
ckts came out in a later rfc
need to check the ospf database router LSA - should show the net type, or
find
the rfc
(kind of a guess)
John.K.Feuerherd@WellsFargo.COM wrote:
> Sanjay,
> An OSPF Virtual link is considered a point-to-point circuit but it
> is run as a demand circuit. This means that once the adjacency is
> established the OSPF hellos and LSA refresh functions are suppressed. I
> guess OSPF treats Virtual links as a bandwidth sensitive link.....
>
> You can see this happen if you set it up in a lab and before you do the
> "area X virtual-link x.x.x.x" command do a "debug OSPF adj" and you will
see
> the hellos suppressed after the link reaches a FULL state.
>
> Thanks,
> JF
>
> -----Original Message-----
> From: Chuck Larrieu [mailto:chuck@cl.cncdsl.com]
> Sent: Thursday, August 16, 2001 6:57 PM
> To: sanjay; ccielab@groupstudy.com
> Subject: RE: OSPF Virtual Link question
>
> I'll let you look up RFC 1793 and report back to us what you discover. ;->
>
> Good Reading!
>
> Chuck
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
> sanjay
> Sent: Thursday, August 16, 2001 5:31 PM
> To: Chuck Larrieu; ccielab@groupstudy.com
> Subject: Re: OSPF Virtual Link question
>
> Look at the info on the bottom. I have 2 routers connected back to back
via
> serial inter. R1 has an interface in Area 0 and the serial interface in
Area
> 2. On R2 a loopback interface is in Area 4 and the serial link is in Area
2.
> I have configured the Loopback interface with "ip ospf network
> point-to-point" .
>
> r3#sh ip ospf virtual-links
>
> Virtual Link OSPF_VL0 to router 170.100.4.1 is up
> Run as demand circuit
> DoNotAge LSA allowed.
> Transit area 2, via interface Serial2/0:1, Cost of using 74
> Transmit Delay is 1 sec, State POINT_TO_POINT,
> Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
> Hello due in 00:00:07
> Adjacency State FULL (Hello suppressed)
>
> r3#
>
> ----- Original Message -----
> From: "Chuck Larrieu" <chuck@cl.cncdsl.com>
> To: "sanjay" <ccienxtyear@hotmail.com>; <ccielab@groupstudy.com>
> Sent: Thursday, August 16, 2001 4:56 PM
> Subject: RE: OSPF Virtual Link question
>
> > it operates as a point to point link. why? because the RFC says so. :->
> >
> > from RFC 2328:
> >
> > The virtual link is treated as if it were an unnumbered point-to-
> > point network belonging to the backbone and joining the two area
> > border routers. An attempt is made to establish an adjacency over
> > the virtual link. When this adjacency is established, the virtual
> > link will be included in backbone router-LSAs, and OSPF packets
> > pertaining to the backbone area will flow over the adjacency. Such
> > an adjacency has been referred to in this document as a "virtual
> > adjacency".
> >
> > the cost of the link is the sum of the costs of the links across the
> transit
> > area.
> > for the real low down check out ftp://ftp.isi.edu/in-notes/rfc2328.txt
> >
> > HTH
> >
> > Chuck
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
> > sanjay
> > Sent: Thursday, August 16, 2001 4:41 PM
> > To: ccielab@groupstudy.com
> > Subject: OSPF Virtual Link question
> >
> >
> > Does the virtual link run as a demand circuit and why ?
> >
> > -sanjay
> > **Please read:http://www.groupstudy.com/list/posting.html
> > **Please read:http://www.groupstudy.com/list/posting.html
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