Subrat,

Subrat Mohanty wrote:

> Sina!I have some more questions on u r clarifications. I have listed
> it below. Can u pl. answer those?
>
> -----Original Message-----
> From: Mailing List [mailto:OSPFdiscuss.microsoft.com]On
> Behalf Of Sina Mirtorabi
> Sent: Wednesday, July 04, 2001 4:11 AM
> To: OSPFdiscuss.microsoft.com
> Subject: Re: Doubt in Virtual Link
>
> Subrat
>
> actually there is a virtual link between R10 and R11 in
> order to link the area 3 to the backbone
> if the link between R6-R10 goes down R10 won't be ABR any
> more and area 3 will be disconnected from the backbone
> [Subrat Mohanty] Even if u have a virtual link between R10
> and R7, can I say that Area 3 is connected to BB as there is
> an virtual link between R11-R10 and R10-R7 and R7 is
> connected to BB?
>

absolutely, since R11 has a virtual link to an ABR ( R10 ) through a
Transit area, it doesn't matter if this ABR is connected to the backbone
or has itself a Virtual link to another ABR

> The basic condition for establishing a virtual link is the
> routers should be ABR.
>

and having a non-backbone area ( excluding Stub and NSSA ) in common

> if the link between R6-R10 goes down R10 won't be ABR.
> Should the virtual link still exist?
>

right but if R10 has itself a virtual link to R7 when the link between
R6-R10 goes down R10 will remain ABR

>
> here is another example
>
> R2
> / \
> / \
> / \
> R1--------------------------R3
> | |
> | |
> | |
> R5 R4
>
>
> R1-R5 is in area 1
> R1-R3 area 0
> R3-R4 area 2
> R1,R2,R3 area 3
>
> if the backbone link between R1-R3 is broken you need a
> virtual link through R2 in order to maintain the
> connectivity
> [Subrat Mohanty] From the above scenario, I feel tha R1 and
> R3 are connected to a common network (assuming that it's a
> LAN). When u say that BB connectivity, does it mean that all
> the networks in the BB area, should be connected physically
> via some router or the other?
>

note that BB router is not necessarily an ABR since it can have all its
interface in area 0
area 0 need Not physically to be connected you could maintain the
connectivity logically via virtual link

> If that is the case, I can conclude that the Topology of a
> OSPF network should be such that atleast networks in BB will
> be connected. if the backbone link between R1-R3 is broken ,
> then I can still forward the data traffic thru R2 without
> having a virtual link. Because R1 and R3 already would have
> the reachability info between them. Then what is the extra
> benifit I have having a virtual link?
>

in the above scenario R1 and R3 will still be ABR if the link between
them in area 0 goes down ( for RFC ABR definition )
so they will generate summary for their attached area but an ABR will
only consider the summary from the backbone which means that R1 will
ignore the summary of R3 for area 2 and your routing connectivity is
broken ie : area 1 and 2 could not communicate
by configuring a virtual link between R1 and R3 your backbone is re
stored since virtual link is part of area 0

>
>
> virtual link could also be used in order to have control
> over the traffic as intra area path are always preferred
> over Inter area path by playing with virtual link you could
> make the traffic go through the desired path
> [Subrat Mohanty] Can u pl. give me an example of it? Or
> refer to any doc where I will have the example?
>
>

R1-------------R2
 | / |
 | / |
 | / |
 | / |
 | / |
R4 ------------R3

ok R4-R1-R2 are in area 1 and R2-R3-R4 are in area 0 ( so R2 and R4 are
ABR )
now we want the link R2-R4 be either in area 1 or area 0
if we put in area 1 then area 1 will have the optimal path ( R4 will
send directly to R2 ) but area 0 will have non optimal path as R4 goes
through R3 to reach R2
if we put in area 0 then area 0 will have the optimal path ( R4 will
send directly to R2 ) but area 1 will have non optimal path as R4 goes
through R1 to reach R2 ( intra area path are preferred over Inter area
path )

by putting the link in area 1 and configuring a virtual link between R2
and R4 both area will send directly from R4 to R2 since virtual link
path here is the same as physical path and is part of area 0.

Sina

>
> Sina
>
>
> Subrat Mohanty wrote:
>
> > I have a basic doubt in Virtual Link. I am still not clear
> > about the use or
> > benifit of the VLink.
> >
> > Pl. Check RFC-2328 Fig-6. Sec-3.4. See the last line which
> > states
> >
> > "Note that a failure of the line between Routers RT6 and
> > RT10 will cause the
> > backbone to become disconnected. Configuring a virtual
> > link between Routers
> > RT7 and RT10 will give the backbone more connectivity and
> > more resistance to
> > such failures. "
> >
> > Anyway RT7 and RT10 are connected thru network-6 and all
> > the routers are
> > having this info so that when the link fails between
> > RT6-RT10 fails they can
> > still do the routing. What is the benifit of the Vlink?
> >
> > Can u pl. give me a scenario where I will have an
> > advantage of a VLink?
> >
> > Regards Subrat
>

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