Re: OSPF Problem

A virtual-link is an extension of area 0 to solve a few specific problems --
bridging together multiple area 0 backbones or bridging area 0 together with
an area not directly connected to area 0

A sham-link is sort of like a virtual-link through an MPLS service provider
and is there to solve a different problem -- When you run OSPF as a CE/PE
routing protocol the LSAs you receive on your CE will be either inter-area
type 3 LSAs or external type 5 LSAs depending on configuration of the OSPF
domain ID. The rules of OSPF dictate that intra-area routes are always
preferred over inter-area and external routes. Thus if you have a backup
"back door" link between your sites, but you want traffic to flow over the
MPLS by default you have a problem. The Sham-Link fixes that specific
problem by allowing your CE to see the inter site routes as intra area LSAs
instead of inter-area or external LSAs.

If I remember correctly, the sham-link does not HAVE to be in area 0
though....it can be a member of any area you tell it to be. I believe you
are just extending that area across the service provider cloud, but I MIGHT
be wrong on that. I would have to double check. Perhaps somebody else can
elaborate on that point.

On Tue, Jul 5, 2011 at 11:17 AM, Routing Freak <routingfreak_at_gmail.com>wrote:

> Hey mates,
> I have a little confusion. What is the difference between Sham link and
> Virtual Link?
>
> I know that Virtual Link is to extend ur backbone
> Sham link is used as a false link between two PE's . Is it extending ur
> Area to other area and fool them that they r in the same area??
>
> Because Sham link mainly used as a virtual cable which sends Intra area
> LSA's ( which is the Type 1 LSA's) to other PE .
>
> What does exactly the difference betwen Virtual links and Sham links
>
>
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Regards,
Joe Astorino
CCIE #24347
Blog: http://astorinonetworks.com
"He not busy being born is busy dying" - Dylan
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