It is an extension of the area. It extends the flooding domain, which in turn changes the route calculation process. Not only does this modify which paths will be preferred, but it also affects how changes in the OSPF topology at one VPN site will affect the others.
Normally the MPLS network is considered the "super backbone" for OSPF, because even if the PE-CE links are running OSPF area 0, the flooding domain is not extended over the MPLS network. This means from the perspective of the CE or C routers, the PEs look like ABRs. They originate either Type 3 Network Summary LSAs, Type 4 ASBR Summary LSAs, or Type 5 External LSAs to describe other VPN sites.
The key is that in OSPF design, the ABR is in charge of topology summarization. If a change in the OSPF topology occurs at VPN site 1, it will not necessarily result in an SPF run at other VPN sites. Instead it results in what's known as a Partial SPF or Partial Route Calculation (PRC). This is why OSPF is sometimes described as distance vector between areas, while it is only link-state inside the area. When a sham-link is used to extend the area over MPLS though, this behavior changes.
Per the RFC "The sham link is an unnumbered point-to-point intra-area link and is advertised as a type 1 link in a type 1 LSA." This means that the sham-link is considered a normal portion of the SPF graph of the area, and must be used for flooding. Like a virtual-link, the sham-link runs as a demand circuit. This means that periodic flooding will not occur, but normal flooding must occur if there is a change in the topology.
The end result of this is that sham-links, just like virtual-links, limit the scalability of the network. If you were to run sham-links between all of your sites it would basically be the same as running one flat area everywhere. Changes anywhere in the topology will require an SPF run, or at least an iSPF run, everywhere in the network.
Brian McGahan, CCIE #8593 (R&S/SP/Security)
bmcgahan_at_INE.com
Internetwork Expert, Inc.
http://www.INE.com
-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of -Hammer-
Sent: Tuesday, July 05, 2011 11:00 AM
To: Routing Freak
Cc: ccielab_at_groupstudy.com
Subject: Re: OSPF Problem
It's probably not the best answer but I hope it helps. Lastly, you keep referencing (for the sham link) extending a non-backbone to the backbone. I'm pretty sure a sham link is between two routers regardless of backbone state. In other words, it's not like a virtual link where your whole purpose is to get to area 0. In a sham link, you are just making routes available between two PE routers regardless of where area
0 is. Sham links are not depending on particular OSPF areas as much as they are tied to specific OSPF processes.
Anyone jump in if I'm misunderstanding this.
-Hammer-
"I was a normal American nerd"
-Jack Herer
On 07/05/2011 10:56 AM, Routing Freak wrote:
> hmm Thanks Hammer for ur valuable info
>
> I m looking for some GREAT's response
>
> On Tue, Jul 5, 2011 at 9:23 PM, -Hammer- <bhmccie_at_gmail.com
> <mailto:bhmccie_at_gmail.com>> wrote:
>
> There are certainly more qualified folks here that may speak up
> but I do not look at it as extending a non backbone link. I look
> at it as a method to enforce more preferred (the cloud) routing
> between PEs. I look at it more or less as a pvc or a tunnel
> between the PEs. I'm sure that's not the best way but it works for
> me to think thru it.
>
> -Hammer-
>
> "I was a normal American nerd"
> -Jack Herer
>
>
>
>
> On 07/05/2011 10:48 AM, Routing Freak wrote:
>> Hey Hammer, Good one
>>
>> But stilll I hava a problem, Is sham link extending ur non
>> backbone link to the backbone link ( ie ur Super Backbone)
>>
>>
>> On Tue, Jul 5, 2011 at 9:12 PM, -Hammer- <bhmccie_at_gmail.com
>> <mailto:bhmccie_at_gmail.com>> wrote:
>>
>> Dude I didn't mean to simplify it. You were asking for the
>> difference and I thought a clearer explanation might have
>> helped. I don't look at these as being in the same realm and
>> that's why maybe I am missing where you are confused. Yes,
>> V-Links are an "extension" of area 0. Whereas sham links are
>> used to "bridge" together multiple PEs in the same MPLS VPN
>> backbone. The sham link gets you past the default OSPF rules
>> for selecting intra-area routes instead of inter-area. The
>> sham link also allows the sites to communicate over the MPLS
>> VPN instead of via any external paths not provided via the
>> cloud. Does that help?
>>
>> -Hammer-
>>
>> "I was a normal American nerd"
>> -Jack Herer
>>
>>
>>
>>
>> On 07/05/2011 10:33 AM, Routing Freak wrote:
>>> Dude i know how to configure ospf sham link . I need to know
>>> about the difference technically speaking differences
>>>
>>> Share some theory behind both, What r the similarities and
>>> what r the differences?
>>>
>>>
>>> On Tue, Jul 5, 2011 at 8:55 PM, -Hammer- <bhmccie_at_gmail.com
>>> <mailto:bhmccie_at_gmail.com>> wrote:
>>>
>>> This helped me understand sham links better a while back....
>>>
>>>
>>> http://blog.ipexpert.com/2010/01/20/introduction-to-ospf-sham-link/
>>>
>>> -Hammer-
>>>
>>> "I was a normal American nerd"
>>> -Jack Herer
>>>
>>>
>>>
>>> On 07/05/2011 10:17 AM, Routing Freak 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
>>> >
>>> >
>>> > Blogs and organic groups at http://www.ccie.net
>>> >
>>> >
>>> _______________________________________________________________________
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>>>
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>>>
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Received on Tue Jul 05 2011 - 11:50:07 ART
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