Re: OSPF Problem

A virtual-link is a unicast area 0 adjacency between two ABRs using a non-backbone transit area. A sham-link is a unicast OSPF adjacency of an area of your choice between two PEs using VPNv4 BGP learned routes and MPLS for transit.

The sham link is used in cases where a backdoor link exists between two L3VPN sites in MPLS where OSPF is used as the PE to CE protocol. Due to the encoding of the OSPF LSA types in a VPNv4 update, the backdoor link would be preferred over the MPLS network in most cases. The sham link works around this problem by extending the flooding domain of an OSPF area over the MPLS network. Normally the flooding domain terminates at the PE-CE link.

Check section 4.2.7 of http://www.faqs.org/rfcs/rfc4577.html for the exact specification of sham links.

HTH,

Brian McGahan, CCIE #8593 (R&S/SP/Security)
bmcgahan_at_INE.com
 
Internetwork Expert, Inc.
http://www.INE.com

On Jul 5, 2011, at 10:44 AM, "Routing Freak" <routingfreak_at_gmail.com> 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> 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
>>>
>>>
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Received on Tue Jul 05 2011 - 10:58:35 ART