OSPF Sham Links are required when you try to use a backdoor link between two CE routers in an MPLS VPN PE CE scenario where you use OSPF as the PE-CE routing protocol. This is best explained with an example, take a look at the following topology:
I go with C and not D. According to this article we may not even need OSPF in the MPLS underlay; look at the configuration example at the end of the document. We just need reachability between the two looback interfaces. However this reachability doesn't need to be provided by OSPF in the backbone (PE to PE). Instead we have reachability between the two loopbacks, and placing them in ospf vrf address-family, the same vrf pointing to the CE routers (implying PE to CE ospf). The answer seems to be C.
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/xe-16/iro-xe-16-book/iro-sham-link.html
In an MPLS network, the Provider Edge (PE) routers are typically located in different OSPF areas, with the Provider (P) routers in between them. To enable OSPF routing between the PE routers, a sham-link is created across the MPLS network, connecting the two PE routers and simulating a direct link between them.
You are wrong my friend la awser correct is C:
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/xe-16/iro-xe-16-book/iro-sham-link.html
In that same document:
A sham-link overcomes the OSPF default behavior for selecting an intra-area backdoor route between VPN sites instead of an interarea (PE-to-PE) route. A sham-link ensures that OSPF client sites that share a backdoor link can communicate over the MPLS VPN backbone and participate in VPN services.
Actually, the correct answer is B. The purpose of an OSPF sham-link is to allow intra-area routing when OSPF is used as the PE-CE connection protocol in an MPLS VPN network. This ensures that OSPF treats the paths across the MPLS VPN backbone as intra-area paths, facilitating the exchange of routing information as if the routers were directly connected.
B is correct because sham-links fundamentally enable intra-area routing between PE routers across the MPLS VPN.
C is also correct because this intra-area routing is the mechanism used to prevent backdoor routing.
The reason why B might be considered more accurate is that it focuses on the core function of the sham-link (enabling intra-area routing), while C describes the consequence of that function (preventing backdoor routing).
C is correct
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/xe-16/iro-xe-16-book/iro-sham-link.html#GUID-663C82AA-E6A1-4D95-9EAE-ED1E41E43F78:~:text=Although%20OSPF%20PE,based%20on%20policy.
Answer cannot be D sham link is used because the CE prefer the backdoor link that the CE got instead of the MPLS itself because when you use MPLS OSPF is a link state protocol, the LSAs that the PE will transport to the other end are gonna be TYPE 3 LSAs the CE backdoor link will be TYPE 1 LSA so you will prefer the backdoor link.
Maybe you are asking yourself why do OSPF will send LSA of type 3 instead of 1? The extened communities are not doing their job? Answer is yes they are, the point is that OSPF is smart! It undertand that there is something strange there and it will understand that the area 0 between the two CE routers is not a common area it smells that there is mpbgp there.... So this area will be transformed in a SUPERBACKBONE AREA. By default this area will send type 3 LSAs even if it is an area 0
I said something wrong there sorry, the ospf area 0 of course is between the PE and the CE and then the PE is going to redistribute the OSPF routes into mpls using multiprotocol BGP.
Vice versa the OSPF routes of the other end are gonna be received via multiprotocol bgp and then redistributed in OSPF from the PE and then the PE is going to send the OSPF packet to the CE (the PE and the CE have an OSPF or adj between them of course).
Hope that I explained well the concept, sorry it was late night ;)
For solve this there is sham link, basically you create a sort of virtual link that is gonna hide the fact that you are in a backbone super area, so that the LSAs that will be sent are gonna be TYPE 1 LSAs
Its c, key is on the question because this feature is used to overcome the backdoor issue. Where is that backdoor issue? between the customer and the PE.
Answer E : Sham link allowed between PE & CE only and not PE to PE.
Chat GPT answer :
No, OSPF sham links are typically used within an OSPF routing domain to connect non-adjacent areas through the backbone area. Provider Edge (PE) routers, on the other hand, are routers located at the edge of a service provider's network and are responsible for connecting customer networks to the provider's network.
Sham links are generally not used between PE routers because they are not part of the OSPF routing domain within the provider's network. The PE routers are typically running a different routing protocol, such as Border Gateway Protocol (BGP), to exchange routing information with other provider routers and establish connectivity between customer networks.
The purpose of sham links is to establish connectivity and enable OSPF routing between non-adjacent areas within the OSPF routing domain. PE routers, being at the edge of the network, are not within the OSPF domain itself and are usually responsible for exchanging routing information with other networks using different protocols like BGP.
From Cisco
A sham-link overcomes the OSPF default behavior for selecting an intra-area backdoor route between VPN sites instead of an interarea (PE-to-PE) route
The sham link is a logical link, similar to a virtual link. It allows you to create a point-to-point connection between the two PE routers. The PE routers are then able to flood LSAs across the MPLS VPN backbone. You don't have to configure anything on the CE routers.
team 100% is C
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/xe-16/iro-xe-16-book/iro-sham-link.html#GUID-663C82AA-E6A1-4D95-9EAE-ED1E41E43F78
Answer is c, the shame link is configured between 2 PE routers to correct the backdoor routing between PE-CE.
check below for more description.
https://networklessons.com/mpls/mpls-layer-3-vpn-pe-ce-ospf-sham-link
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