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NO.1 Which protocol is used to hide customer VLANs inside the provider backbone network?
A. 802.1ad
B. 802.1x
C. 802.1q
D. 802.1ap
Answer: A
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NO.2 Which type of VPN requires a full mesh of virtual circuits to provide optimal site-to-site
connectivity?
A. MPLS Layer 3 VPNs
B. Layer 2 overlay VPNs
C. GET VPNs
D. peer-to-peer VPNs
Answer: B
642-889 reference 642-889 examinations
Explanation:
http://etutorials.org/Networking/MPLS+VPN+Architectures/Part+2+MPLSbased+Virtual+Pri
vate+Networks/Chapter+7.+Virtual+Private+Network+VPN+Implementation+Options/Overl
ay+and+Peer-to-peer+VPN+Model/
Two VPN implementation models have gained widespread use: The overlay model, where the service
provider provides emulated leased lines to the customer. The service provider provides the customer
with a set of emulated leased lines. These leased lines are called VCs, which can be either constantly
available (PVCs) or established on demand (SVCs). The QoS guarantees in the overlay VPN model
usually are expressed in terms of bandwidth guaranteed on a certain VC (Committed Information
Rate or CIR) and maximum bandwidth available on a certain VC (Peak Information Rate or PIR). The
committed bandwidth guarantee usually is provided through the statistical nature of the Layer 2
service but depends on the overbooking strategy of the service provider The peer-to-peer model,
where the service provider and the customer exchange Layer 3 routing information and the provider
relays the data between the customer sites on the optimum path between the sites and without the
customer's involvement. The peer-to-peer VPN model was introduced a few years ago to alleviate the
drawbacks of the overlay VPN model. In the peer-to-peer model, the Provider Edge (PE) device is a
router (PE-router) that directly exchanges routing information with the CPE router. The Managed
Network service offered by many service providers, where the service provider also manages the CPE
devices, is not relevant to this discussion because it's only a repackaging of another service. The
Managed Network provider concurrently assumes the role of the VPN service provider (providing the
VPN infrastructure) and part of the VPN customer role (managing the CPE device).
The peer-to-peer model provides a number of advantages over the traditional overlay model: Routing
(from the customer's perspective) becomes exceedingly simple, as the customer router exchanges
routing information with only one (or a few) PE-router, whereas in the overlay VPN network, the
number of neighbor routers can grow to a large number. Routing between the customer sites is
always optimal, as the provider routers know the customer's network topology and can thus establish
optimum inter-site routing. Bandwidth provisioning is simpler because the customer has to specify
only the inbound and outbound bandwidths for each site (Committed Access Rate [CAR] and
Committed Delivery Rate [CDR]) and not the exact site-to-site traffic profile. The addition of a new
site is simpler because the service provider provisions only an additional site and changes the
configuration on the attached PE-router. Under the overlay VPN model, the service provider must
provision a whole set of VCs leading from that site to other sites of the customer VPN.
Prior to an MPLS-based VPN implementation, two implementation options existed for the peer-to-
peer VPN model: The shared-router approach, where several VPN customers share the same PE-
router. The dedicated-router approach, where each VPN customer has dedicated PE-routers. Overlay
VPN paradigm has a number of drawbacks, most significant of them being the need for the customer
to establish point-to-point links or virtual circuits between sites. The formula to calculate how many
point-to-point links or virtual circuits you need in the worst case is ((n)(n-1))/2, where n is the
number of sites you need to connect. For example, if you need to have full-mesh connectivity
between 4 sites, you will need a total of 6 point-topoint links or virtual circuits. To overcome this
drawback and provide the customer with optimum data transport across the Service Provider
backbone, the peer-to-peer VPN concept was introduced where the Service Provider actively
participates in the customer routing, accepting customer routes, transporting them across the Service
Provider backbone and finally propagating them to other customer sites.
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