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Exam Name: Implementing Cisco Service Provider Next-Generation Egde Network Services
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NO.1 A network engineer is tasked to implement an AToM VPN for a given
customer to emulate a
Frame Relay virtual circuit over the MPLS-enabled core
network. Which command enables Frame
Relay to forward frames from the
attachment circuit over the emulated session in regular Cisco
IOS
Software?
A. frame-relay intf-type dce
B. encapsulation
frame-relay
C. ip route 0.0.0.0 0.0.0.0 tailend_router_ip
D. frame-relay
switching
Answer: D
642-889 Real Questions 642-889 Training
Materials
NO.2 Which type of VPN requires a full mesh of virtual
circuits to provide optimal site-to-site
connectivity?
A. peer-to-peer
VPNs
B. Layer 2 overlay VPNs
C. GET VPNs
D. MPLS Layer 3
VPNs
Answer: B
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Tests
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.