Ethernet Virtual Private Line - Cisco ASR 9000 Series Configuration Manuallines

Ethernet Virtual Private Line

Ethernet Virtual Private Line
Ethernet Virtual Private Line (EVPL) is similar to EWS in that it offers point-to-point connectivity. The key
differentiation between EWS and EVPL is that an EVPL uses a VLAN tag to multiplex several,
non-same-destination pseudowires to one port. That is, unlike EPL and EWS, EVPL is a one-to-many
multiplexed service. Service multiplexing simply means that multiple pseudowires utilize a single access
interface or UNI. These circuits can terminate within an L2VPN or on, for example, an Internet gateway.
From the service user's perspective, this service multiplexing capability offers more efficient interface utilization,
simplification of cable plant, and reduced maintenance costs associated with additional interfaces.
Using the same example as above, where a router connects to n other routers, the source router only needs
one port for the service instead of n, as is the case with an EWS. The service need not be port-to-port, but can
be logical-pseudowire-to-logical-pseudowire. In the case of an EVPL, each circuit can terminate at a different
remote location, whereas using EWS, all frames are mapped to a single circuit and therefore a single egress
point.
Figure 2: EVPL Service Multiplexing Example: One Port (Left) Can Be Used for All Destinations (Right)
Like Frame Relay, EVPL allows a customer device to access multiple connections through a single physical
port attached to the service provider network. The service offered by EVPL can be thought of as being similar
in concept to Frame Relay, in that a VLAN number is used as a virtual circuit identifier in a similar fashion
to Frame Relay data link connection identifier (DLCI). Unlike EWS, EVPL does not forward BPDUs, because
IEEE 802.1Q (VLAN tagging) only sends BPDUs on a default VLAN. In a hub-and-spoke network, only one
spoke at most would receive BPDUs, thus breaking the spanning tree in the rest of the network. Therefore,
an EVPL does not transmit any BPDUs and runs routing protocols instead of Ethernet Spanning Tree. The
routing protocols give the customer and provider greater flexibility, traffic determination characteristics, and
value-added services.
Ethernet LAN Service
An Ethernet LAN Service (E-LAN) differs from EWS and ERS in that an E-LAN provides a multipoint
connectivity model. It should be noted that an E-LAN service definition is still under review within the IETF
Multipoint Layer 2 Services working group. Although E-LAN uses a multipoint model, it can forward unicast
packets to single destinations; that is, it also supports point-to-point connections. To the end user, the network
looks like a giant Ethernet switch where each customer has their own VLAN or broadcast domain, rather than
end-to-end pseudowire link(s).
E-LAN Example
An E-LAN does not map an interface or VLAN to a specific point-to-point pseudowire. Instead, it models
the operation of a virtual Ethernet switch: E-LAN uses the customer's MAC address to forward frames to the
correct egress UNI within the service provider's network. An E-LAN emulates the service attributes of an
Ethernet switch and learns source MAC to interface associations, floods unknown broadcast and multicast
frames, and (optionally) monitors the service user's spanning tree protocol. One important point to note is that
although the service provider may utilize spanning tree within the transport network, there is no interaction
with the service user's spanning tree.
This service works similar to an MPLS VPN, except it functions at L2 instead of L3. While a VPLS E-LAN
is a viable solution, its scalability and QoS control are suspect compared to that of MPLS VPNs. In addition,
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
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The Carrier Ethernet Model