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22. Transparent LAN Services (TLS)
Introduction
Service providers are discovering significant new revenue opportunities with Layer 2 services
that extend customer LANs across geographically dispersed sites. Using metro Ethernet
technology, service providers can offer services that connect multiple enterprise customer
offices at Ethernet’s 10-Mbps up to 1-Gbps LAN speeds.
Deploying these services called "Transparent LAN Services (TLS)" requires network
operators to transport a large number of customers’ virtual LANs (VLANs) while keeping
traffic in each VLAN secured from other customers’ VLANs. To do so, they can use the TLS
feature, which segregates VLANs in a way that also overcomes management and scalability
obstacles.
Large business customers usually create 802.1Q VLANs within their enterprises to segregate
certain traffic flows running across their Ethernet inter-networks. They define a certain
number of internal VLANs, which act like “Ethernet tunnels,” to separate groups of users (the
engineering department from the corporate marketing department, for example). Each VLAN
supports a population of users with something in common, such as those who share the same
set of network access rights.
Service providers can use the TLC feature to offer services that provide the same high-speed,
VLAN-based experience that customers enjoy in the LAN across the metropolitan-area
network (MAN) and the WAN. The TLC feature adds a VLAN header to the packet with
EtherType field that is different from the 802.1Q tag of the customer traffic in the switch at
the edge of the service provider’s network.
If the switch receives an untagged packet, it adds a tag header to the packet with EtherType
that is not the standard EtherType (0x8100). If the switch receives a packet with a tag header,
it adds another tag header with EtherType that is not the standard EtherType (0x8100).
Feature Overview
Transparent LAN Services (TLS) implies Layer 2 connectivity offered by a service provider
to multiple customer sites in a manner that is transparent to the customer edge (CE) devices.
The switch is positioned on the edge of the provider network. It is connected to customer edge
(CE) switches, and interfaces to the provider network.
To provide TLS, Ethernet frames originated by the source CE switch are received at the
provider edge (PE) switch, encapsulated and transported across the provider network, where
the PE switch removes the encapsulation and delivers the unmodified frame to the destination
CE switch (see Figure 22-1).
