On Metro Ethernet Services: TLS, DVS, TSL and VLANs

Which service provider metro Ethernet service uses the customer's VLAN IDs to map to a destination site?

1. TLS
2. DVS
3. ISL
4. VLAN

Direct from Transcender, my daily Cisco question source. First step: decrypt the alphabet soup.

TLS is the acronym for Transparent LAN Services, one of the services defined in the Metro Ethernet Services framework. According to the Cisco definition, it "... provides Ethernet connectivity among geographically separated customer locations, creating a VLAN that spans those locations". It's a simple service that gives the user a direct Ethernet connection between the sites, as if all sites were physically connected in the same LAN. The user does not need to know that there is a service provider-owned network, and also, he isn't required to deploy routers or other typical WAN-oriented equipment at their premises; a direct connection to the LAN switch at every location is enough.

As a side note, TLS is also the acronym for Transport Layer Security, and is part of the SSL framework (in fact, TLS is the IETF name for SSL, but for historic reasons, everyone just says "SSL"). Be warned about the potential confusion.

DVS, or Direct VLAN Services, is a term used by Cisco to describe a slightly more complex TLS-like service. In this service, the endpoints see the service provider network as a single LAN switch, with support for VLANs. Note the difference: in a plain TLS, the curstomer gets a single LAN segment, with no support for VLAN switching. In a DVS, the customer VLANs are carried by the service provider, in such a way that separate VLANs can be transported over the Metro Ethernet network.

Note: It seems that the term DVS is being deprecated in favor of the more generic TLS name, whsoe meaning was expanded to include VLAN support. It makes sense, because it's one acronym less to remember, and also because nowadays any decent Metro Ethernet switch will support DVS anyway. One evidence is that the DVS name is not used in the Cisco documentation anymore, with the exception of the certification material.

ISL (Inter Switch Link) is a old Cisco name for a pre-802.1q VLAN trunking mechanism. It was superseded by 802.1q compliant trunks, but it's still supported by Cisco. For the most part, both protocols have comparable features. It's sometimes useful to deploy ISL inside the service provider network to configure non-802.1q trunks, which are useful to avoid some kinds of side effects in situations where VLAN Tunneling, or 1q-in-1q, is being deployed. Cisco has extensive design tips for this scenario in the 1q-in-1q-related chapters for its switches.

VLAN is the acronym for Virtual LAN, as defined by the IEEE 802.1q standard. Iit's now a standard feature for any professional switch, including most low-end devices. VLANs allow for network segmentation, and it's useful to deploy separate logical LANs over a single network infrastructure.

Now for the question at hand. The answer is (b): DVS, which uses the customer VLAN info to deliver Ethernet frames for each endpoint. (a) is a simpler service, which is not able to use the customer VLAN info. (c) and (d) are terms which define some of the technologies involved, but are not Metro Ethernet service names.