Metro Ethernet and Carrier Ethernet services: E-Line, E-LAN and E-Tree

Why Ethernet moved into the WAN

For years, linking two offices meant serial circuits, special interface cards and protocols such as PPP or Frame Relay. Ethernet was the language of the LAN only. Metro Ethernet changed that: the provider hands you an ordinary Ethernet port (copper or fibre) at each site and carries your frames between sites over its own network. Your router or switch plugs in exactly as it would to a LAN switch.

The term started with city-wide ("metropolitan") fibre rings, but the same services now span whole countries, so the industry also calls it Carrier Ethernet. The MEF (formerly the Metro Ethernet Forum) defines the service names and attributes that most providers use, which makes quotes from different carriers easier to compare.

The building blocks

• UNI (user-network interface): the physical Ethernet port where your equipment meets the provider. It is the demarcation point.
• EVC (Ethernet virtual connection): the logical link that joins two or more UNIs. Frames can only travel between UNIs in the same EVC.
• Bandwidth profile: the speed you pay for, usually a CIR (committed information rate) that is guaranteed and sometimes an EIR (excess information rate) that is carried only if spare capacity exists. A 1 Gbps port with a 200 Mbps CIR is common.

The service types

Port-based services give one EVC per port and carry every frame, tagged or not, transparently. VLAN-based services map different VLAN IDs on the same port to different EVCs, so one head-office fibre can reach several branches over separate virtual circuits. That saves ports, but you must agree the VLAN IDs with the provider.

Inside the provider network, the frames usually ride over MPLS (for example VPLS or pseudowires) or over EVPN. You do not see that; you only see Ethernet at both ends.

Connecting your routers

An E-LAN behaves like one big switch, so all your routers share one subnet and can form routing adjacencies directly. With an EVPL you normally use a router sub-interface per VLAN. A head-office router reaching two branches over VLAN-based E-Lines might look like this:

interface GigabitEthernet0/0/1
 description Metro-E UNI to provider
 no ip address
interface GigabitEthernet0/0/1.110
 description EVPL to Branch-North
 encapsulation dot1Q 110
 ip address 10.250.110.1 255.255.255.252
interface GigabitEthernet0/0/1.120
 description EVPL to Branch-South
 encapsulation dot1Q 120
 ip address 10.250.120.1 255.255.255.252

Tip: the port may run at 1 Gbps while your CIR is much lower. Shape outbound traffic to the CIR on the router (for example with a hierarchical QoS policy) so the provider does not drop your excess frames at random. See QoS for voice, video and business applications.

Also check the MTU. Many providers support frames larger than 1500 bytes; if yours does not, routing protocols and VPN tunnels running over the service must allow for it.

Real-world example: a council joins its libraries

A district council runs a town hall and eight small libraries in the same county. They used to pay for eight separate leased lines into the town hall. When the contracts ended, the network team bought one E-Tree service: the town hall is the root on a 1 Gbps port with a 500 Mbps CIR, and each library is a leaf with 100 Mbps. Libraries reach the town hall's servers and internet breakout, but cannot reach each other directly, which suits their security policy. Adding a ninth library later only needed a new leaf on the existing EVC, not a new circuit into the town hall.

Strengths and limits

• Strengths: familiar Ethernet everywhere, high speeds, simple upgrades (often just a change of bandwidth profile), and a provider SLA.
• Limits: availability depends on fibre reaching your sites, the traffic is separated but not encrypted, and wide E-LANs make every site part of one Layer 2 domain, so keep routing at the edges.

Today Metro Ethernet is often one of several transports underneath an SD-WAN overlay; see SD-WAN vs MPLS vs IPsec VPN.