I was reading a product sheet about a Metro-Ethernet backhaul service which mentions, which is described as a mechanism to connect two datacentres within the same region (ie, city) via point-to-point Ethernet, deployed as an ‘MPLS Pseudowire’. It mentions that "the service is available at any of our on-net datacentres and in a choice of capacities including: 100Mbps, gigE and 10G – as well as fractional 10G."

What is a fractional port in this context? My understanding is that its a port split in multiple VLANs (trunk port) and the customer is assigned with a VLAN at a capacity that is a fraction of the original physical port capacity. For example, in that case the customer can get a 2G fractional port through VLANs mapped to the physical port?

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The fractional service is something less than the full line rate. The specific offerings of a carrier will vary by the carrier. For example, Carrier 1 may offer fractional 10 Gbps in increments of .5 Gbps, Carrier 2 in increments of 1 Gbps, Carrier 3 in increments of 2 Gbps, and Carrier 4 in 2, 5, 8 or 10 Gbps.

Some carriers will offer this as a layer-2 link on which you could connect the two sites with VLANs, and some require layer-3 connections. You need to discuss with a specific carrier what it offers.

In most cases with sub-rate connections, you need to properly implement QoS to take into account the actual throughput, otherwise your equipment will assume the physical interface is full line rate, and you can run into problems.

For example, if you have 2 Gbps service on a 10 Gbps interface, and you want VoIP to have a 20% priority queue, you need to make sure that the bandwidth statement on the interface is set to 2 Gbps, otherwise the VoIP could monopolize the link. In fact, VoIP has very small packets with a lot of overhead, so you probably want to set the bandwidth statement to something like 85% of 2 Gbps. The smaller the packets, the more overhead of packet headers, etc.

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