Home > CCNP, ONT > ONT – Layer 2 Marking

ONT – Layer 2 Marking

Following the QoS basics, the next step is Classification and Marking.

Quick definition reminder:

  • Classification – Inspecting one or more aspects of a packet to see what the packet is carrying. This is a local process affecting one router.
  • Marking – Writing information to a packet to easily identify it on other network devices. This is a global process running on one router but affecting any router on the network.

Whenever possible we should use marking because it has big impact on the network performance. Big example for the benefits of marking is when marked traffic leave the LAN and enter the ISP’s cloud. Using classification the same traffic will get to the ISP and will have no information attached to it while marking pass the data to the ISP. In this example your ISP can apply priority and other rules on the data and offer you the service you need.

Two other terms we should be familiar with (both in the QoS spirit):
CoS = Class of Service => L2 marking used on Ethernet
ToS = Type of Service => L3 marking = 1byte (8bits)

Frame Relay DE bit: 0 or 1. value of 0 = no, value of  1 = maybe, possible.
DE = Discard Eligible. You can mark the traffic that will become DE.

MPLS Exp bits: similar to CoS.

Ethernet trunk CoS: 3 bits.
CoS work by using 3bits -> only working on trunk connections (isl \ dot1q)
Ethernet CoS can get to 8 levels of service:

000 = best effort
001 = low data -> like web traffic
010 = high data -> apps like Citrix that need constant connection
011 = voice signal -> hold music
100 = video -> video streaming
101 = voice -> top class of traffic
110 = reserved -> that is for routing update traffic, STP etc
111 = reserved -> used by the router by default

Summary: L2 marking  is stripped at every router and consume more resources. L3 marking is going all the way, marking on one router and using it all over the network. It is a much more important type of marking and will get its own (next) post.

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