October 2012
Intermediate to advanced
419 pages
10h 47m
English
Content preview from Arista WarriorBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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Chapter 17. Quality of Service
Quality of Service (QoS) on a switch can be a confusing subject, especially if you’re used to dealing with things like low-latency queuing on Cisco routers. It doesn’t help that a lot of the documentation for QoS on switches isn’t great (from any vendor), so I’m going to try my best to explain it here in a way that’s easy to understand.
QoS on an Arista (and many other) switches is different than it is on a router because, aside from the obvious fact that switches aren’t routers, they operate primarily at layer-2. When dealing with routers, we usually work with either IP precedence or Differentiated Services Code Point (DSCP) fields, both of which reside in the layer-3 headers of packets. Switches operate primarily at layer-2, and while typical Ethernet packets don’t include a field for QoS, packets encoded with 802.1Q do. When an Ethernet frame includes the 802.1Q tag, we usually think of it as being VLAN tagged for use in a trunk, but these tags have other uses as well.
The 802.1Q tag is not just a VLAN tag. While I’m not usually a fan of digging into packet formats, it’s important to understand the method used for tagging CoS into non-VLAN-tagged frames. The entire 802.1Q tag is four bytes in length, and includes the following four fields:
- Tag Protocol Identifier (TPID)
This 16-bit field contains the value 0v 0x8100 to identify the frame as 802.1Q.
- Priority Code Point (PCP)
A three-bit field used for frame priority. This field contains the values used ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.