What Is 802.1p?


802.1p refers to an IEEE specification for giving Layer 2 switches the ability to prioritize traffic as well as perform dynamic multicast filtering. Basically, it provides a mechanism for implementing Quality of Service at the MAC (Media Access Level) level. Quality of Service (abbreviated QoS) is a mechanism that allows for better handling of data that passes through the network.

The prioritizing specification works at the MAC framing layer of the Open System Interconnection (OSI) model. Layer 2 switches, to be compliant with 802.1p, must have the capability of grouping incoming Local Area Network (LAN) packets into separate traffic classes. So far, there are eight classes defined by the 802.1p and are expressed through the 3-bit-user priority field in an IEEE 802.1Q header under the frame. The manner in which the traffic is treated when assigned to any particular class is undefined and left to implementation, and therefore the network managers must determine the actual mappings. The highest priority is seven, and thus it might go to network-critical traffic such as Routing Information Protocol and Open Shortest Path First table updates.


Other priorities represented by values five and six might be for delay-sensitive applications such as interactive video and voice. Classes represented by four to one range from controlled load applications like the streaming multimedia and business critical traffic (such as that carrying SAP data, for example) down to “loss eligible” traffic. The last value (zero) is used as best-effort default, invoked automatically when no other value is set. When in operation, 802.1p requires the use of priority fields within the packet to signal the switch of the priority-handling requirements.


This is how the priority is set for 802.1p (this specifically is for the HP station). The IP packets are classified and marked into different priorities levels. The markings are then transported through a type of service (ToS) octet in the IPv4 header and a traffic class field in the IPv4 header. The HP-UX end station transmits the IPv4 ToS values but it does not enforce priority. The end station performs the ToS-to- 802.1p conversion and vice versa for the IP packets, and this depends on how the VLAN overrides are set. The station also allows 802.1p priority settings for non-IP packets.


As already alluded to before, the priority may be set by the user, or by the VLAN. And the same case applies with regard to destination address, input port, output port and access priority. The table below shows the eight levels of IEEE 802.1precommended user priorities.
IEEE 802.1p USER PRIORITY. TRAFFIC TYPE.
7 Network Management
6 Voice.
5 Video
4 Controlled load
3 Excellent effort
0 (routinetraffic) Best effort
2 Undefined
1 (lowest) Background

Its good to note that 802.1p is not backward compatible and can lead to instability on networks with non-802 .1p switches. This happens because older switches are likely to misinterpret th header used by the 802.1p protocol. So the network administrator should ensure that the switches, the Ethernet cards and device drivers are all 802.1p compatible.

 

Patricia Johnson writes on topics such as www.tech-faq.com/802.1p.shtml">802.1p , www.tech-faq.com/mac-address.shtml">MAC Address and www.tech-faq.com/token-ring.shtml">Token Ring for The Tech FAQ.

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