Ethernet port configuration
This article explains the available settings for Ethernet Ports found e.g. in the New Switch dialog window, see the image below. We begin with discussing some relevant information related to Ethernet layer-2 priority support:
When an Ethernet frame enters an Ethernet Switch via an ingress port, one of the first things that happens is that the ingress interface associates a priority with that particular packet. Depending on the Switch port configuration, the priority is selected from one of the following sources:
VLAN ID: A switch can be configured to give a specific priority to the traffic assigned to a specific VLAN. For example, in an ordinary office network, the network administrator could assign all IP telephony traffic to a dedicated VLAN and give it a higher priority for improved user quality of experience.
Priority based on VLAN ID has precedence over all priority classifications described below.
VLAN tag: For packets carrying a VLAN tag, the packet’s priority can be based on content of the priority bits inside the VLAN tag. The VLAN tag is useful to carry packet priority information on inter-switch links. Use of VLAN tag priority can be configured per port (see below).
IP ToS/DiffServ: For IP packets, the priority can be classified based on the content of the IP ToS bits (IPv4) or the IP TC bits (IPv6). In a traditional office network, classification based on the IP ToS/Diffserv bits could typically be used to provide higher priority to delay sensitive applications, such as IP telephony and remote login, than to bulk data applications such as file transfer. However, a requirement would be that that those applications could set the IP ToS/Diffserv bits appropriately.
Use of IP ToS/DiffServ priority can be configured per port (see below).Port Priority: Priority can be classified based on the inbound Switch port.
Use of port priority can be configured per port. Furthermore, when priority classification is configured to be based on VLAN tag (or IP ToS/DiffServ), priority will be based on the port priority for untagged (or non-IP respectively) packets.
The priority assigned to a packet will take a value in the range 0-7 and be represented by 3 bits (IEEE 802.1p). If the egress interface of an Ethernet port has, for example, only four traffic class queues, a 802.1p priority (8 values) must be mapped to a traffic class corresponding to one of the four output queues. Different priority to traffic class mappings are used depending on the selected scheduling policy of the port. These mappings are shown in the tables below.
The following mappings are currently static and cannot be reconfigured. However, in an upcoming release, the traffic class tables will be made manageable, so that you can define your own mappings.
Scheduling Policy: Strict Priority Scheduling (SPS)
IEEE 802.1P priority | Queue number / Traffic class |
|---|---|
0, 1 | 0 (lowest) |
2, 3 | 1 |
4, 5 | 2 |
6, 7 | 3 (highest) |
Scheduling Policy: Audio Video Bridging (AVB)
IEEE 802.1P priority | Queue number / Traffic class |
|---|---|
0, 1, 2, 3 | 0 (lowest) |
4 | 2 |
5 | 3 (highest) |
6, 7 | 1 |
Scheduling Policy: Time-Aware Shaper (TAS)
IEEE 802.1P priority | Queue number / Traffic class |
|---|---|
0, 1, 2, 3, 4 | 0 (lowest) |
5 | 1 |
6 | 2 |
7 | 3 (highest) |
The priority selection of a Switch port is governed by the two fields Port Priority and Port Priority Mode, see below.
After a priority has been associated with a newly arrived Ethernet frame, the frame is then routed to the appropriate output port via an internal switch fabric. In the egress interface of the output port, the priority associated with the Ethernet frame determines into which traffic class queue the Ethernet packet is placed.
We now go through the individual configuration parameters of an Ethernet port that are available, for example, in the New Switch dialog window:
Speed
The available Ethernet port speeds are currently 10 Mbit/s, 100 Mbit/s, 1 Gbit/s and 5 Gbit/s.
Priority
Specifies the (IEEE 802.1p) priority associated with the port. Packets coming in on a specific port will receive this priority unless priority is based on VLAN ID, VLAN tag or IP ToS/DiffServ bits, see Port Priority Mode below.
Mode
The port priority mode. Determines if the priority classification for this port is based on content of VLAN tag (IEEE 802.1p priority bits), content of IP ToS/Diffserv bits, or the port priority configured for this port.
VLAN priority settings will have precedence over port priority mode settings.
tag: (Default) The packet’s priority is based on the content of the VLAN tag (802.1p priority bits) of the incoming packet. For packets coming in untagged, the priority is based on the priority associated with the port, see Port Priority above.
ip: The packet’s priority is based on the content of the IP ToS/Diffserv bit of the incoming packet. For non-IP packets coming in on the port (e.g., ARP packets), the priority is based on the priority associated with the port, see Port Priority above.
port: The packet’s priority is based on the priority associated with the port, see Port Priority above.
Scheduling Policy
Supported scheduling policies are currently Strict Priority Scheduling (SPS), the Credit Based Shaper (CBS) algorithm used by Ethernet AVB switches and Time-Aware Shaper (TAS) for scheduled traffic.
Queue Count
The number of available traffic class queues in the egress interface of a port. Currently, we recommend limiting the number of queues to four.
Fraction A and B
These are the bandwidth fractions that are reserved for stream reservation (SR) classes, which are traffic classes whose bandwidth can be reserved for audio and video (AV) traffic.
When the Credit Based Shaper algorithm is the chosen scheduling policy, packets belonging to SR class A and B are currently expected to be assigned priorities 5 and 4, respectively, so you need to specify the reserved bandwidth fraction as a percentage of the total bandwidth for queue number 3 (Fraction A) and queue number 2 (Fraction B) when Queue Count is 4 and queue number 5 (Fraction A) and queue number 4 (Fraction B) when Queue Count is 8.
When the Time-Aware Shaper algorithm is selected as the frame selection transmission algorithm, packets belonging to SR class A and B are currently expected to be assigned priorities 6 and 5, respectively, so you need to specify the reserved bandwidth fraction as a percentage of the total bandwidth for queue number 2 (Fraction A) and queue number 1 (Fraction B) when Queue Count is 4 and queue number 6 (Fraction A) and queue number 5 (Fraction B) when Queue Count is 8.