Westermo RedFox Series - Manual (Page 167)

Westermo OS Management Guide
Version 4.17.0-0
8.1.3 Flow control
The ports can be configured to use flow control, i.e., to dynamically limit inbound
traffic to avoid congestion on outbound ports.
When flow control is enabled on a full duplex port, the switch will send pause
frames (IEEE 802.3x) to limit inbound traffic on this port, if that traffic is causing
congestion when sent out on another switch port.
When flow control is enabled on a half duplex port, the switch will use a tech-
nique known as back-pressure to limit inbound traffic on this port, if that traffic
is causing congestion when sent out on another switch port. (The back-pressure
technique enables a switch to force its neighbour to slow down by sending jam-
ming signals on that port, thus emulating a packet collision.)
8.1.4 Layer-2 priority support
Each Ethernet port has four output queues, enabling layer-2 priority support with
four traffic classes. The queues are serviced according to strict priority schedul-
ing, i.e., when there are traffic in multiple queues, the packets in the queue with
higher priority is serviced first.
A packet’s priority is determined when it enters on a port, and can be classified
based on:
❼ VLAN ID: The switch can be configured to give specific priority to certain
VLANs. This can be useful to, e.g., when providing IP telephony via a dedi-
cated VLAN. Priority based on VLAN ID has precedence over all priority clas-
sifications described below.
VLAN ID priority settings are further described in chapter 13 .
❼ 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 sections 8.2 and 8.3 ).
❼ 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). Classification based
on the IP ToS/Diffserv bits can be useful to provide higher priority to de-
lay sensitive applications, such as IP telephony and remote login, than to
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