Network load control
10.4
QoS/Priority
124
UM Config GRS
Release
8.0
09/2019
For data packets with VLAN tags, the device evaluates the following information:
Priority information
When VLANs are configured, VLAN tagging
Figure 21: Structure of the VLAN tagging
Data packets with VLAN tags containing priority information but no VLAN information (VLAN ID =
0), are known as Priority Tagged Frames.
Note:
Network protocols and redundancy mechanisms use the highest traffic class 7. Therefore,
select other traffic classes for application data.
When using VLAN prioritizing, consider the following special features:
End-to-end prioritizing requires the VLAN tags to be transmitted to the entire network. The
prerequisite is that every network component is VLAN-capable.
Routers are not able to send and receive packets with VLAN tags through port-based router
interfaces.
10.4.4
IP ToS (Type of Service)
The Type-of-Service field (ToS) in the IP header was already part of the IP protocol from the start,
and is used to differentiate different services in IP networks. Even back then, there were ideas
about differentiated treatment of IP packets, due to the limited bandwidth available and the
unreliable connection paths. Because of the continuous increase in the available bandwidth, there
was no need to use the ToS field.
Only with the real-time requirements of today’s networks has the ToS field become significant
again. Selecting the ToS byte of the IP header enables you to differentiate between different
services. However, this field is not widely used in practice.
t
4 Octets
User Priority
, 3 Bit
Canonical Forma
t Id
entifier
1 Bit
VLAN Identifier
12 Bit
Tag Protocol Identifier
2 x 8 Bit
Table 18: ToS field in the IP header
Bits (0-2): IP Precedence Defined Bits (3-6): Type of Service
Defined
Bit (7)
111 - Network Control
0000 - [all normal]
0 - Zero
110 - Internetwork Control
1000 - [minimize delay]
101 - CRITIC / ECP
0100 - [maximize throughput
Bits
0
1
3
2
4
5
6
7
MBZ
Precedence
Type of Service
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