Network load control
20
8.4
QoS/Priority
Basic - L3P
Release
4.0
11/07
Although VLAN prioritizing is widespread in the industry sector, it has a num-
ber of limitations:
X The additional 4-byte VLAN tag enlarges the data packets. With small
data packets, this leads to a larger bandwidth load.
X End-to-end prioritizing requires the VLAN tags to be transmitted to the en-
tire network, which means that all network components must be VLAN-ca-
pable.
X Routers cannot receive or send packets with VLAN tags via port-based
router interfaces.
8.4.3 IP ToS / DiffServ
TYPE of Service
The Type of Service (ToS) field in the IP header (see table 11) has been
part of the IP protocol from the start, and it is used to differentiate various
services in IP networks. Even back then, there were ideas about differen-
tiated 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.
Bits
0
1324567
MBZPrecedence Type of Service
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