CTC Union FRM220-100AS-1, Руководство пользователя

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CHAPTER 3
PROVISIONING VIA TEXT MENU
becomes the root port (RP) of the bridge. Different technologies have different default costs for
network segments. An administrator can configure the cost of traversing a particular network
segment. The default
root path cost in 100AS-1 is 0.
The switch sends a
BPDU (Bridge Protocol Data Units) frame using the unique MAC address of the
port itself as a source address, and a destination address of the STP multicast address
01:80:C2:00:00:00.
There are three types of BPDUs:
1. Configuration BPDU (CBPDU), used for Spanning Tree computation
2. Topology Change Notification (TCN) BPDU, used to announce changes in the network
topology
3. Topology Change Notification Acknowledgment (TCA)
Hello Time is the time interval (number of seconds between) at which the root bridge transmits
configuration BPDUs.
When a device is first attached to a switch port, it will not immediately start to forward data. It will
instead go through a number of states while it processes BPDUs and determines the topology of
the network. The time spent in the listening and learning states is determined by a value known as
the
Forward Delay (default 15 seconds and set by the root bridge).
Maximum Age
, referred to by the switch as Max Age , is the amount of time a switch will retain a
BPDU's contents before discarding it. The default is 20 seconds. It also influences the bridge table
aging timer during the Topology Change Notification process.
Remember that these timers should not be changed lightly. If you decide to change any or all of
these timers, that change must be configured on the root bridge! The root bridge will inform the
non-root switches of the change via BPDUs.
3.6.5 Class of Service
Class of Service (COS) is a technique used to deliver Quality of Service (QoS) in a network. CoS is a
way of classifying and prioritizing packets based on classification. A "first class" priority label is
assigned to data applications - such as mission-critical data transactions, or video or voice
transmissions - which require faster turnaround, while a lower-priority label is assigned to less
time-sensitive traffic, such as e-mail and Web surfing.
CoS, as defined in IEEE 802.1p, uses Layer 2 VLAN Tagging and makes use of three bits in the
Ethernet frame header that can be used to specify priority. The field specifies a priority value of
between 0 and 7 inclusive that can be used by Quality of Service (QoS) disciplines to differentiate
traffic.
CoS operates only on Ethernet at the data link layer.