Advanced Reference Guide
Fast SwitcHub-8mi SNMP
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Communication Mode
- If you use auto-negotiation, be sure the attached device also
supports this mode. If any device attached to the switch runs at full duplex, but does
not support auto-negotiation, you will need to manually set the communication mode to
full duplex. However, once set, the communication mode setting will remain unchanged,
even if the link is broken (e.g., by unplugging the cable, or by a power reset).
To restore auto-negotiation after manually selecting half or full duplex, use the Configure
button, on-board management program, or SNMP network management software to set
the communication mode to auto-negotiation.
Notes
: 1. Full duplex can only be used for a dedicated link. When connecting to a
shared collision domain (e.g., a repeater hub) be sure the transmission mode
is set for half-duplex.
2. The de facto standard for auto-negotiation is used in this switch.
Switching Methods
- The SwitcHub employs a dynamic forwarding architecture that
enables it to support the following switching modes, "adaptive cut-through," "cut-
through," "fragment-free cut-through," and "store-and-forward." The function of each
of these switching methods can be described as follows.
Cut-through:
A packet is forwarded as soon as the 6-byte destination address in
the packet header has been scanned, it does not wait until the entire packet has been
received. This technique, which requires the least processing time, reduces the
latency of packet transmission to 20 microseconds or less.
Fragment-free cut-through:
A packet is forwarded only after the first 64 bytes
have been received. This prevents runts from being passed along, effectively
cleaning up the data stream. This mode is beneficial when it is probable that the
collision rate will be high (i.e., when the switch is used to interconnect several
shared segments, each having a large number of end stations).
Store-and-forward:
An entire packet is received into a buffer so it can be checked
for validity before being forwarded to another port. This prevents errors from being
propagated throughout the network.
Adaptive cut-through:
This method utilizes Accton's smart algorithm to dynamically
change between switching modes based on the current CRC error rate. Switching
starts with standard cut-through for a clean data environment, changes to fragment-
free cut-through for a higher error rate, and then to store-and-forward for a highly
contentious environment.
To ensure that you can manage any network load without manual intervention, adaptive-
cut-through is used as the default switching method. By using this method, the
SwitcHub delivers the best network performance under any environment.
Using the Spanning Tree Algorithm
The Spanning Tree Algorithm (i.e., the STA configuration algorithm as outlined in IEEE
802.1D) can be used to detect and disable network loops, and to provide link backup.
This allows the switch to interact with other bridging devices (i.e., STA compliant
switch, bridge or router) in your network to ensure that only one route exists between
any two stations on the network. If redundant paths or loops are detected, one or more
ports are put into a blocking state (i.e., stopped from forwarding packets) to eliminate
the extra paths. Moreover, if one or more of the paths in a stable spanning tree
topology fail, this algorithm will automatically change ports from blocking state to
forwarding state to reestablish contact with all network stations.
The STA uses a distributed algorithm to select a bridging device (i.e., STA compliant
switch, bridge or router) that serves as the root of the spanning tree network. It
selects a root port on each bridging device (except for the root device) which incurs
the lowest path cost when forwarding a packet from that device to the root device.
Then it selects a designated bridging device from each LAN which incurs the lowest
path cost when forwarding a packet from that LAN to the root device. All ports
connected to designated bridging devices are assigned as designated ports. After
determining the lowest cost spanning tree, it enables all root ports and designated
ports, and disables all other ports. Network packets are therefore only forwarded
between root ports and designated ports, eliminating any possible network loops.
The following figure gives an illustration of how the Spanning Tree Algorithm assigns
bridging device ports.
Configurable STA parameters can be seen in the figure below and are described in the
following table.
Spanning Tree Configuration
Spanning Tree Algorithm Yes
Priority 32768 Maximum Message Age 20
Hello Time 2 Forward Delay 15
Port Priority Path Cost
-----------------------------------------------------
1 128 10
2 128 10
3 128 10
4 128 10
5 128 10
6 128 10
7 128 10
8 128 10
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