System information

8 IBM and Cisco LAN Switching: Interoperability and Migration Guide
• Auto-speed negotiation
• Flow control
• Spanning tree protocol
• VLAN functions
• Traffic prioritization
• Link aggregation
• Layer-3 routing
In addition we make some general observations that were made during the
course of interoperability testing as they relate to each of these criteria.
Other functional characteristics such as modularity, scalability, management,
number of MAC addresses, and performance are obviously relevant when
choosing an Ethernet switch. But since these qualities have no direct bearing
on interoperability, they are left out of this discussion.
2.1.1.1 Auto-speed negotiation
Auto-speed negotiation is firmly based on the IEEE 802.3 standard. All tests
were passed successfully. However, connecting differently configured ports,
such as auto-speed to fixed speed, could cause negotiation to take a long
time or fail.
2.1.1.2 Flow control
To prevent overflow in case of output congestion, early Ethernet switches
provided back pressure by transmitting a random carrier signal. This works
fine on half-duplex ports, but would have no effect on full duplex ports. IEEE
802.3 specifies a flow control technique whereby a receiving port can signal
to the transmitting port to stop or resume sending frames - somewhat
analogous to the XON/XOFF signaling we know from asynchronous
communication.
IEEE 802.3x flow control between IBM and Cisco switches was verified
successfully.
When connecting two 10/100 switch ports configured to auto-speed, they
will almost always negotiate the highest speed and full duplex. However, if
you want to be sure that the best performance is achieved, you must set
both ports to the desired fixed configuration. Do not connect a port with
auto configuration to a port with a fixed configuration.
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