The ATM1600 has been extended with the capability to define multiport interfaces (MPIFs). Multiport interfaces consist of two or more ATM ports whose bandwidth is combined, using flow by flow inverse multiplexing, for use as a single interface to provide very high speed trunking capability between ATM1600 IP switches.
MPIFs are created by combining several ports on the ATM1600 and having them appear as a single interface to the switch controller. As the switch controller directs the ATM1600 to switch flows on the multiport interface, the ATM1600 distributes the flows across the individual ports. The bandwidth and performance of the multiport interface approaches that of a single interface having bandwidth equal to the sum of that of the individual ports; in other words, an OC12c-equivalent interface can be built by combining four OC3c interfaces into a multiport interface of OC12c bandwidth.
The multiport interface is useful in relieving network congestion that might result from trunk connections between IP Switches. Such congestion might arise when traffic from many fast Ethernets on a FAS1200 must cross a trunk before reaching an important server or server farm.
Creation and deletion of multiport interfaces is done at the ATM1600 console. Configuration information is stored in non-volatile memory on the ATM1600.
Three console commands are available for configuring and managing multiport interfaces. These are the define mpif, show mpif and delete mpif commands.
The following example shows a multiport interface consisting of ports eight, nine, ten, and twelve being combined as a single interface. The resulting interface will be numbered number 8, and will be available at the switch controller as s1p1c0s8p1c0. The individual ports of the multiport interface are known as subports.
Using interface numbers that correspond to real ATM1600 ports permits additional ports to be added as subports of a multiport interface as bandwidth requirements increase without the need to change the configuration of the switch controller.
After defining one or more multiport interfaces, the switch and controller must restart their communication in order to exchange the new list of available interfaces.
This is done by restarting the ATM1600 using the reboot command:
Multiport interfaces support fault-tolerant operation. Failure of one or more of the subports making up the multiport interface will cause the ATM1600s at each end to move flows carried on the failed subports(s) to operational subports of the interface. This will result in some packet loss, but will not necessarily cause the switch controller to change its network topology due to loss of routing adjacency across the interface.
As individual subports making up the multiport interface are restored, new flows will make use of the available bandwidth on the recovered subports.
Inverse multiplexing across the subports of a multiport interface is accomplished on a flow-by-flow basis. The net result is traffic being distributed rather evenly across the subports. This is accomplished as follows.
Periodically, the ATM1600 measures the level of traffic on each port. Each new flow is placed on the subport with the lowest level of traffic. The lowest level of traffic is derived as follows:
1. Use the port having the fewest outgoing cells in the measured
period.
2. If all available subports are running at full line rate then the
new flow is added to the subport with the shortest queue of waiting traffic
at the given priority level.
3. If all subports are running at less than full line rate, but are
running at equal outgoing cell rates (highly unlikely at cell rates other
than zero) then the lowest numbered port will carry the flow.
The flow-distribution algorithm treats all channels, including the default channel, as any other flow and its path follows the distribution algorithm above.
If you do not want the flow to be balanced across all of the specified ports, but instead want the current port to be fully loaded before adding the next port, use Weighted Multiport Interfaces (WMPIFs). WMPIFs are configured in the same way as MPIFs except the commands use WMPIF instead of MPIF.
The commands are: define wmpif, show wmpif and delete wmpif. For example, to define a WMPIF similar to the MPIF shown in the above example, you enter the following command:
In this case, when port 8 is running at full line rate, port 9 will
be opened. When port 9 is running at full line rate, port 10 will be opened
and so forth.