Comtech EF Data CDM-760, Installation And Operation Manual

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D–1
Appendix D. ETHERNET
NETWORK CONFIGURATION
D.1 Overview
For operations requiring Ethernet-based terrestrial data handling, it is important to stress the
importance of avoiding Ethernet looping connection problems – with or without use of the
CDM-760 Advanced High-Speed Trunking Modem in redundancy. These issues are specifically
addressed with a CDM-760 redundancy configuration that uses Comtech EF Data’s CRS-170A
(L-Band) or CRS-180 (70/140 MHz) 1:1 Redundancy Switches, or a CRS-500 M:N Redundancy
System in 1:N redundancy.
Use this appendix to review the differing methods for Ethernet-based data handling with the
CDM-760.
D.2 Ethernet Routers and Switches
Routers and switches allow connection of one or more computers or networked devices to
other computers or network devices. Each has two or more connectors, called physical ports, in
which cables connect to other network devices.
An Ethernet switch examines the traffic that comes across it, and learns where particular MAC
addresses are. An Ethernet switch maintains what is known as a C ontent A ddressable M emory
( CAM ) table, listing the MAC addresses for each switch port. The Ethernet switch uses the CAM
table to determine where to forward Ethernet frames. By default, Ethernet switches will update
the CAM table automatically; for example, if an Ethernet switch sees traffic from ‘Machine A’
coming in on ‘Port 2’, it now knows that ‘Machine A’ is connected to that port, and that traffic
destined for ‘Machine A’ needs to only be sent to that port and not any of the others.
An Ethernet router determines where to forward IP traffic based upon the destination IP address
and the Route table entries in the router. An Ethernet router can be programmed to understand
and route the data it is directed to handle; for example, broadband routers include the ability to
"hide" computers behind a type of firewall, which involves slightly modifying the packets of
network traffic as they traverse the device. All routers include some kind of user interface for
configuring how the router will treat traffic: larger routers include the equivalent of a full-blown
programming language to describe how they should operate, as well as the ability to communicate