Appendix C: Ethernet Port Operation; Ethernet Port Operation - CTI Janus 2500C-J Series Installation And Operation Manual

APPENDIX C: ETHERNET PORT OPERATION

The following sections describe how the ports on the Janus controller work and how Alternate IP subnets are
applied.

Ethernet Port Operation

Ethernet (layer 2) is responsible for sending and receiving Ethernet frames, which contain the TCP/IP protocol and
related data. Among other information, Ethernet frames contain a source address, which identifies the originator
of the frame and a destination address, which indicates how the frame will be delivered. The source address
contains the MAC (Media Access Control) address of the originator. The destination address may contain a unicast
address (the MAC address of the intended recipient), a multicast address, or an address that indicates a broadcast
address.
Microprocessor
(SoC)
Ethernet
Interface
Embedded
Switch
A B C D
You can enable forwarding between external ports for unusual applications.
Frames entering the external ports are forwarded to the internal Ethernet interface if the Ethernet destination
address is the same as the MAC address of the interface (Unicast) or when the destination address is a multicast or
broadcast address. The switch does not forward frames that do not meet these criteria, preventing them from
consuming microprocessor resources.
So, when a frame is sent from the internal Ethernet interface, how does the switch determine which port to
forward it to? The answer to this is in the way Ethernet switches work. Ethernet switches learn the MAC
address(es) of the device(s) connected to the ports. Once the port associated with a MAC address is known, the
switch will forward a frame whose destination address matches this address to this port only.
CTI Janus Programmable Automation Controller IOG
The Ethernet ports on the Janus controller are connected to an Ethernet switch on the
Janus circuit board. This switch is also connected to an internal Ethernet interface of
the Janus microprocessor. By default, the external ports (PORTS A, B, C, and D) are
isolated from each other. Frames entering one port are never forwarded to the other
ports. This is important when connecting ports to different Ethernet networks. It
prevents traffic (especially broadcast and multicast traffic) on one network from
propagating to other networks. This is also important to prevent network loops, which
can cause broadcast storms, when connecting multiple ports to the same Ethernet
network, as you might do when designing redundant networks.
See Section 4.2.3 Internal Ethernet Switch
NOTE
Configuration.
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