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4 - 8 ONline Ethernet Management Module Installation and Operation Guide
The EMM generates packets on the network (channel) in the following
three instances:
❑ When establishing and maintaining a remote login session (RCP or
Telnet session).
❑ When resolving a name tag (or IP Address) to a network address
(TAG, ARP).
❑ When the EMM is configured to support SNMP, and send Alert
messages.
In all other cases, the EMM does not generate any network activity. Traffic
statistics are collected by operating in promiscuous mode, that is, listening
to each packet on the network. Management of the other modules in the
concentrator is accomplished by utilizing a separate control bus on the
ONline backplane.
Note that when the EMM is isolated from the backplane, it cannot
generate, nor receive, any Ethernet traffic.
3Com Protocols
In order to support the remote login mechanism, 3Com has defined a thin
protocol layer called RCP (Remote Character Protocol). This protocol uses
the IEEE 802.2 link control layer protocol. This implies that remote login
sessions can exist across bridges, but not across a router.
Also, 3Com has defined a support protocol similar to ARP that resolves
names to addresses. This “tag resolution” occurs when a user initiates the
REMOTE_LOGIN or PING command, using the EMM name.
The entire 3Com protocol packet is structured as follows:
802.3 Header 802.2 Header 3Com Protocol Header Data