Ethernet Interface Redundancy; Nat (Network Address Translation) Support - 3Com V7122 User Manual

Ethernet Interface Redundancy

The gateway supports the following redundancy scheme:
At the beginning of the start-up procedure, the gateway tests whether the 'Primary' Ethernet
interface is connected by checking the existence of the Ethernet link carrier. If it is
connected, the start-up procedure commences as usual. If not, the start-up application tries
the 'Secondary' Ethernet interface. If this interface is connected, the whole start-up
procedure is performed using it. If both interfaces are out of order, the start-up procedure
commences using the parameters, tables, and software residing in the gateway's non-
volatile memory. Note that Ethernet switchover occurs only once during the start-up
procedure (at its beginning). If the Ethernet interface fails after the selection is made, the
gateway does not switch over to the second port.
After start-up has completed and the operational software is running, the gateway continues
to use the Ethernet port used for program load. The gateway switches over from one
Ethernet port to the other one every time an Ethernet link carrier loss is detected on the
active Ethernet port, and if the Ethernet link of the other port is operational. Switchover takes
place only once per link loss (that is, the 'secondary' interface stays the active one even if
the 'primary' interface has returned to life).
After start-up, the gateway generates a gratuitous ARP message each time a switchover
occurs.
For correct functionality of the redundancy mechanism, it is recommended to configure both
links to the same mode. It is essential that both link partners (the primary link partner and the
secondary link partner) have the same capabilities, so that whenever a switchover occurs the
gateway is able to provide at least the same Ethernet services as were provided prior to the
switchover.
For correct functionality of the redundancy mechanism, it is recommended to set the physical
secondary link prior to the gateway being reset (since the MAC configuration cannot be
changed thereafter).
Because the two Ethernet ports use the same MAC address, the external
switches connected to the gateways can in some cases create a noticeable
switchover delay due to their internal switching logic, though on the gateway level,
the switchover delay is minimal (milliseconds).

NAT (Network Address Translation) Support

Network Address Translation (NAT) is a mechanism that maps a set of internal IP addresses
used within a private network to global IP addresses, providing transparent routing to end
hosts. The primary advantages of NAT are (1) Reduction in the number of global IP
addresses required in a private network (global IP addresses are only used to connect to the
Internet); (2) Better network security by hiding its internal architecture.
Figure 81 illustrates the supported NAT architecture.
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