272 V7122 GatewayUser GuideEthernet Interface RedundancyThe gateway supports the following redundancy scheme:At the beginning of the start-up procedure, the gateway tests whether the ‘Primary’ Ethernetinterface is connected by checking the existence of the Ethernet link carrier. If it isconnected, the start-up procedure commences as usual. If not, the start-up application triesthe ‘Secondary’ Ethernet interface. If this interface is connected, the whole start-upprocedure is performed using it. If both interfaces are out of order, the start-up procedurecommences using the parameters, tables, and software residing in the gateway’s non-volatile memory. Note that Ethernet switchover occurs only once during the start-upprocedure (at its beginning). If the Ethernet interface fails after the selection is made, thegateway does not switch over to the second port.After start-up has completed and the operational software is running, the gateway continuesto use the Ethernet port used for program load. The gateway switches over from oneEthernet port to the other one every time an Ethernet link carrier loss is detected on theactive Ethernet port, and if the Ethernet link of the other port is operational. Switchover takesplace only once per link loss (that is, the ‘secondary’ interface stays the active one even ifthe ‘primary’ interface has returned to life).After start-up, the gateway generates a gratuitous ARP message each time a switchoveroccurs.For correct functionality of the redundancy mechanism, it is recommended to configure bothlinks to the same mode. It is essential that both link partners (the primary link partner and thesecondary link partner) have the same capabilities, so that whenever a switchover occurs thegateway is able to provide at least the same Ethernet services as were provided prior to theswitchover.For correct functionality of the redundancy mechanism, it is recommended to set the physicalsecondary link prior to the gateway being reset (since the MAC configuration cannot bechanged thereafter).Because the two Ethernet ports use the same MAC address, the externalswitches connected to the gateways can in some cases create a noticeableswitchover delay due to their internal switching logic, though on the gateway level,the switchover delay is minimal (milliseconds).NAT (Network Address Translation) SupportNetwork Address Translation (NAT) is a mechanism that maps a set of internal IP addressesused within a private network to global IP addresses, providing transparent routing to endhosts. The primary advantages of NAT are (1) Reduction in the number of global IPaddresses required in a private network (global IP addresses are only used to connect to theInternet); (2) Better network security by hiding its internal architecture.Figure 81 illustrates the supported NAT architecture.
