User`s manual

Mediant 1000
H.323 User's Manual 280 Document #: LTRT-83401
2. 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 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 occurs only once per link loss (that is, the ‘secondary’ interface remains
the active one even if the ‘primary’ interface has returned to "life"). In addition, the
gateway generates a gratuitous ARP message each time a switchover occurs.
For correct functionality of the redundancy mechanism, it is recommended to configure the
same mode for both links. 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. In addition, it is recommended to configure the physical secondary
link prior to the gateway being reset (since the MAC configuration cannot be changed
thereafter).
Note: Since 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 at the gateway
level, the switchover delay is minimal (milliseconds).
9.3 NAT (Network Address Translation) Support
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 9-1 illustrates the NAT architecture.
Figure 9-1: NAT Functioning
The way H.323 is designed creates a problem for VoIP traffic to pass through NAT. H.323
uses IP addresses and port numbers in its message body. The NAT server can’t modify
H.323 messages and therefore, can’t change local to global addresses.
Two different streams traverse through NAT: signaling and media. A gateway (located
behind a NAT) that initiates a signaling path will have problems in receiving incoming
signaling responses (they will be blocked by the NAT). Furthermore, the initiating gateway
must notify the receiving gateway where to send the media to.