User`s manual

Mediant 2000 & TP-1610 & TP-260
SIP User's Manual 230 Document #: LTRT-68805
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).
Note that as 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).
9.3 NAT 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) reduces the number of global
IP addresses required in a private network (global IP addresses are only used to connect
to the Internet); (2) provides a better network security by hiding its internal architecture.
Figure
9-1 below illustrates the supported NAT architecture.
Figure 9-1: NAT Functioning
The way SIP is designed creates a problem for VoIP traffic to pass through NAT. SIP uses
IP addresses and port numbers in its message body. The NAT server can’t modify SIP
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.
To solve these problems the following mechanisms are available:
 STUN (refer to Section 9.3.1 below).
 First Incoming Packet Mechanism (refer to Section 9.3.2 on page 232)
 RTP No-Op packets according to the avt-rtp-noop draft (refer to Section 9.3.3on page
232).
 For SNMP NAT traversal, refer to Section 15.10 on page 326.