Typical Installations; Prerequisites For Configuring Modem Dial Backup - Avaya G250 Administration

Configuring WAN interfaces

Typical installations

The Avaya G250 and G350 Media Gateways were designed for small branch offices of a larger
enterprise. Consequently, the same RAS may serve many branch offices, and therefore many
G250/G350s. A reasonable assumption is that not all branch office would need modem dial
backup at the same time. Therefore, the ratio of modem channels at the RAS to G250/G350s at
branch offices can be less than 1:1. There are several practical ways to configure the RAS
server for use with modem dial backup Dialer interfaces:
The RAS can assign an IP address to the calling G250/G350. This would require the RAS
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to identify the call gateway using the PAP/CHAP username, and install an appropriate
static route to the branch office subnets accordingly. The username, password, and static
route can be configured in an external RADIUS/TACACS+ server.
The RAS server can use OSPF to learn the branch office subnets. This is much simpler to
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configure as all branch offices can share the same username and password. The G250/
G350 is configured to advertise the branch office subnets with OSPF. This feature requires
the use of unnumbered IP addresses at the G250/G350 and the RAS. Since the Dialer and
the primary interfaces are not expected to be up in the same time, the RAS server can use
passive-OSPF-interface and the G250/G350 can use static via routes.
The G250/G350 can call an ISP RAS (which is likely to assign it a dynamic IP address)
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and open an IPSec VPN tunnel to an enterprise-owned VPN gateway.
While using OSPF and calling an ISP RAS are expected to be the most common scenarios,
they involve complex interaction with IP routing and the remote RAS server. For more detailed
configuration examples, see Application Note - VoIP Network Resiliency.

Prerequisites for configuring modem dial backup

At least one dialer string, which determines the phone number(s) of the remote modem(s)
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dialed by the Dialer interface
A configured interface to be backed up
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Read/write or admin access level
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A Multitech MultiModem ZBA (MT5634ZBA) or MultimodemUSB (MT5634ZBA-USB)
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modem
RAS properties:
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- A dialer string
- Authentication parameters (username, password, PAP/CHAP)
- IP addressing (static, dynamic, or unnumbered)
- Routing (static, RIP, or OSPF)
- IPSec VPN, with all necessary parameters configured
236 Administration for the Avaya G250 and Avaya G350 Media Gateways