If a coordinator has several access IP addresses (for example, each one for a different communications channel), then, on another ViPNet host, you can configure priorities for the channels you use to connect to this coordinator. If the channel with the highest priority is unavailable, then the communications channel will be selected according to the priority specified for other channels. When the highest-priority channel becomes available, the connection will be established anew via this channel.
Note: This can be effective only if the host establishes connection to the coordinator via various channels, for example, across the Internet and a dedicated network (in other words, when the connection is routed via different gateways).
Priority of channels is defined by specifying a metric for each access IP address of the coordinator. By default, metrics are assigned automatically. When assigning metrics, you should stick to the following rules:
An IP address metric defines a delay (in milliseconds) before sending test IP packets to detect the IP address accessibility. The connection is established using the first address that appears to be accessible during the poll.
The polling is performed in the following cases:
o At ViPNet Monitor startup.
o When you check connection to the host manually.
o Periodically. You can specify the polling interval on the coordinator in the Options dialog box, in
the Private Network > Additional Parameters section. By default, the interval of a coordinator polling other coordinators is 15 minutes; the interval of polling its coordinator by a client is 5 minutes.
The IP address with the least metric has the highest priority. This address is always used to establish connections if it is available.
If metrics for all access IP addresses are assigned automatically, then the value of all metrics is 0. If metrics for all access IP addresses are assigned automatically, then the value of all metrics is 0. If for some IP addresses metrics are assigned manually, and for others automatically, then the value of automatic metrics is always 100 milliseconds greater than the maximum value of any manually assigned metric.
The greater the difference between the least metric and other metrics is, the less the chance is that the connection will be established via a low-priority channel in case of a short-time failure. If the connection is established via a low-priority channel, the host will be able to switch to the highest- priority channel quicker, when it becomes available.
checked only when the connection via the current channel is lost. The same mechanism is used if the connection is established via the highest-priority channel.
If at least one IP address metric is assigned manually, and its value is not the highest one, then the availability of other communications channels, including the one with the highest priority, will be checked as well.
When the ViPNet Monitor starts and when the connection with the coordinator is checked (see
Checking Connection to a ViPNet Host on page 217), the availability of all communications channels is always checked in order to select the channel with the least metric.
When a channel is selected, the current access IP address is displayed in the coordinator's properties window, on top of the list on the Firewall tab.
To specify metrics for access IP addresses of a coordinator:
1 In the navigation pane of the main ViPNet Monitor window, select Private Network.
2 In the Private Network section, double-click the coordinator you are going to specify the access IP addresses priority for.
3 In the ViPNet Host Properties dialog box, click the Firewall tab.
4 If necessary, configure the firewall parameters for the coordinator (see Configuring Access to ViPNet Hosts on page 102).
5 To specify a metric for an IP address, select the address from the list and click Edit.
Figure 46. Specifying a metric
6 In the displayed window, check the Set metrics check box and, in the corresponding box, enter the metrics value in milliseconds (valid values range from 1 to 9999). Then click OK.
Consider the following scenario. Suppose, a coordinator has four IP addresses allowing you to access it via channels A, B, C, and D. You need to specify metrics for these channels. You need to specify metrics for these channels.
Let the channels have the following priority:
1 A is the fastest and the most secure channel. Should be used in the first place. 2 C and D are secure but slower channels. Should be used if channel A is unavailable. 3 B is a less secure channel. Should be used last of all.
To set the highest-priority level for channel A, specify the minimum metrics value for it, for example, 1. Specify the maximum metrics value (9999) for channel B because using this channel is undesirable. Specify equal metrics for channels C and D so that they do not differ much from the channel A metrics,
With such metrics values, channel A will always be used if it is available. If channel A becomes unavailable or its traffic rate decreases, connection to the coordinator will be established via channel C or channel D. In case of emergency, when channels A, B, and C are all unavailable, channel B will be used.
If connection is established via channel B, C, or D, then your host will try to establish connection via channel A when the polling period is finished, on ViPNet Monitor startup, or when the connection to the coordinator is checked. The less the polling period is, the faster the channel will be switched in case of a failure and will then revert to a channel with a higher priority.