ISDN is a fantastic option as a backup in cases where your primary link has failed. However often your primary line has much more bandwidth than a single ISDN line (Channel). The ISDN BRI B-Channels run at 56kb/s or 64 kb/s (depending on country) and although this is a good start, often you need more bandwidth to accommodate the traffic.
ISDN BRI (Basic Rate Interface) interfaces have 2 B-channels. By default when you create a connection using ISDN only one of these channels will dial. In order for us to use the additional B-channel we need to insert an additional command under our BRI interface (PPP Multilink)
Similarly ISDN PRI (Primary Rate Interface) interfaces have (23 B-Channels on T1, 30 B-Channels on E1) each of the channels on a PRI line run at 64kb/s and often we would like to use more than just one channel for our backup.
Although we can have all channels connect immediately when the ISDN becomes active; this results in all lines been billed by the PSTN. Instead we would rather have additional lines been brought up one at a time when the traffic demands it. We can achieve this by defining a load threshold that the line must be under before bringing up additional channels.
The load-threshold command is on a scale from 1 to 255 where 255 is equal to 100% utilisation.
AOIP.ORG(config)# interface bri 2/0
AOIP.ORG(config-if)# ppp multilink
AOIP.ORG(config-if)# dialer load-threshold 128 either
In the above example, I have set a threshold of 128 (50%) and this is based on traffic either inbound or outbound. In order to only monitor traffic inbound, replace ‘either’ with inbound. The same applied to outbound traffic.
NOTE: In order to use PPP multilink, both sides of the link need to be configured for its use.