ISDN trunks

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Aggregate ISDN trunks

When connecting using multiple ISDN BRI interfaces (I've not tested this with PRI, but should work), it's easier to use ISDN trunks than creating a dial-peer for each voice port.

We start by creating a trunk:

trunk group TRK-PSTN
 max-retry 5
 voice-class cause-code 1
 description ** Trunk group to PSTN **
 hunt-scheme sequential

The max-retry is the number of times the trunk tries to establish a call before it considers it a fail. A rule of the thumb is: "number of voice ports"-1, ie, with 4 BRIs the trunk should retry 3 times in case of BRI failures. The hunt-scheme defines the hunt algorithm your trunk will search for an available channel.

What we define as fails is defined with this:

voice class cause-code 1

This effectively makes the the trunk consider failures when the disconnect cause is no-circuit (ISDN is down), temp-fail (temporary failure at the far end) or no-channel (no channel is available). If we don't define any of this cause-codes, if a trunk cannot make a call because of this reasons, it will deliver "destination unreachable" to the dial-peer and the call will not be made, even if you have available working BRIs across the trunk.

This is very useful when you have several BRIs, and one of them fails. The trunk will continue to try the remaining BRIs.

Do not forget to include each BRI into the trunk:

interface BRI0/1/1
 no ip address
 isdn switch-type basic-net3
 isdn point-to-point-setup
 isdn incoming-voice voice
 isdn send-alerting
 isdn sending-complete
 isdn static-tei 0
 trunk-group TRK-PSTN

The only point that is important here is the trunk-group command. All the others are network dependent and are not related to the trunk assignment.

Remember that when using ISDN trunk-groups, the trunk becomes the voice-port. The voice-ports for each BRI are not mentioned any more, but only the trunk. However, if there are any capabilities or compandings, this still must be configured on each BRI voice-port.

voice-port 0/0/1
 compand-type a-law
 bearer-cap Speech

Finally, the simplified (and single) dial-peer (for all ports, of course):

dial-peer voice 10 pots
 trunkgroup TRK-PSTN
 description ** PSTN Emergency
 translation-profile outgoing TO-PSTN
 destination-pattern 0011[1-3]
 forward-digits all

For outgoing calls, issue the trunkgroup command on the dial-peer.

With this method, you can effectively simplify your dial-peer usage and aggregate all the BRI voice-ports into a single trunk.