The Acme Packet SBC supports the ability to load balance SIP traffic several difference ways. Simply stated, a SIP invite will ingress to the SBC. The SBC will lookup the destination and if it results in a Session Agent Group (SAG) configuration with 2 or more Session Agents, the SBC will egress the SIP invite using one of the selected load balancing algorithms such as Least Busy.

The SBC also supports DNS SRV records for load balancing. For those not aware of what DNS SRV means, it’s a form of DNS that allows a single domain name (associated with a service and protocol) to resolve to many possible destinations. The destinations may be equal or weighted differently. The following is an example of a DNS SRV record for SIP over UDP looks like. In this case all records have an equal weight of 10.

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The domain name will resolve to 5 different A records. The lookup must be a DNS SRV lookup for this to resolve correctly. In this case we are using SIP over UDP as the service which has been defined in Bind.

Here is how to enable DNS SRV load balancing on the SBC. Note this complies with RFC 3263 “Locating SIP Servers”. Once the config has been saved and actives the SBC will ping the address resolved by to see which IP’s are in service.

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Now when a SIP Invite ingresses the SBC and matches a local-policy where the destination next-hop utilizes DNS SRC load balancing the traffic will be balanced across all nodes.

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  1. Raj says:

    while creating local policy, Do we need to create SAG and then the domain as destination or directly directly give the next hop as domain name?

  2. Petr says:

    …unfortunately seems that DNS SRV method not work on SBC9200, resp. load balancing fails (traffic is send only to first IP)

  3. Mark says:

    I believe it was only implemented in C-Series code. The 9200 runs D-Series