Architecting SIP Trunk Load Balancing across Multiple SBC Clusters using DNS SRV Records
What This Guide Covers
This guide details the configuration of a Genesys Cloud SIP Trunk that utilizes DNS SRV records to resolve destination endpoints across multiple on-premises Session Border Controller (SBC) clusters. The end result is an architecture where call traffic automatically distributes across SBCs based on priority and weight, with seamless failover capabilities during cluster outages without requiring manual Genesys Cloud reconfiguration.
Prerequisites, Roles & Licensing
- Licensing Tier: Genesys Cloud CX Professional or Enterprise license with SIP Trunking capability enabled. Standard licenses do not support custom SIP trunk destinations via API/Architect for this specific high availability pattern.
- Permissions:
Telephony > Trunks > Edit,Telephony > Trunks > Create,Admin > Users > Read. You must possess thesip_trunkscope if using the API for provisioning. - External Dependencies: Access to a public or private DNS server capable of handling SRV queries (e.g., Microsoft AD DNS, BIND, CloudDNS). A functional SBC environment (Cisco VCS/Expressway, Oracle UC SBC, Avaya Session Manager) configured with multiple clusters.
- Network Requirements: Open UDP/TCP port 5061 for TLS or 5060 for UDP on all SBC listening interfaces. Firewall rules must allow traffic from Genesys Cloud IP ranges to the SBC VIPs or specific cluster IPs.
The Implementation Deep-Dive
1. DNS SRV Record Architecture Design
Before configuring the telephony platform, you must design the DNS resolution strategy. This is the foundation of the load balancing logic. You cannot simply point a FQDN to an IP; you must define service priority and weight.
The DNS SRV record format for SIP follows the IETF RFC 2782 standard. The syntax places the service name _sip._tcp or _sip._udp as the subdomain, followed by the domain. For Genesys Cloud to route effectively, you must define at least two hostnames representing your SBC clusters.
The Trap:
A common architectural error is assigning equal priority values (e.g., Priority 10) to all SBC clusters without considering capacity or geographic proximity. If all clusters have the same priority, Genesys Cloud will rotate round-robin based on the DNS response order, which may not align with your traffic distribution goals or disaster recovery requirements. This leads to uneven load and potential overload of a single cluster during a failover event because the secondary cluster was never intended to handle 100% of the load instantly.
Architectural Reasoning:
We use Priority levels to dictate failover order. Lower numbers indicate higher preference. We use Weight values to dictate load balancing among clusters sharing the same priority level. For example, a primary cluster might have Priority 10 and Weight 100. A secondary cluster in a different region might also have Priority 10 and Weight 50. If both are healthy, traffic splits roughly 2:1. If the primary fails, the DNS still returns the secondary, but because of the priority logic, Genesys Cloud will attempt the next priority level if available (though SRV records for SIP often resolve all hosts at a given priority).
Actionable Configuration:
Create an SRV record in your authoritative DNS server. The record must point to the FQDNs of your SBC clusters.
_sip._tcp.example.com. IN SRV 10 50 5061 primary-sbc-cluster-1.example.com.
_sip._tcp.example.com. IN SRV 20 50 5061 secondary-sbc-cluster-2.example.com.
In this example:
- Priority: 10 for Primary, 20 for Secondary.
- Weight: 50 for both (equal distribution at the same priority level).
- Port: 5061 (Standard TLS port for secure SIP).
- Target: The FQDN of the SBC listening interface.
Ensure that TTL (Time To Live) is set low, typically between 300 and 900 seconds. A high TTL causes DNS caches to retain stale records during a failure, extending outage duration significantly. Genesys Cloud resolvers cache these results aggressively; therefore, DNS propagation speed is critical for failover performance.
2. Provisioning the SIP Trunk Destination
Once the DNS layer is validated, you configure the Genesys Cloud side to utilize this SRV endpoint. You can create this via the User Interface or the REST API. For production environments, API provisioning ensures consistency and allows for automation scripts.
UI Configuration Path:
Navigate to Admin > Telephony > Trunks. Click Add SIP Trunk. In the Destination field, enter the FQDN used in your SRV record (e.g., sip.example.com). Do not enter an IP address. Genesys Cloud will perform a DNS lookup and follow the SRV resolution chain.
API Configuration Payload:
For programmatic deployment, use the following JSON payload. Ensure the uri field contains the domain name that resolves via your SRV records.
{
"name": "Enterprise-SBC-Load-Balanced-Trunk",
"protocol": "SIP",
"transportType": "TLS",
"destination": {
"type": "sip",
"uri": "sip.example.com"
},
"mediaEncryption": "mandatory",
"ipAddresses": [],
"authenticationType": "none",
"status": "active"
}
The Trap:
A critical misconfiguration occurs when users populate the ipAddresses field in the API payload while also setting a FQDN URI. If you specify explicit IP addresses alongside a DNS URI, Genesys Cloud may prioritize the static IPs over the DNS resolution logic, effectively bypassing your load balancing architecture and creating a single point of failure. Additionally, if the transportType is set to TCP but the SRV record points to UDP (or vice versa), call setup will fail immediately. The transport protocol must match between the Genesys Cloud trunk settings and the SBC cluster listening ports.
Architectural Reasoning:
We avoid hardcoding IP addresses because SBC clusters often utilize Dynamic NAT or floating IPs that change during maintenance or cloud migration events. By relying on DNS resolution, you decouple the telephony topology from the network layer addressing. This allows Network Engineers to update backend routing without touching Telephony configuration. It also enables DDoS mitigation strategies at the DNS level (e.g., Geo-DNS) before traffic even reaches the Genesys Cloud ingress points.
3. Configuring SBC Cluster Response Logic
The final component is ensuring your on-premises SBC clusters are prepared to receive these requests and report their status correctly via DNS. The SBCs must be configured to listen on the ports specified in the SRV record (e.g., 5061).
Configuration for Oracle UC SBC:
In the Oracle UC SBC web interface, navigate to Routing > SIP Trunk Groups. Define a Virtual Gateway that listens on the specific port. Ensure the SIP Server configuration allows incoming TLS handshakes from Genesys Cloud IP ranges. You must also configure the DNS Zone File entries in your authoritative DNS server to match the target FQDNs used in the SRV record.
Configuration for Cisco VCS:
Use the following CLI snippet to define a SIP Trunk that accepts calls from the defined domain and responds appropriately to registration keep-alives.
xconfiguration CallRouting: Enable = true
xconfig callrouting sip-trunk 1 destination "sip.example.com"
xconfig callrouting sip-trunk 1 tls-port 5061
xconfig callrouting sip-trunk 1 allow-registration = true
The Trap:
The most frequent failure mode involves the SBC not responding to DNS queries for the SRV record itself. If your SBC is behind a NAT device, you must ensure that the Internal DNS Zone and External DNS Zone are synchronized correctly. A common issue arises when the SBC registers with an external SIP Provider but fails to answer internal DNS queries because the firewall blocks port 53 (DNS) or the DNS server is not authoritative for the domain used in the SRV record. This results in a “NXDOMAIN” error from Genesys Cloud, leading to immediate trunk failure.
Architectural Reasoning:
We configure the SBCs to handle TCP and UDP traffic explicitly on port 5061. SIP requires distinct handling for signaling (SIP) and media (RTP). While SRV records define the signaling path, ensure your firewall rules allow RTP/UDP ports 10000-20000 (or your defined range) between Genesys Cloud IP ranges and the SBC clusters. If you restrict this range too narrowly on the SBC side, calls will connect but media will fail, resulting in “One-way audio” which is often harder to troubleshoot than a full registration failure.
Validation, Edge Cases & Troubleshooting
Edge Case 1: DNS Propagation Latency During Failover
The Failure Condition:
During a simulated outage of the primary SBC cluster (Priority 10), calls fail for approximately five minutes before routing to the secondary cluster (Priority 20).
The Root Cause:
This is caused by high TTL values on the DNS SRV record. Genesys Cloud resolvers cache the DNS response based on the TTL. If you set the TTL to 86400 seconds (24 hours), the cached failure state persists for that duration even if you switch traffic in DNS immediately.
The Solution:
Audit your DNS provider settings. For production telephony SRV records, set the TTL to 300 seconds (5 minutes) or lower during maintenance windows. Monitor the dig command output from a Genesys Cloud region to verify cache expiration times.
dig +nocmd @8.8.8.8 sip.example.com SRV +noall +answer
Edge Case 2: TLS Handshake Failures with Mixed Transport Types
The Failure Condition:
Trunk status shows “Registered” or “Active”, but all outbound calls result in a SIP 503 Service Unavailable error.
The Root Cause:
The SRV record specifies _sip._tcp, but the Genesys Cloud Trunk is configured for UDP, or vice versa. Alternatively, the SBC cluster requires TLS 1.2, but the Genesys Cloud trunk setting defaults to an older version due to legacy configuration profiles.
The Solution:
Verify the transport protocol in the SRV record matches the transportType field in the Genesys Cloud Trunk JSON payload exactly. Update the SBC cluster certificates to ensure they are trusted by the Genesys Cloud platform and vice versa. Check the System Logs > SIP Logs for handshake rejection codes (e.g., 403 Forbidden).
Edge Case 3: Asymmetric Load Distribution
The Failure Condition:
Traffic concentrates heavily on one SBC cluster despite equal weight settings in the SRV record.
The Root Cause:
DNS Round-Robin behavior is not always strictly enforced by intermediate resolvers or client-side caching mechanisms. Genesys Cloud may cache a specific IP address from the DNS response for the duration of its own resolver cache, ignoring subsequent weights.
The Solution:
Implement Geo-DNS routing at your DNS provider level if you have clusters in different regions. Configure the SRV record to return hosts based on the location of the Genesys Cloud ingress point. Alternatively, implement a weighted load balancer (e.g., F5 BIG-IP) in front of the SBCs that receives all traffic from a single VIP and distributes it internally based on real-time CPU/Memory metrics.