4.2 Route Patterns, Route Groups & Route Lists

4.2 Route Patterns, Route Groups & Route Lists

Outbound call routing in Cisco Unified Communications Manager (CUCM) directs dialed numbers from internal endpoints to external Public Switched Telephone Network (PSTN) gateways, Session Border Controllers (such as Cisco Unified Border Element, or CUBE), and inter-cluster SIP trunks. The routing hierarchy is structured into three tiered elements: Route Patterns, Route Lists, and Route Groups, complemented by Call Hunting mechanisms for incoming call distribution.

---

1. Route Patterns and Dial Plan Wildcards

A Route Pattern defines a string of digits and wildcards that match a dialed telephone number. Unlike Translation Patterns, which rewrite digits and restart digit analysis internally, a Route Pattern represents an egress routing decision, pointing directly to a Route List, a Gateway, or a SIP Trunk.

Wildcard Syntax Reference

CUCM digit analysis supports a comprehensive set of wildcards and special characters for constructing precise dial patterns:

---

2. Urgent Priority and Interdigit Timeout (T.302) Mechanics

When a user enters digits off-hook on a Cisco IP phone without pressing # or a "Call" softkey, CUCM receives digits one-by-one via Keypad Markup Language (KPML) or Skinny Station Protocol. As each digit arrives, CUCM determines whether the dialed string matches an active pattern.

The T.302 Interdigit Timer

If the entered string matches a pattern, but longer matching patterns exist in the dial plan, CUCM cannot immediately route the call. It starts the T.302 Interdigit Timeout timer (default: 15,000 milliseconds, or 15 seconds) to wait for subsequent digits:

• If the user dials another digit before T.302 expires, the timer resets, and digit analysis continues.
• If the user enters no further digits and T.302 expires, CUCM routes the call based on the best matching pattern accumulated so far.
• If the user dials #, CUCM terminates dialing immediately without waiting for T.302.

Urgent Priority Behavior

The Urgent Priority setting is a checkbox available on Route Patterns, Translation Patterns, and Directory Numbers. When enabled, Urgent Priority instructs the Digit Analysis engine to halt the T.302 interdigit timer immediately upon reaching an exact match, routing the call without waiting for potential longer patterns.

Scenario: Emergency Calling vs. Variable-Length International
------------------------------------------------------------------------
Pattern A: 911     (Urgent Priority = ENABLED)
Pattern B: 9.!     (Variable-length international, e.g., 9011...)

Caller Dials: 9 -> 1 -> 1
Result: Because Urgent Priority is ENABLED on 911, CUCM terminates digit
        analysis instantly and dispatches the call to the emergency gateway.
        If Urgent Priority were DISABLED, CUCM would pause for 15 seconds
        (T.302) after the second '1' to see if the caller intended to dial
        a longer string matching 9.!

Architectural Pitfall: Premature Digit Truncation

Applying Urgent Priority incorrectly can severely disrupt internal dial plans. For example, consider an enterprise with both 4-digit extensions (3XXX) and 5-digit extensions (3XXXX). If an administrator enables Urgent Priority on the 3XXX pattern, callers can never dial 5-digit extensions starting with 3. As soon as the fourth digit is entered, CUCM matches 3XXX, immediately terminates digit analysis, and routes the call to the 4-digit target, truncating the fifth digit.

---

3. Discard Digits Instructions (DDIs)

Public carriers expect standard numbers (such as 10-digit local or 11-digit national) without internal enterprise steering digits (such as an external access code 9). Discard Digits Instructions (DDIs) remove these steering digits at the egress boundary before the call is delivered to a Route Group or Gateway.

Standard Discard Digits Instructions

In addition to DDIs, administrators can configure Prefix Digits (Outgoing Calls) to prepend digits (such as prepending an international carrier access code 011) after the DDI has stripped the designated prefix.

---

4. Route Groups and Distribution Algorithms

A Route Group is an ordered, logical aggregation of one or more physical voice gateways or trunks (such as Cisco Unified Border Elements, MGCP gateways, or SIP trunks). Route Groups do not contain dial patterns; they represent pools of egress hardware interfaces.

Distribution Algorithms

When multiple gateways or trunks are placed inside a Route Group, CUCM distributes outbound calls across those devices using one of two distribution algorithms:

1. Top Down:
   • CUCM always attempts to route calls out the first device listed at the top of the Route Group.
   • If the first device is busy (all DS0 channels or SIP call legs exhausted) or unreachable, CUCM rolls over to the second device, and so forth down the list.
   • Use Case: Preferred when one egress path has lower carrier costs or higher quality (e.g., primary fiber SIP trunk over an ISR 4451, failing over to a secondary T1/E1 PRI circuit).
2. Circular (Round-Robin):
   • CUCM distributes calls evenly across all operational gateways in the Route Group by cycling through the list in a round-robin sequence.
   • Each new call setup request is offered to the gateway immediately following the device that received the preceding call.
   • Use Case: Preferred when distributing load across multiple equal-cost Session Border Controllers or parallel SIP trunks to maximize hardware utilization and prevent bottlenecking.

---

5. Route Lists and Path Failover Logic

A Route List is an ordered collection of Route Groups. While a Route Group selects the physical gateway, a Route List establishes the hierarchical failover strategy across different Route Groups and applies path-specific digit manipulations.

[Route Pattern: 9.1[2-9]XX[2-9]XXXXXX]
               |
               v
     [Route List: RL_Outbound_WAN]
               |
               +---> Route Group 1: RG_HQ_CUBE_SIP      (Primary path - SIP)
               |        |---> CUBE-01 (HQ)
               |        |---> CUBE-02 (HQ Backup)
               |
               +---> Route Group 2: RG_HQ_PRI_GATEWAY   (Secondary path - T1 PRI)
                        |---> ISR4331-MGCP-PRI

Failover Conditions and PSTN Cause Codes

CUCM automatically advances to the next Route Group in a Route List under specific network and signaling conditions:

• Circuit Congestion / Busy: The gateway returns ISDN Cause Code 34 (Circuit/Channel Congestion) or SIP 503 Service Unavailable.
• Temporary Failure: The gateway returns ISDN cause 41 (Temporary Failure), which SIP devices typically signal as 503 Service Unavailable.
• Unreachable Interface: Layer 3 connection failure or SIP OPTIONS keepalive failure on the trunk.

Critical Distinction: If a carrier rejects a call due to an invalid dialed number (e.g., ISDN Cause Code 1 Unallocated/Unassigned Number or SIP 404 Not Found), CUCM does not fail over to the next Route Group (the Stop Routing on Unallocated Number Flag service parameter, True by default). An unallocated number indicates the destination does not exist; retrying the same invalid number across alternate carrier circuits would needlessly waste PSTN resources.

Path-Specific Digit Manipulation

Route Lists allow administrators to configure digit transformations on a per-Route Group member basis. For example, if Route Group 1 routes over a SIP trunk expecting standard 11-digit numbers (14085550100), while Route Group 2 routes over an older legacy PBX expecting 7 digits (5550100), the Route List can apply PreDot on Route Group 1 and a Called Party Transform Mask stripping the area code on Route Group 2.

---

6. Call Hunting Architecture

Call Hunting distributes incoming calls to a designated group of internal directory numbers when a single shared department number is dialed (such as Technical Support, IT Helpdesk, or Billing).

Hunting Component Hierarchy

Call hunting in CUCM is constructed using three layered objects:

1. Hunt Pilot: The virtual telephone number (with an assigned partition and CSS) dialed by callers (e.g., extension 4500). The Hunt Pilot points to a single Hunt List.
2. Hunt List: An ordered collection of one or more Line Groups. It defines the sequence in which different agent tiers are engaged.
3. Line Group: An ordered collection of individual Directory Numbers (endpoints) that ring according to a selected distribution algorithm.

Line Group Distribution Algorithms

Hunting Timers and Native Queuing

• RNA Reversion Timeout: Configured on the Line Group, the time (default 10 seconds) that CUCM rings a member before treating the attempt as Ring No Answer (RNA) and advancing to the next member.
• RNA Reversion Destination: The fallback destination (such as a general voicemail box or external answering service) if all members across all Line Groups fail to answer.
• Native Call Queuing: Built-in CUCM capability allowing callers to wait in queue when all Line Group members are busy. Features include maximum queue depth (e.g., 32 callers), periodic queue position and wait-time announcements, and Music on Hold (MoH) streaming.