Introduction
When KNX IP routing stops working, the system often looks partially alive:
- ETS connects successfully
- Devices appear online
- Downloads may work on one line
- But group communication between lines fails
This creates confusion, finger-pointing, and wasted time.
The truth is simple:
KNX IP routing fails in very specific and repeatable ways.
This article provides a step-by-step method to identify exactly why KNX IP routing is not working and how to fix it, based on real commissioning and service cases.
First Reality Check: What “Routing Not Working” Really Means
KNX IP routing is failing if:
- Telegrams do not pass between KNX lines
- Group monitor shows traffic on one line but not another
- Cross-line scenes don’t work
- Status feedback never returns
ETS connectivity alone does not prove routing is working.
STEP 1: Confirm You Are Using IP Routers (Not Interfaces)
This sounds obvious, but it’s a very common mistake.
✔ Device must be a KNX IP Router, not an IP Interface
✔ Routing capability must exist in the hardware
✔ Interface-only devices cannot route telegrams
If even one segment uses an interface instead of a router, routing will fail.
STEP 2: Verify Routing Is Enabled in Device Parameters
Many IP routers ship with routing disabled by default.
✔ Open device parameters in ETS
✔ Confirm “KNX IP Routing” is enabled
✔ Multicast address left at default (unless documented otherwise)
If routing is disabled, the device behaves like an interface only.
STEP 3: Check ETS Topology Placement
ETS topology must match physical reality.
✔ IP routers placed correctly under lines
✔ Correct area and line numbers
✔ No router accidentally placed on wrong line
A router placed on the wrong line will never forward telegrams correctly.
STEP 4: Confirm Individual Addresses
Routing depends on proper addressing.
✔ Every IP router has a unique individual address
✔ Address structure matches topology (Area.Line.Device)
✔ No duplicate addresses
Duplicate or misplaced addresses silently break routing.
STEP 5: Verify All IP Routers Are on the Same IP Network
KNX IP routing does not cross IP subnets by default.
✔ All routers in the same IP subnet
✔ Same VLAN
✔ No firewall or L3 routing between them
If routers are in different VLANs, routing will fail — even if ETS connects.
STEP 6: Confirm Multicast Is Allowed (Most Critical Step)
KNX IP routing depends on multicast.
✔ Multicast enabled on switches
✔ UDP multicast traffic not blocked
✔ No “unknown multicast suppression”
If multicast is blocked:
- ETS still works (unicast)
- Routing fails completely
This is the #1 real-world cause of routing failure.
STEP 7: Review IGMP Snooping Configuration
IGMP snooping can either help or destroy KNX IP routing.
✔ If IGMP snooping enabled → IGMP querier must exist
✔ Snooping must not prune KNX multicast groups
✔ If unsure → disable IGMP snooping on KNX VLAN
Many enterprise switches break KNX routing with default IGMP settings.
STEP 8: Disable Energy Efficient Ethernet (EEE)
EEE causes random and intermittent routing failures.
✔ Disable EEE on all KNX IP router ports
✔ Especially uplink and backbone ports
If routing works after a reboot but fails later, EEE is often the reason.
STEP 9: Check Managed Switch Security Features
Look for:
- Storm control
- Broadcast suppression
- Multicast rate limiting
- Port security rules
✔ Disable or relax these features for KNX VLAN
KNX multicast traffic is often mistaken for “noise” by IT switches.
STEP 10: Verify Filter Tables
Routing can be electrically perfect but logically blocked.
✔ Group addresses correctly linked
✔ Filter tables generated
✔ Filters downloaded to routers and couplers
Over-filtering blocks telegrams silently.
STEP 11: Test with Group Monitor (Correctly)
Use two Group Monitors if possible:
- One on sending line
- One on receiving line
✔ Telegram visible on source line
✔ Telegram appears on destination line
If it stops at the router → network issue
If it passes router but not actuator → filtering or application issue
STEP 12: Simplify to Isolate
If still not working:
✔ Temporarily connect routers via simple unmanaged switch
✔ Remove VLANs temporarily
✔ Eliminate IT network from path
If routing works in isolation, the issue is 100% network configuration.
STEP 13: Check Recent Changes
Routing failures often appear after:
- Switch firmware updates
- IT security changes
- Network expansion
- Device replacement
Always ask: “What changed recently?”
Most Common Root Causes (From Real Projects)
1️⃣ Multicast blocked
2️⃣ IGMP snooping misconfigured
3️⃣ Routers in different VLANs
4️⃣ Routing disabled in parameters
5️⃣ EEE enabled
6️⃣ Wrong topology in ETS
Fixing these solves most routing issues.
What Routing Problems Are NOT
❌ Not usually device defects
❌ Not ETS bugs
❌ Not “KNX is unstable”
KNX IP routing is extremely reliable when the network respects its requirements.
Preventive Design Rules
✔ Dedicated KNX VLAN
✔ Multicast tested early
✔ EEE disabled by default
✔ Clean topology in ETS
✔ Document switch configuration
Routing should be validated before full commissioning.
Conclusion
When KNX IP routing is not working, guessing wastes time.
A structured, step-by-step approach finds the root cause quickly.
In almost every case, the failure lies in:
- Network configuration
- Multicast handling
- ETS topology alignment
Once these are corrected, KNX IP routing becomes rock-solid and predictable.
In modern KNX systems, routing health equals system health.
