Multicast & IGMP
Summary
IGMP v3 snooping is enabled on the CRS326 to manage multicast traffic for both PTP timing and media streams. The CRS326 acts as the IGMP querier on the bridge. Multicast is fundamental to SMPTE 2110 — all media streams use multicast groups in the 239.1.0.x range, and PTP uses the well-known multicast groups 224.0.1.129 (PTP announce/sync) and 224.0.0.107 (peer delay).
A critical interaction exists between IGMP snooping and PTP on MikroTik switches: IGMP snooping dynamically creates MDB entries that exclude the bridge interface from PTP multicast groups, breaking boundary clock synchronization. The fix is mandatory static MDB entries for PTP groups on all PTP-enabled ports plus the bridge interface.
QoS/DSCP configuration is planned for Phase 3 to prioritize PTP (DSCP 56/CS7) over media (DSCP 46/EF for audio) over best-effort traffic.
Timeline
- 2026-04-03: IGMP v3 snooping identified as requirement during switch research.
- 2026-04-03: PTP + IGMP conflict discovered and workaround documented.
- 2026-04-03: QoS/DSCP plan drafted for PTP and Dante audio.
Current State
Not yet configured — waiting for CRS326 delivery. Configuration commands are drafted.
Planned CRS326 IGMP config:
/interface bridge set [find name=bridge] igmp-snooping=yes
/interface bridge set [find name=bridge] multicast-querier=yes
/interface bridge mdb add group=224.0.1.129 bridge=bridge ports=ether1,ether2,ether4,bridge
/interface bridge mdb add group=224.0.0.107 bridge=bridge ports=ether1,ether2,ether4,bridge
Planned QoS (Phase 3):
| Traffic | DSCP | Priority |
|---|---|---|
| PTP clock | 56 (CS7) | Highest |
| Dante/AES67 audio | 46 (EF) | High |
| Dante low-priority | 8 (CS1) | Low |
Key Decisions
- 2026-04-03: IGMP v3 over v2 — required for source-specific multicast (SSM) support used by some 2110 implementations.
- 2026-04-03: CRS326 as IGMP querier — simplest setup, no external querier needed.
- 2026-04-03: Static MDB entries for PTP — mandatory workaround for MikroTik IGMP+PTP conflict.
Experiments & Results
| Experiment | Status | Finding | Source |
|---|---|---|---|
| IGMP v3 membership reports (Wireshark) | Planned | Capture join/leave for 239.1.0.x | Phase 1 plan |
| PTP multicast with IGMP snooping | Planned | Verify static MDB fix works | Switch research |
| QoS DSCP marking verification | Phase 3 | Wireshark DSCP field check | Architecture doc |
Gotchas & Known Issues
- Static MDB entries are mandatory — without them, IGMP snooping breaks PTP boundary clock sync on MikroTik.
- Bridge interface must be in MDB entry — not just the physical ports. This is the non-obvious part of the fix.
- PTP multicast on SFP+ uplink — must NOT reach USW-16-PoE. PTP disabled on SFP+ ports prevents leakage.
- If PTP ports use VLANs — also add bridge as untagged port in Bridge VLAN Table for every PTP VLAN entry.
Open Questions
- Will IGMP snooping on the CRS305 media switch cause any issues with 2110 media multicast?
- How many simultaneous multicast groups can the CRS326 handle before MDB table limits matter?
- Should media multicast groups also get static MDB entries, or is dynamic IGMP sufficient?