CRS305-1G-4S+IN: Unicast IPv6 Routing Works, but IPv6 Multicast Streams Fail Across VLANs

RouterOS Scripting
1

Hello MikroTik Community,

We are setting up a hardware-in-the-loop testing environment using a CRS305-1G-4S+IN running RouterOS v7. We are encountering an issue where IPv6 Unicast traffic routes perfectly between two VLANs, but IPv6 Multicast stream traffic fails to reach destination receivers across subnets.

System Topology & Addressing Overview

  • Router/Switch: MikroTik CRS305-1G-4S+IN

  • Interface 1 (sfp-sfpplus1): Connected to Receiver / Host (VLAN 4, Subnet fd53:7xxx:xxx:4::/64)

  • Interface 2 (sfp-sfpplus2): Connected to Transmitter / ECU (VLAN 2, Subnet fd53:7xxx:xxx:2::/64)

  • Routing Type: Inter-VLAN Layer 3 IPv6 Routing (No NAT)

Multicast Signal Details

  • Source IPv6 Address: fd53:7xxx:xxx:2::10f

  • Destination Multicast Group 1: ff14::63

  • Destination Multicast Group 2: ff14::7f

The Problem

  1. Unicast IPv6 (Diagnostic Traffic): Works flawlessly. TCP/UDP unicast packets route between VLAN 2 and VLAN 4 with low latency and zero drop rates.

  2. Multicast IPv6 (High-Frequency Data Streams): The Transmitter at fd53:7xxx:xxx:2::10f emits IPv6 Multicast streams targeted at destination groups ff14::63 and ff14::7f. Receivers on VLAN 4 subscribe to these groups, but the multicast packets are dropped or fail to traverse the VLAN/subnet boundary through the switch.

Current Configuration Summary

We have created VLAN interfaces attached to SFP ports, assigned /64 IPv6 addresses as default gateways, and configured Neighbor Discovery (ND) on both interfaces:

Code snippet

VLAN Interfaces

/interface vlan add interface=sfp-sfpplus1 name=vlan4-tester vlan-id=4
/interface vlan add interface=sfp-sfpplus2 name=vlan2-ecu vlan-id=2

Gateways

/ipv6 address add address=fd53:7xxx:xxx:4::1/64 interface=vlan4-tester advertise=no
/ipv6 address add address=fd53:7xxx:xxx:2::1/64 interface=vlan2-ecu advertise=no

Firewall Filter Rules

/ipv6 firewall filter
add chain=forward action=accept protocol=icmpv6 comment="Allow ICMPv6"
add chain=forward action=accept src-address=fd53:7xxx:xxx:4::/64 dst-address=fd53:7xxx:xxx:2::/64 comment="Route Subnet 4 to 2"
add chain=forward action=accept src-address=fd53:7xxx:xxx:2::/64 dst-address=fd53:7xxx:xxx:4::/64 comment="Route Subnet 2 to 4"

Specific Multicast Rules

add chain=forward action=accept dst-address=ff14::63/64 comment="Allow Multicast Group 63"
add chain=forward action=accept dst-address=ff14::7f/64 comment="Allow Multicast Group 7f"
add chain=forward action=accept src-address=fd53:7xxx:xxx:2::10f/128 dst-address=ff00::/8 comment="Allow Multicast Source IP"

"Instead of routing multicast traffic across subnets, what is the best way to configure a single L2 Multicast VLAN across both SFP ports on the CRS305? We want to isolate the high-frequency UDP multicast streams (ff14::63 / ff14::7f) to Layer 2 so the switch chip handles them in hardware, while keeping unicast diagnostic traffic routed on L3 VLANs."

Any insight or working config snippets for inter-VLAN IPv6 multicast on CRS300 series hardware would be greatly appreciated!

Thanks in advance.
2

Hi,

Shouldn't that "prompt" be sent to any AI istead of the forum as whole post looks as an chat formatted "answer"?

3

Did you also configure PIM-SM (with a rendez-vous point for the ff14::/16 group)? You can ask your AI for example.

4

Thank you guys for the reply..

I have tried with two option PIM-SM and VLAN Bridging.

VLAN Bridging configuration works perfectly. Here the PDU Multicast Traffic: Configured in LabVIEW to egress on VLAN 2 directly (fd53:7xxx:xxx:2::10f) with destination addresses ff14::7f / ff14::63

Code snippet

/interface bridge vlan # Trunk VLAN 2 on both SFP ports and CPU (br-automotive) for Layer 2 Multicast PDU add bridge=br-automotive tagged=sfp-sfpplus1,sfp-sfpplus2,br-automotive vlan-ids=2