Version
VyOS 1.5-rolling-202503030030

Summary
It seems like enabling multicast inside a GRE VPN tunnel interface is not possible, or at least not functioning properly. This breaks routing protocols configured inside the tunnel.

Symptomps Discovered

I enable multicast with following command,

set interfaces tunnel tun3 enable-multicast

Then i configure for example MPLS LDP with following parameters:

set protocols bgp interface tun3 mpls forwarding
set protocols mpls interface 'tun3'
set protocols mpls ldp discovery transport-ipv4-address '192.168.0.1'
set protocols mpls ldp interface 'tun3'
set protocols mpls ldp router-id '192.168.0.1'

But LDP won't form neighbor relationship over the tunnel interface, unless using targeted LDP. Similar issues happened with OSPF and I suspect it is related to this.

I figured out that the configuration in FRR doesn't show that multicast is enabled in the running configuration under the tunnel interface, because it seems to be the default. But it's not on by default in VyOS mode? Anyway, a "show interface tunX" from FRR shows that multicast is indeed enabled:

LBS-RO1# show int tun3
Interface tun3 is up, line protocol is up
  Link ups:       4    last: 2025/03/19 22:45:59.76
  Link downs:     3    last: 2025/03/17 19:24:02.23
  vrf: default
  index 21 metric 0 mtu 1400 speed 0 txqlen 1000
  flags: <UP,LOWER_UP,RUNNING,MULTICAST>
  MPLS enabled
  Ignore all v6 routes with linkdown
  Type: GRE over IP
  inet 192.168.2.1/32 unnumbered
  inet6 fe80::fc21:62ff:fe51:2629/64
  Interface Type GRE
  Interface Slave Type None
  Link Interface eth1.3004
  protodown: off 
  Parent interface: eth1.3004

but, when turning on "terminal monitor" from FRR, these messages shows up constantly:

LBS-RO1# terminal monitor 
2025-03-19 22:52:05.473 [WARN] ldpd: [HD2R1-Z7JJT] send_packet: error sending packet to 224.0.0.2
2025-03-19 22:52:10.478 [WARN] ldpd: [HD2R1-Z7JJT] send_packet: error sending packet to 224.0.0.2

Steps to reproduce

Configure a tunnel interface with GRE encapsulation. Enable a routing protocol, for example BGP between two routers. Also enable mpls ldp on the tunnel.

Alternatively configure OSPF point-to-multipoint or broadcast mode and see if you get similar multicast errors.

Full configuration can be found on my blog that will be released on April 14th, but I can let you have the secret preview link:
https://opensourceisfun.substack.com/p/37bf68c3-53a3-466e-8446-31b030b7a59b