FRR in Ubuntu 20.04 does not forward multicast packet

[hoangphhut]

Hi
I am using frr 7.2.1 on ubuntu 20.04 as a router. The router has 2 interfaces: enp0s9 to receive UDP multicast and forward to another interface enp0s8. The multicast routing config as following:

$ sudo nano /etc/frr/frr.conf
frr version 7.2.1
frr defaults traditional
hostname R1
log syslog informational
no ipv6 forwarding
service integrated-vtysh-config
!
interface enp0s8
 ip pim
!
interface enp0s9
 ip pim
 ip mroute enp0s8 226.96.1.1
!
line vty
!

I check the mroute and it seems OK:

# show ip mroute
 Source         Group       Flags  Proto   Input   Output  TTL  Uptime
 *              226.96.1.1  -      STATIC  enp0s9  enp0s8  1    00:22:50

Then, I send a UDP stream by iperf from client to enp0s9. By using tcpdump I got UDP packets arrived on that interface:

$ sudo tcpdump -i enp0s9 -n
23:36:20.423001 IP 192.168.1.120.55761 > 226.96.1.1.5001: UDP, length 1470
23:36:20.434268 IP 192.168.1.120.55761 > 226.96.1.1.5001: UDP, length 1470
23:36:20.444920 IP 192.168.1.120.55761 > 226.96.1.1.5001: UDP, length 1470

but nothing come out from interface enp0s8:

$ sudo tcpdump -i enp0s8 -n
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on enp0s8, link-type EN10MB (Ethernet), capture size 262144 bytes

Ubuntu kernel does not forward any multicast packet (but it forward IP unicast packets, I did set net.ipv4.ip_forward=1). Did I forget any configuration? I did the same config with Ubuntu 18.04 and it worked!!!

Anyone can help me please.
Thank you

[donaldsharp]

this was asked in slack. The ttl of the multicast packet was 1 when it hit the router. Thus it was dropped.