mpls router not forward pakcets with mpls labels in some circumstance

[ChengChung]

related linux kernel version: 5.11.0-1017-aws, 5.10.0-1045-oem and some other versions
all instances are run as virtual machine
ubuntu version: 20.04 with all packages updated

recently when I try to use ubuntu as mpls router, I found that in some circumstance, the packets will be dropped or discarded.
I have set all related interfaces with the rp_filter value as 0, and ipv4 forwarding enabled, also mpls input enabled

I always set
net.core.default_qdisc = fq
net.ipv4.tcp_congestion_control = bbr
in /etc/sysctl.conf in most of my VMs, and find with these two settings, or more exactly, with
net.core.default_qdisc = fq
, the router will not forward mpls packets.

Assume I have 3 VMs, named A, B, C, they are connected as A-B-C via virtual links.
I use frrouting as routing software, and use ospf as iGP, with MPLS-te and segment-routing enable with no-php-flag.
if A-B and B-C are connected with GRE tunnel, in this circumstance with default_qdisc set to fq, if I send an ICMP packet from A to C, I can see mpls packets forwared to interface B-C on VM-B, but the interface B-C dropped packet(as the counter increase), and C-B interface on VM C don't see any income packets and none of the releated packets can be seen on ethernet interfaces.
But when I reboot without setting default_qdisc to fq, the packet can be successfully forwarded from A to C by B.
if A-B and B-C are connected with tap tunnel(for example, I use tinc), I cannot see packet forwarded to interface B-C at all, and without default_qdisc set to fq, I can see packets successfully forwarded to C.

As the mpls routing table and normal IP route table seems nothing wrong, so I don't think it's caused by frrouting.
And I am not sure what exactly the conditions to reproduce this bug, but at least 'net.core.default_qdisc = fq' can be used to reproduce it.

[Ubuntu Foundations Team Bug Bot (crichton)]

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