MTU for ip6gre tunnel?

Bugs
1

Using version 1.4-RC1

I set up two GRE tunnels, one across an IPv4 network (gre) and another across IPv6 (ip6gre). Both tunnels will carry dual stack traffic.

Without specifying MTU, VyOS set both to 1476 because they are on a link with 1500 MTU. However for the ip6gre tunnel the MTU seems to be too large.

On the gre tunnel the MTU is found to be 1476 as set:

vyos@vyos:~$ ping 10.0.0.5 do-not-fragment size 1460
PING 10.0.0.5 (10.0.0.5) 1460(1488) bytes of data.
/bin/ping: local error: message too long, mtu=1476

On the ip6gre tunnel, the MTU is found to be 1448:

$ ping 10.0.0.1 do-not-fragment size 1430
PING 10.0.0.1 (10.0.0.1) 1430(1458) bytes of data.
/bin/ping: local error: message too long, mtu=1448

is 1448 the expected MTU for an ip6gre tunnel on a 1500 MTU link? If so this should be set as the default when creating an ip6gre tunnel.

2

Provide set of commands to reproduce

3

Create an ip6gre tunnel with minimal options. I assigned an IPv4 and two IPv6 addresses.

set interfaces tunnel tun0 address '10.0.0.2/30'
set interfaces tunnel tun0 address '2xxx:...::2/64'
set interfaces tunnel tun0 address 'fe80::10/128'
set interfaces tunnel tun0 encapsulation 'ip6gre'
set interfaces tunnel tun0 remote '2xxx:...::1'
set interfaces tunnel tun0 source-address '2607:...:a5a'

Other side of the tunnel is also VyOS, IPv4 address is 10.0.0.1.

sh int shows,

tun0         10.0.0.2/30                               n/a                default   1476  u/u
             2xxx:...::2/64

so MTU is 1476 as set up by default.

Now ping across the ip6gre tunnel, should fit but does not:

$ ping 10.0.0.1 do-not-fragment size 1430
PING 10.0.0.1 (10.0.0.1) 1430(1458) bytes of data.
/bin/ping: local error: message too long, mtu=1448