The following can be used to confirm MTU inconsistency between BGP peers:
> get logical router
> vrf <VRF/UUID/Name of the impacted Tier-0 SR>
> ping <neighbour_IPv4> source <local_IPv4> dfbit enable size <MTU minus 28>
> ping6 <neighbour_IPv6> source <local_IPv6> size <MTU minus 48>
Output below shows local failure (MTU on the external interface is not big enough to accommodate the ping):edge02(tier0_sr[1])> ping 192.168.132.254 source 192.168.132.2 dfbit enable size 8873
PING 192.168.132.254 (192.168.132.254) from 192.168.132.2: 8873 data bytes
36 bytes from 192.168.132.254: frag needed and DF set (MTU 8900)
Vr HL TOS Len ID Flg off TTL Pro cks Src Dst
4 5 00 22c5 0000 0 0000 40 01 8de6 192.168.132.2 192.168.132.254
36 bytes from 192.168.132.254: frag needed and DF set (MTU 8900)
Vr HL TOS Len ID Flg off TTL Pro cks Src Dst
4 5 00 22c5 0000 0 0000 40 01 8de6 192.168.132.2 192.168.132.254
Failure due to insufficient MTU on the uplink profile or BGP neighbor is less than the MTU on the external interface:edge02(tier0_sr[1])> ping 192.168.132.254 source 192.168.132.2 dfbit enable size 8872
PING 192.168.132.254 (192.168.132.254) from 192.168.132.2: 8872 data bytes
^C
--- 192.168.132.254 ping statistics ---
6 packets transmitted, 0 packets received, 100.0% packet loss