In a NE40E router with VRP V300R002C05B020, a vpn instance is configured with a name with more than 19 characters. (In the NE40E the vpn nstance name can have up to 31 characters).
When using the "display arp vpn-instance" command to check the ARP mapping table of the vpn instance, it is found that the name can only have up to 19 characters:
<mde-lba-ne40e>dis arp vpn-instance ?
STRING<1-19> VPN-Instance name of MPLS VPN
Beacause of this limitation the command can't be used and the ARP table can't be checked.
In order to solve this problem it is required to upgrade the VRP to V300R002C06B260 version or later. Also the problem can be avoided using names with less than 19 characters.
This problem is caused by VRP version limitation.