why asbr-pe rewrite vpn label because next-hop changed ,why.
if vpn label is not rewritten, is there any problem? what problem it is.
The problem arises when forwarding the packet, because of the construction of the label stack. In general the VPN label MUST be assigned by the BGP next hop of a VPNv4 route. Assume an ASBR-PE with next hop IP 184.108.40.206 will not rewrite a label learned f.e. VPN label 12345 and the ASBR-PE has no label 12345 assigned himself.
When the label stack is created by another PE in this AS in general it will look like this:
Top label: LDP label for 220.127.116.11/32 assigned by a P router
VPN label 12345
This will be sent towards the ASBR-PE. as the top label points to 18.104.22.168/32 typically the ASBR-PE will assign an implicit null label, i.e. penultimate hop popping occurs. This means that the penultimate hop will remove the top label and send the rest, i.e. VPN label 12345. This arrives at the ASBR-PE and no matching LFIB entry in my example would lead to packet drops.
So again, VPN labels MUST be assigned by the BGP next hop of a VPNv4 prefix or the packets will be dropped or forwarded to the wrong LSP.
This any modification of the BGP next hop needs to go along with a rewrite of the VPN label.
Hope this helps! Please rate all posts.