The MPLS WG Archive

[Juha Heinanen <jh@lohi.eng.telia.fi>]

Metz, E.T. writes:

 > When CE and PE configuration are
 > not aligned on this point, the CE may expect to connect to one site on a
 > particular circuit, while the PE actually connects it to another one.

in fr or atm world, the ce learns via lmi or ilmi about new or deleted
labels.  then it sends an inarp message along the vc and learns who is
behind that vc and starts to exchange ospf messages with it.  the vc
topology doesn't need to be full mesh and still all the vpn members can
share a common ip subnet.

as long as the ce-pe protocol remains fr or atm and mpls is used only in
the provider network, there should be no changes whatsoever to the above
described ce operation.

but if the ce-pe protocol is native mpls, then the standard fr or atm
inarp procedure stops working, because (a) mpls vcs are unidirectional
and (b) can be merged.  one possibility might be that the destination ce
that receives an inarp request for an unkown source, sends the reply
back to all vcs that are still unknown to it.

-- juha