LC-ATM and VC Merge

Below are some points about LC(Label Controlled)-ATM interfaces and VC Merge;
  • In cell-mode MPLS networks, the label is encoded as the VPI/VCI field from the ATM header.
  • The number of virtual circuits supported by router and switch hardware is severely limited • Conclusion: Labels in cell-mode MPLS are a scarce resource.
  • Unlabeled IP packets cannot be propagated across LC-ATM interfaces.
  • Forwarding tables are not built until the labels are assigned to destinations in IP routing tables.
  • Each ATM switch acts as an IP router in control plane, exchanging IP router information.
  • Routing tables are built only after an ordered sequence of requests, from the upstream side, have been answered from downstream routers.
  • An ATM switch can allocate an incoming label only if it already has a corresponding outgoing label.
  • An egress ATM edge LSR allocates a label and replies to requests from upstream neighbors.
  • LDP uses an additional TLV to count the number of hops in an LSP.
  • The TTL field in the IP header or label header is decreased by the number of hops by the ingress ATM edge LSR before being forwarded through an LVC. If the TTL field is 0 or less, the packet is discarded.
  • Because it is possible to have two virtual circuits merge into one virtual circuit, the interleaving of cells is a potential problem.
  • VC merge solves the cell interleaving issue by buffering incoming cells from a new packet until all of the cells from the first packet have been forwarded.
  • Per-interface label allocation prevents label spoofing.
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