How MPLS IP VPN Works
MPLS IP VPN works by prefixing VOIP
Phone Service packets with an MPLS header, containing
one or more 'labels'. Called a label stack. Each entry
contains four fields:
The VOIP Phone Service entry and
exit points of an MPLS IP VPN network are called Label
Edge Routers (LER), which, respectively, push an MPLS IP
VPN label onto an incoming VOIP Phone Service packet and
pop it off the outgoing packet. Routers that perform
routing based only on the label are called Label Switch
Routers (LSR).
Labels are distributed between VOIP
Phone Service LERs and LSRs using the “Label
Distribution Protocol” (LDP)[4]. Label Switch Routers in
an MPLS VOIP Phone Service network regularly exchange
label and reachability information with each other using
standardized procedures in order to build a complete
picture of the network they can then use to forward
packets. Label Switch Paths (LSPs) are established by
the network operator for a variety of purposes, such as
to create network-based IP Virtual Private Networks or
to route VOIP Phone Service traffic along specified
paths through the network. In many respects, LSPs are
not different from PVCs in ATM or Frame Relay networks,
except that they are not dependent on a particular Layer
2 technology.[5]
These MPLS IP VPN labeled packets
are switched after a Label Lookup/Switch instead of a
lookup into the IP table. As mentioned above, when MPLS
IP VPN was conceived, Label Lookup and Label Switching
were faster than a RIB lookup because they could take
place directly within the switched fabric and not the
CPU.
In the specific context of an
MPLS-based Virtual Private Network (VPN), LSRs that
function as ingress and/or egress routers to the VPN are
often called PE (Provider Edge) routers. Devices that
function only as transit routers are similarly called P
(Provider) routers. See RFC 2547.[6] The job of a P
router is significantly easier than that of a PE router,
so they can be less complex and may be more dependable
because of this.
When an unlabeled VOIP Phone Service packet
enters the ingress router and needs to be passed on
to an MPLS IP VPN VOIP Phone Service tunnel, the
router first determines the forwarding equivalence class
(FEC) the packet should be in, and then inserts one or
more labels in the packet's newly-created MPLS IP VPN
header. The packet is then passed on to the next hop
router for this tunnel.
