Traditional IP forwarding techniques
analyze the destination IP address contained in the network layer header for
every packet at each hop in the network. This process is called
hop-by-hop destination-based routing. The route that packets take is
based solely on the destination unicast address. Layer 3 routing protocols
do not traditionally have any interaction with Layer 2 network
characteristics, making the implementation of Quality of Service (QOS) and
loading features difficult.
Multiprotocol Label Switching (MPLS) is
a vendor-independent protocol (based on Cisco's tag-switching protocol) that
applies labels to packets providing QOS and advance route selection
functions. This week's newsletter introduces MPLS concepts.
There are
several terms used in MPLS implementations.
Label Header
applied to a packet by an edge label switch router (edge LSR) and used by
label switch routers (LSR) to forward packets.
Label forwarding
information base (LFIB) Table that indicates where and how to forward
frames. Created by label switch-capable devices, the LFIB contains a
list of entries consisting of an ingress and one or more egress subentries
(outgoing label, outgoing interface, outgoing link-level components). The
LFIB is constructed based on information the LSRs gain from interaction with
the routing protocols.
Label Switch Router
(LSR) Device (switch or router) that forwards based upon label
values. Neighboring LSRs establish a TCP connection to transfer label
bindings.
Edge Label Switch Router (edge LSR) Device on
the edge of a label network that applies or ultimately removes labels from
the packet.
Label-switched Path (LSP) The end-to-end
path defined by labels on a packet.
Label virtual circuit
(LVC) An LSP through an ATM system.
Label Switch
Controller (LSC) An LSR that communicates with an ATM switch to
provide label information.
Label Distribution Protocol
(LDP) IETF standard label binding protocol used to distribute
label information to LSRs.
MPLS has two major components. The control
component (also known as the control plane) is responsible for bindings
between the labels and the network layer routes. It creates and
maintains the LFIB. The forwarding component uses the LFIB to perform
forwarding based on the labels contained in the packets. Labels are
distributed using LDP.
MPLS can be implemented over any media-type and
can be used with point-to-point, multipoint and ATM links. MPLS can be
used with different network layer protocols (hence the term
multiprotocol) by using a control component specific to the desired
protocol.
Labels can be applied to packets in different ways depending
on the network type. In ATM, labels are applied to the layer 2 header of
the packets (cells). IPv6 supports label switching via the Flow Label field.
For most other protocols, the label is applied as a small, shim label header
between the Layer 2 and Layer 3 headers.
This week we covered the
basic components of MPLS. If you want to know more on this fascinating
topic, check out the links below.
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