or incapable of analysing these headers with
sufficient high speed.- Assigning a packet to an FEC, the ingress
router may use information about the packet
that goes beyond the content of the packet
header, like the interface. Hence, assignment
to FECs can be a more involved process, with-
out impacting all routers in the network. - Forwarding decisions within a network may
be made depending on which ingress router a
packet used. Then a packet may be forced to
follow a particular route explicitly chosen, cir-
cumventing the ordinary routing.
Some central terms for MPLS are given in Box B.
6.1 MPLS Formats and Terms
To some extent, the use of Label Switched Paths
(LSPs) can be considered as introducing tun-
nelling as seen from the IP layer. That is, when
an LSP is introduced an intermediate node
would not examine the IP header information in
order to decide upon the proper handling of the
packets arriving in that LSP. That is, with MPLS
the classification of packets into FECs is only
performed at the ingress to the MPLS domain.
The packet is then mapped to an LSP by encap-
sulation of an MPLS header. The LSP is identi-
fied locally by the header, see Figure 22. Based
on the value of the label the packet is mapped
to the next hop. In successive routers within the
MPLS domain the label is swapped (therefore it
can have only local significance) and the packet
is mapped to the next hop.
An LSP can be considered as a path created by
concatenation of one or more hops, allowing a
packet to be forwarded by swapping labels from
an incoming to an outgoing side of the MPLS
node. An MPLS path is frequently referred to as
layer 2 1/2 in the OSI model. That is, it may be
considered as a tunnel as mentioned above. In
order to introduce a tunnel, a “header” is att-
ached to the IP packet as shown in Figure 22 for
the Point-to-Point Protocol (PPP) case (e.g. IP
over SDH). When the IP packets are carried by
ATM the label may be identical to the VPI/VCI
fields in the ATM cell header. The MPLS archi-
tecture is described in [RFC3031].
Referring to Figure 22, the fields in the MPLS
header can be used as follows:- Label – contains a 20 bit tag identifying an
LSP; - Exp – contains 3 bits (originally not allocated,
intended for experimentation) which can refer
to a certain service class, e.g. in analogy to the
DiffServ classes;
Figure 22 MPLS header and
placement in layer “2 1/2”Box B Selected MPLS terminology (from [RFC3031])
Label– A short fixed length physically continuous identifier used to identify an
FEC, of local significance.
Label merging– The replacement of multiple incoming labels for a particular
FEC with a single outgoing label.
Label swap– The basic forwarding operation consisting of looking up an in-
coming label to determine the outgoing label, encapsulating, port, and other
data handling information.
Label swapping– A forwarding paradigm allowing streamlined forwarding
of data by using labels to identify classes of data packets which are treated
indistinguishably when forwarding.
Label switched hop– The hop between two MPLS nodes, on which forwarding
is done by use of labels.
Label switched path– The path through one of more Label Switched Routers
(LSRs) at one level of the hierarchy followed by a packet of a particular FEC.
Label stack– An ordered set of labels.
Merge point– A node at which label merging is done.
MPLS domain– A continuous set of nodes which operate MPLS routing and
forwarding and which are also in one routing or administrative domain.
MPLS edge node– An MPLS node that connects an MPLS domain with a node
which is outside of the domain, either because it does not run MPLS, and/or
because it is in a different domain. Note that if an LSR has a neighbouring host
which is not running MPLS, that LSR is an MPLS edge node.
MPLS egress node– An MPLS edge node in its role in handling traffic as it
leaves an MPLS domain.
MPLS ingress node– An MPLS edge node in its role in handling traffic as it
enters an MPLS domain.
MPLS label– A label which is carried in a packet header, and which represent
the packet’s FEC.
MPLS node– A node which runs MPLS. An MPLS node will be aware of MPLS
control protocols, will operate one or more layer 3 routing protocols, and will be
capable of forwarding packets based on labels.Label (20) Exp (3) S (1) TTL (8)e.g. PPP header MPLS header IP header • • • • •