xStack
®
DES-3810 Series Layer 3 Managed Ethernet Switch Web UI Reference Guide
57
When a packet enters the MPLS domain, the LER is responsible for adding a label to the packet. Also when a packet
leaves the MPLS domain, the LER is responsible for removing the label. Inside the MPLS domain, packets will be
transferred based on their label. The path that a packet follows, in and out of an MPLS domain, is known as a
Label
Switch Path (LSP)
. The LSP, under normal circumstances, is unidirectional. The first LER, in the LSP, is known as
the LSP’s
ingress
and the last LER, in the LSP, is known as the LSP’s
egress
. There can be only one ingress and
egress in an LSP.
When an unlabeled packet enters the ingress router and needs to be passed on to an LSP, the ingress LER first
determines the
Forwarding Equivalence Class (FEC)
the packet should be in, and then inserts one or more labels in
the packet's MPLS header. The packet is then passed on to the next LSR.
Labels are distributed between LERs and LSRs using the
Label Distribution Protocol (LDP)
. The LDP defines the
bi-directional communication between the ingress and the egress LERs of a specific MPLS tunnel, through the MPLS
domain. LERs, using LDP, will build and maintain an LSP database that will be used to forward traffic through the
MPLS network. Thus, the two peer LERs will constantly exchange this information to effectively control the traffic flow.
Labels are always distributed, through the LSP, in the opposite direction of the dataflow. In other words, the label
distribution takes place in an upstream direction. The main function of LDP is to
classify FEC
s,
distribute labels
, and
create and maintain LSP
s.
Static LSP
– Users can configure the LSP manually by physically defining the outgoing labels of upstream LSRs and
incoming labels of downstream LSRs. Static LSPs are configured without the need for LDP or exchange
control packets. This configuration has very little data overhead and is suitable for small-scale networks
only, where the network layout is simple and static.
Dynamic LSP
– Users can configure the LSP to initiate automatically with the use of LDP. Additionally, the
Interior
Gateway Protocol (IGP)
, the
Border Gateway Protocol (BGP)
, and the
Resource Reservation
Protocol (RSVP)
can also be extended to distribute MPLS labels adding the routing functionality. This
configuration is suitable for large-scale networks and is also used for VPN services.
Structure of an MPLS packet header:
Figure 4-3 MPLS Packet Header
The label contains the following fields:
Parameter
Description
Label
This indicates the value field of the label. The length is 20 bits.
EXP
This indicates the bits used for extension. The length is 3 bits. Normally this field is used
for the Class of Service (CoS) service.
S
This indicates the bottom of the label stack value. The length is 1 bit. When this value is
set, in other words configured as ‘1’, it means that this entry is at the bottom of the label
stack.
TTL
This indicates the time-to-live (TTL) value. The length is 8 bits. This field is similar to the
TTL in IP packets.
Labels are always encapsulated between the data link layer and the networking layer. This means that encapsulation
labels support all protocols available in the data link layer.
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