Understanding Mpls Label Operations On Ex Series Switches; Mpls Label Switched Paths And Mpls Labels On Ex Series Switches - Juniper JUNOS OS 10.4 - FOR EX REV 1 Manual

Complete Software Guide for Junos

Understanding MPLS Label Operations on EX Series Switches

MPLS Label Switched Paths and MPLS Labels on EX Series Switches

3650
®
OS for EX Series Ethernet Switches, Release 10.4
Configuring Policers to Control Traffic Rates (CLI Procedure) on page 3300
In the traditional packet-forwarding paradigm, as a packet travels from one switch to
the next, an independent forwarding decision is made at each hop. The IP network header
is analyzed and the next hop is chosen based on this analysis and on the information in
the routing table. In an MPLS environment, the analysis of the packet header is made
only once, when a packet enters the MPLS tunnel (that is, the path used for MPLS traffic).
When an IP packet enters a label-switched path (LSP), the ingress provider edge (PE)
switch examines the packet and assigns it a label based on its destination, placing the
label in the packet's header. The label transforms the packet from one that is forwarded
based on its IP routing information to one that is forwarded based on information
associated with the label. The packet is then forwarded to the next provider switch in
the LSP. This switch and all subsequent switches in the LSP do not examine any of the
IP routing information in the labeled packet. Rather, they use the label to look up
information in their label forwarding table. They then replace the old label with a new
label and forward the packet to the next switch in the path. When the packet reaches
the egress PE switch, the label is removed, and the packet again becomes a native IP
packet and is again forwarded based on its IP routing information.
MPLS Label Switched Paths and MPLS Labels on EX Series Switches on page 3650
Reserved Labels on page 3651
MPLS Label Operations on EX Series Switches on page 3651
Penultimate Hop Popping on page 3652
When a packet enters the MPLS network, it is assigned to an LSP. Each LSP is identified
by a label, which is a short (20-bit), fixed-length value at the front of the packet. Labels
are used as lookup indexes for the label forwarding table. For each label, this table stores
forwarding information. Because no additional parsing or lookup is done on the
encapsulated packet, MPLS supports the transmission of any other protocols within the
packet payload.
NOTE: MPLS for EX Series switches supports only single-label packets.
Figure 90 on page 3651 shows the encoding of a single label. The encoding appears after
data link layer headers, but before any network layer header.
Copyright © 2010, Juniper Networks, Inc.