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MPLS Architecture
511
Figure 122 MPLS basic principle
Forwarding Labeled
Packets
At the ingress, the packets entering the network are classified into FECs according
to their characteristics. Usually, packets are classified into FECs according to the IP
address prefix or host address. Packets in the same FEC pass through the same
path (that is, LSP) in MPLS area. LSR assigns a short label of fixed length for the
incoming FEC packet, and then forwards it through the corresponding interface.
to their characteristics. Usually, packets are classified into FECs according to the IP
address prefix or host address. Packets in the same FEC pass through the same
path (that is, LSP) in MPLS area. LSR assigns a short label of fixed length for the
incoming FEC packet, and then forwards it through the corresponding interface.
On the LSR along the LSP, the mapping table of the import/export labels has been
established (the element of this table is referred to as Next Hop Label Forwarding
Entry (NHLFE)). When the labeled packet arrives, LSR only needs to find the
corresponding NHLFE from the table according to the label and replace the
original label with a new one, and then forwards the labeled packet. This process
is called Incoming Label Map (ILM).
established (the element of this table is referred to as Next Hop Label Forwarding
Entry (NHLFE)). When the labeled packet arrives, LSR only needs to find the
corresponding NHLFE from the table according to the label and replace the
original label with a new one, and then forwards the labeled packet. This process
is called Incoming Label Map (ILM).
At the ingress, MPLS specifies a FEC for a specific packet, and the following
routers only need to forward the packet by label switching, therefore this method
is much simpler than general network layer forwarding and increases the
forwarding speed.
routers only need to forward the packet by label switching, therefore this method
is much simpler than general network layer forwarding and increases the
forwarding speed.
Establishing LSP
Actually, the establishment of LSP refers to the process of binding FEC with the
label, and then advertising this binding to the adjacent LSR on LSP. This process is
implemented through LDP, which regulates the message in interactive processing
and message structure between LSRs as well as routing mode.
label, and then advertising this binding to the adjacent LSR on LSP. This process is
implemented through LDP, which regulates the message in interactive processing
and message structure between LSRs as well as routing mode.
LDP working process
Through sending Hello message periodically, an LSR finds its neighbor and then
establish LDP session with the newly discovered adjacent LSR. By LDP session, the
adjacent LSRs advertise such information as label switching mode, label space,
session Keepalive timer value to each other. LDP session is a TCP connection,
which needs to be maintained through LDP message. In case there is not any other
LDP message during the time period specified by the session Keepalive timer value,
and then it is necessary to send session Keepalive message to maintain the
existence of LDP session. Figure 123 illustrates the diagram of LDP label
distribution.
establish LDP session with the newly discovered adjacent LSR. By LDP session, the
adjacent LSRs advertise such information as label switching mode, label space,
session Keepalive timer value to each other. LDP session is a TCP connection,
which needs to be maintained through LDP message. In case there is not any other
LDP message during the time period specified by the session Keepalive timer value,
and then it is necessary to send session Keepalive message to maintain the
existence of LDP session. Figure 123 illustrates the diagram of LDP label
distribution.
LSP
MPLS core router (LSR)
Ingress
Egress
MPLS edge router (LER)
LSP
MPLS core router (LSR)
Ingress
Egress
MPLS edge router (LER)
LSP
MPLS core router (LSR)
Ingress
Egress
MPLS edge router (LER)
LSP
MPLS core router (LSR)
Ingress
Egress
MPLS edge router (LER)