Alcatel-Lucent 6850-48 Network Guide
MPLS Overview
Configuring MPLS
page 10-12
OmniSwitch AOS Release 6 Network Configuration Guide
September 2009
MPLS Overview
MPLS directs a flow of IP packets along a Label Switched Path (LSP). LSPs are simplex, meaning that the
traffic flows in one direction (unidirectional) from an ingress router to an egress router. Two LSPs are
required for duplex traffic. Each LSP carries traffic in a specific direction, forwarding packets from one
router to the next across the MPLS domain.
traffic flows in one direction (unidirectional) from an ingress router to an egress router. Two LSPs are
required for duplex traffic. Each LSP carries traffic in a specific direction, forwarding packets from one
router to the next across the MPLS domain.
When an ingress router receives a packet, it adds an MPLS header to the packet and forwards it to the next
hop in the LSP. The labeled packet is forwarded along the LSP path until it reaches the egress router
(destination point). The MPLS header is removed and the packet is forwarded based on Layer 3 informa-
tion such as the IP destination address. The physical path of the LSP is not constrained to the shortest path
that the IGP would choose to reach the destination IP address.
hop in the LSP. The labeled packet is forwarded along the LSP path until it reaches the egress router
(destination point). The MPLS header is removed and the packet is forwarded based on Layer 3 informa-
tion such as the IP destination address. The physical path of the LSP is not constrained to the shortest path
that the IGP would choose to reach the destination IP address.
MPLS Label Stack
MPLS requires a set of procedures to enhance network layer packets with label stacks which thereby turns
them into labeled packets. Routers that support MPLS are known as Label Switching Routers (LSRs). In
order to transmit a labeled packet on a particular data link, an LSR must support the encoding technique
which, when given a label stack and a network layer packet, produces a labeled packet.
them into labeled packets. Routers that support MPLS are known as Label Switching Routers (LSRs). In
order to transmit a labeled packet on a particular data link, an LSR must support the encoding technique
which, when given a label stack and a network layer packet, produces a labeled packet.
In MPLS, packets can carry not just one label, but a set of labels in a stack. An LSR can swap the label at
the top of the stack, pop the stack, or swap the label and push one or more labels into the stack. Labeled
packet processing is independent of the level of hierarchy. Processing is always based on the top label in
the stack which includes information about the operations to perform on the packet's label stack.
the top of the stack, pop the stack, or swap the label and push one or more labels into the stack. Labeled
packet processing is independent of the level of hierarchy. Processing is always based on the top label in
the stack which includes information about the operations to perform on the packet's label stack.
The processing of a labeled packet is completely independent of the level of hierarchy. The processing is
always based on the top label, without regard for the possibility that some number of other labels may
have been above it in the past, or that some number of other labels may be below it at present.
always based on the top label, without regard for the possibility that some number of other labels may
have been above it in the past, or that some number of other labels may be below it at present.
The label value at the top of the stack is looked up when a labeled packet is received. A successful lookup
reveals:
reveals:
• The next hop where the packet is to be forwarded.
• The operation to be performed on the label stack before forwarding.
In addition, the lookup may reveal outgoing data link encapsulation and other information needed to prop-
erly forward the packet.
erly forward the packet.
Label Switching Routers
LSRs perform the label switching function. LSRs perform different functions based on the position of the
router in an LSP. Routers in an LSP do one of the following:
router in an LSP. Routers in an LSP do one of the following:
• The router at the beginning of an LSP is the ingress Label Edge Router (ILER). The ingress router can
encapsulate packets with an MPLS header and forward it to the next router along the path. An LSP can
only have one ingress router.
only have one ingress router.
• A Label Switching Router (LSR), also referred to as a transit router, is any intermediate router within
the LSP between the ingress and egress routers. An LSR swaps the incoming label with the outgoing
MPLS label and forwards the MPLS packets it receives to the next router in the MPLS path (LSP).
MPLS label and forwards the MPLS packets it receives to the next router in the MPLS path (LSP).
• The router at the end of an LSP is the egress Label Edge Router (ELER). The egress router strips the
MPLS encapsulation which changes it from an MPLS packet to a data packet, and then forwards the