Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) User Manual
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Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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How MPLS works
1
1. The Ingress LER receives a packet and pushes a label onto it.
When a packet arrives on an MPLS-enabled interface, the device determines to which LSP (if
any) the packet are assigned. Specifically, the device determines to which Forwarding
Equivalence Class (FEC) the packet belongs. An FEC is simply a group of packets that are all
forwarded in the same way. For example, a FEC could be defined as all packets from a given
Virtual Leased Line. FECs are mapped to LSPs. When a packet belongs to a FEC, and an LSP is
mapped to that FEC, the packet is assigned to the LSP.
When a packet is assigned to an LSP, the device, acting as an ingress LER, applies (pushes) a
tunnel label onto the packet. A label is a 32-bit, fixed-length identifier that is significant only to
MPLS. Refer to
for specific information about the contents of a
label. From this point until the packet reaches the egress LER at the end of the path, the
packet is forwarded using information in its label, not information in its IP header. The packet’s
IP header is not examined again as long as the packet traverses the LSP. The ingress LER may
also apply a VC label onto the packet based on the VPN application.
On the ingress LER, the label is associated with an outbound interface. After receiving a label,
the packet is forwarded over the outbound interface to the next router in the LSP.
2. A transit LSR receives the labelled packet, swaps the label, and forwards the packet to the next
LSR.
In an LSP, zero or more transit LSRs can exist between the ingress and egress LERs. A transit
LSR swaps labels on an MPLS packet and forwards the packet to the next router in the LSP.
When a transit LSR receives an MPLS packet, it looks up the label in its MPLS forwarding
table. This table maps the label and inbound interface to a new label and outbound interface.
The transit LSR replaces the old label with the new label and sends the packet out the
outbound interface specified in the table. This process repeats at each transit LSR until the
packet reaches the next-to-last LSR in the LSP (for signaled LSPs).
illustrates an example of the label swapping process on a transit LSR.
FIGURE 2
Label swapping on a transit LSR