© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.21-1 MPLS Concepts Introducing MPLS Labels and Label Stacks. - презентация

1 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v MPLS Concepts Introducing MPLS Labels and Label Stacks

2 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v Outline Overview What Are MPLS Labels? What Is the MPLS Label Format? Where Are MPLS Labels Inserted? What Is an MPLS Label Stack? What Are MPLS Label Operations? Summary

3 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v MPLS Labels Are 4 byte identifiers used for forwarding decisions Define the destination and services for a packet Identify a forwarding equivalence class (FEC) Have local significance –Each LSR independently maps a label to an FEC in a label binding. –Label bindings are exchanged between LSRs.

4 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v FEC and MPLS Forwarding An FEC is a group of packets forwarded: –In the same manner –Over the same path –With the same forwarding treatment MPLS packet forwarding consists of: –Assigning a packet to a specific FEC –Determining the next hop of each FEC MPLS forwarding is connection-oriented.

5 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v MPLS Label Format MPLS uses a 32-bit label field that contains the information that follows: 20-bit label (a number) 3-bit experimental field (typically used to carry IP precedence value) 1-bit bottom-of-stack indicator (indicates whether this is the last label before the IP header) 8-bit TTL (equal to the TTL in the IP header)

6 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v MPLS Labels MPLS technology is intended to be used anywhere regardless of Layer 1 media and Layer 2 encapsulation. Frame-mode MPLS is MPLS over a frame-based Layer 2 encapsulation –The label is inserted between the Layer 2 and Layer 3 headers. Cell-mode MPLS is MPLS over ATM. –The fields in the ATM header are used as the label.

7 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v MPLS Labels: Frame-Mode MPLS

8 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v MPLS Label Stack Usually only one label is assigned to a packet, but multiple labels in a label stack are supported. These scenarios may produce more than one label: –MPLS VPNs (two labels): The top label points to the egress router, and the second label identifies the VPN. –MPLS TE (two or more labels): The top label points to the endpoint of the traffic engineering tunnel and the second label points to the destination. –MPLS VPNs combined with MPLS TE (three or more labels).

9 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v Example: MPLS Label Stack The outer label is used for switching the packet in the MPLS network (points to the TE destination). Inner labels are used to separate packets at egress points (points to egress router and identifies VPN).

10 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v Example: MPLS Label Stack Format The PID in a Layer 2 header specifies that the payload starts with a label (labels) followed by an IP header. The bottom-of-stack bit indicates whether the label is the last label in the stack. The receiving router uses the top label only.

11 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v MPLS Label Operations An LSR can perform these functions: –Insert (impose or push) a label or a stack of labels on ingress edge LSR –Swap a label with a next-hop label or a stack of labels in the core –Remove (pop) a label on egress edge LSR

12 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v MPLS Label Operations: Frame Mode On ingress, a label is assigned and imposed. LSRs in the core swap labels based on the contents of the label forwarding table. On egress, the label is removed and a routing lookup is used to forward the packet.

13 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v Summary An MPLS label is a 4 byte identifier used for forwarding decisions. –A MPLS label corresponds to an FEC. MPLS frame-mode labels are inserted between the Layer 2 and Layer 3 headers. MPLS supports multiple labels in one packet, creating a label stack. LSRs can perform these operations: –Insert (impose) a label on ingress edge LSR –Swap a label –Remove (pop) a label on egress edge LSR

14 © 2006 Cisco Systems, Inc. All rights reserved. MPLS v