
- •Acknowledgments
- •Introduction
- •Assessment Test
- •Answers to Assessment Test
- •Service Provider Networks
- •Scalability
- •Traffic Engineering
- •Quality of Service
- •MPLS Label Stack
- •Shim Header
- •MPLS Architecture
- •Control
- •Forwarding
- •MPLS Label Switching
- •MPLS Network Components
- •Device Output
- •Label-Switched Paths
- •MPLS Applications
- •MPLS and ATM
- •Overlay
- •Quality of Service
- •Traffic Engineering
- •Summary
- •Exam Essentials
- •Key Terms
- •Review Questions
- •Answers to Review Questions
- •Routing Review
- •Frame-Mode MPLS Working Example
- •Network Routing Protocol Examples
- •MPLS Step by Step
- •Label Distribution
- •Assigning Labels
- •Troubleshooting and Verification
- •Device Configuration
- •IGP Verification
- •CEF Verification
- •MPLS Verification
- •Label Distribution and Bindings
- •Binding Verification
- •Troubleshooting the Network
- •Hiding Service Provider Devices
- •Summary
- •Exam Essentials
- •Key Terms
- •Review Questions
- •Answers to Review Questions
- •Frame-Mode MPLS and ATM
- •Frame-Mode MPLS and ATM Configuration
- •Cell-Mode MPLS
- •Label Binding with ATM
- •Cell-Mode Label Switching
- •VC Merge
- •Loop Prevention
- •Cell-Mode MPLS Configuration
- •Summary
- •Exam Essentials
- •Key Terms
- •Review Questions
- •Answers to Review Questions
- •VPNs 101
- •Point-to-Point Connections
- •Virtual Private Networks
- •Categories of VPNs
- •VPN Routing
- •Peer-to-Peer VPNs
- •Optimal Routing
- •Peer-to-Peer Security
- •Peer-to-Peer VPN Routing
- •Summary
- •Exam Essentials
- •Key Terms
- •Review Questions
- •Answers to Review Questions
- •Service Provider Configuration
- •MPLS VPNs
- •Virtual Router
- •Virtual Routing and Forwarding Tables
- •MPLS Operational Overview
- •MP-BGP Configuration
- •An MPLS VPN Example
- •Route Distinguisher
- •MP-IBGP Configuration Example
- •Initial Network Configuration
- •MP-IBGP Configuration
- •Verification
- •Summary
- •Exam Essentials
- •Key Terms
- •Review Questions
- •Answers to Review Questions
- •A Review of VPNs
- •Configuring a Simple MPLS VPN
- •Configuring VRF Interfaces
- •Running RIP in an MPLS VPN
- •Configuring RIPv2 with Address-Family ipv4
- •Configuring Redistribution
- •Route Targets
- •Configuring Route Targets
- •A Review of Simple VPN Configuration
- •Configuring MPLS in the Service Provider Network
- •Simple VPN Configuration
- •Configuring the PE-CE Routing Protocol
- •Lab: Configuring an MPLS VPN
- •Configuring POP Routers
- •VPN Configuration
- •Raleigh Running-Config
- •Atlanta Running-Config
- •Peer 1 Running-Config
- •Peer 2 Running-Config
- •Verification with Ping
- •Routing Table Isolation
- •Verifying VRF Routes
- •Summary
- •Exam Essentials
- •Key Terms
- •Review Questions
- •Answers to Review Questions
- •MP-BGP and OSPF
- •A Review of OSPF
- •OSPF Router Types
- •Link State Advertisements
- •OSPF for MPLS VPNs
- •OSPF Super-Backbone
- •Preventing Routing Loops
- •Path Selection
- •MPLS VPN OSPF Lab
- •Summary
- •Exam Essentials
- •Key Terms
- •Review Questions
- •Answers to Review Questions
- •Static Routing
- •Device Configuration
- •VPN Configuration
- •Raleigh Running-Config
- •Atlanta Running-Config
- •Peer Router Configuration
- •Verification with Ping
- •Verifying Static VRF Routes
- •E-BGP and MPLS VPNs
- •Device Configuration
- •E-BGP Operation
- •AS-Override
- •VPN Configuration
- •Raleigh Running-Config
- •Atlanta Running-Config
- •Peer Router Configuration
- •Peer 1 Running-Config
- •Peer 2 Running-Config
- •Verification with Ping
- •Advanced MPLS VPN Topologies
- •Simple VPNs
- •Central Services MPLS VPN Topology
- •Overlay MPLS VPN Topology
- •Summary
- •Exam Essentials
- •Key Terms
- •Review Questions
- •Answers to Review Questions
- •Challenge Lab 1
- •MPLS
- •MP-IBGP
- •Answer to Lab 1.1
- •Answer to Lab 1.2
- •Answer to Lab 1.3
- •Challenge Lab 2
- •Tag Switching
- •MP-IBGP
- •Answer to Lab 2.1
- •Answer to Lab 2.2
- •Answer to Lab 2.3
- •Challenge Lab 3
- •VRF Configuration
- •RIPv2
- •Redistribution
- •Answer to Lab 3.1
- •Answer to Lab 3.2
- •Answer to Lab 3.3
- •Challenge Lab 4
- •VRF Configuration
- •OSPF
- •Redistribution
- •Answer to Lab 4.1
- •Answer to Lab 4.2
- •Answer to Lab 4.3
- •Challenge Lab 5
- •VRF Configuration
- •Static Routes and Redistribution
- •Answer to Lab 5.1
- •Answer to Lab 5.2
- •Challenge Lab 6
- •VRF Configuration
- •E-BGP Configuration
- •Answer to Lab 6.1
- •Answer to Lab 6.2
- •Service Provider Network Configuration with OSPF
- •Router Configuration
- •Routing Tables
- •Tags
- •Service Provider Network Configuration with IS-IS
- •Router Configuration
- •Routing Tables
- •Tag Switching Forwarding Tables
- •Glossary
MPLS Label Switching 15
Label-Switched Paths
Now let’s take a look at the label-switched paths. A label-switched path (LSP) is a unidirectional set of LSRs that the labeled packet must flow through in order to get to a particular destination.
Let’s say that the user on PE1 wants to ping the loopback address of PE2. So, the user types ping 192.168.1.4.
By looking at the labels in the following output of PE1, you can see the outbound label that will be used is 28 and it will be sent out Serial 0/0:
PE1#show mpls forwarding-table |
|
|
|
||
Local |
Outgoing |
Prefix |
Bytes tag |
Outgoing |
Next Hop |
tag |
tag or VC |
or Tunnel Id |
switched |
interface |
|
27 |
27 |
192.168.1.16/30 |
0 |
Se0/0 |
point2point |
28 |
28 |
192.168.1.4/32 |
0 |
Se0/0 |
point2point |
29 |
Pop tag |
192.168.1.2/32 |
0 |
Se0/0 |
point2point |
30 |
29 |
192.168.1.3/32 |
0 |
Se0/0 |
point2point |
32 |
Pop tag |
192.168.1.12/30 |
0 |
Se0/0 |
point2point |
If a labeled packet of 28 arrives on P1, it will be sent out Serial 0/1 with an outbound label of 27, as the following output shows:
P1#show mpls forwarding-table |
|
|
|
||
Local |
Outgoing |
Prefix |
Bytes tag |
Outgoing |
Next Hop |
tag |
tag or VC |
or Tunnel Id |
switched |
interface |
|
27 |
Pop tag |
192.168.1.16/30 |
0 |
Se0/1 |
point2point |
28 |
27 |
192.168.1.4/32 |
0 |
Se0/1 |
point2point |
29 |
Pop tag |
192.168.1.3/32 |
0 |
Se0/1 |
point2point |
31 |
Pop tag |
192.168.1.1/32 |
0 |
Se0/0 |
point2point |
If a labeled packet of 27 arrives on P2, it will be sent out Serial 0/1 unlabeled. The Pop tag, which you can see from the show mpls forwardingtable command on P2, means, “Don’t send this traffic as labeled, but instead send it as unlabeled IP traffic.” You can think of Pop tag as meaning, “The next hop router needs to do a Layer 3 lookup on the packet” or “The next hop router is the destination network or has a connected interface that is in the destination network.” The official name for this process is called penultimate hop popping.
Copyright ©2002 SYBEX, Inc., Alameda, CA |
www.sybex.com |

16 Chapter 1 An Introduction to MPLS
The word penultimate means “next to last.” With penultimate hop popping, the penultimate router in an LSP pops the label and forwards the packet as unlabeled IP to the next hop router.
In this example, the next-to-last router (P2) in the LSP pops the label and forwards the unlabeled packet to its ultimate destination (PE2), as the following output demonstrates:
P2#show mpls forwarding-table |
|
|
|
||
Local |
Outgoing |
Prefix |
Bytes tag Outgoing |
Next Hop |
|
tag |
tag or VC |
or Tunnel Id |
switched |
interface |
|
27 |
Pop tag |
192.168.1.4/32 |
26224 |
Se0/1 |
point2point |
28 |
Pop tag |
192.168.1.2/32 |
29568 |
Se0/0 |
point2point |
30 |
Pop tag |
192.168.1.8/30 |
0 |
Se0/0 |
point2point |
31 |
31 |
192.168.1.1/32 |
0 |
Se0/0 |
point2point |
Figure 1.9 shows the LSP from PE1 to PE2.
F I G U R E 1 . 9 The LSP from PE1 to PE2
|
|
Serial |
0/0 |
0/0 |
Serial |
0/1 |
0/0 |
Serial |
0/1 |
|
0/0 |
|
|||
Serial 0/1 |
|
Serial |
|
|
|
Serial |
|
Serial |
|
Serial 0/1 |
|||||
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|
||
PE1 |
|
|
|
P1 |
|
|
|
P2 |
|
|
|
PE2 |
|||
Serial 0 |
IP |
|
28 |
|
|
IP |
|
27 |
|
IP |
|
|
Serial 0 |
||
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|
|||
CE1 |
|
|
|
|
|
|
|
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|
|
|
|
CE2 |
Now let’s now see what happens when a user on PE1 wants to ping the loopback address of PE2. The user types ping 192.168.1.3.
By looking at the labels of PE1 in the following output, you can see the outbound label that will be used is 29, and it will be sent out Serial 0/0:
PE1#show mpls forwarding-table |
|
|
|
||
Local |
Outgoing |
Prefix |
Bytes tag |
Outgoing |
Next Hop |
tag |
tag or VC |
or Tunnel Id |
switched |
interface |
|
27 |
27 |
192.168.1.16/30 |
0 |
Se0/0 |
point2point |
28 |
28 |
192.168.1.4/32 |
0 |
Se0/0 |
point2point |
29 |
Pop tag |
192.168.1.2/32 |
0 |
Se0/0 |
point2point |
30 |
29 |
192.168.1.3/32 |
0 |
Se0/0 |
point2point |
32 |
Pop tag |
192.168.1.12/30 |
0 |
Se0/0 |
point2point |
Copyright ©2002 SYBEX, Inc., Alameda, CA |
www.sybex.com |