This lesson, which focuses on configuring MPLS, is fourth in a series that will help you learn skills required for the MPLS portion of the Cisco CCIP certification. These skills are tested in the CCIP 642-611 - Implementing Cisco MPLS Exam.
The last few articles have focused on the MPLS theory that will be tested on that portion of the CCIP exam. In addition to MPLS theory, you must be able to configure the routers as well. The configuration can apply to customer edge (CE) routers, provider edge (PE) routers or provider (P) routers. Each of these must be configured in order for MPLS to work within an enterprise's architecture. Although the responsibility for configuration of each router may vary based on where they sit in the architecture (e.g. customer premises or provider network), the exam expects you to understand the configuration of all elements. This article and the next will focus on configuring routers to support MPLS functionality.
Cisco originally developed MPLS type technology and called it tag switching. This technology evolved into MPLS labeled switching. The CCIP exam rquires an understanding of both the Cisco tag switching commands as well as the MPLS commands. Both tag switching and MPLS configurations will be presented.
There are multiple components to configuring the MPLS routers to deliver MPLS services. Standard routing protocols must be configured in order for the PE and P routers to be able to communicate with each other. BGP is used exclusively between the PE routers in order to exchange customer routing information. In addition to IP routing the routers must exchange labels and build label switched paths and segment customer routing information into virtual routing and forwarding tables (VRFs).
Please pay close attention to the syntax as the exam will require you to enter or recognize IOS commands.
Turn on Tag or Label Switching
In order to turn on tag switching, you must configure Cisco Express Forwarding (CEF) and tell the router to advertise tags. In addition, each interface must be enabled with tag switching. Commands entered on the router are in bold.
Tag Switching Configuration
P1(config)# ip cef
P1(config)# tag-switching advertise-tag
P1(config)# interface serial 0
P1(config-if)# tag-switching ip
MPLS Configuration
P1(config)# ip cef
P1(config)# mpls ip
P1(config)# interface serial 0
P1(config-if)# mpls ip
That is all that is required for the routers to begin exchanging labels and building the label switched paths for forwarding traffic across the MPLS backbone. Once tag or MPLS switching has been turned on the routers will begin neighbor discovery and the label exchange. Tag switching uses UDP for neighbor discovery and TCP port 711 for tag exchange. MPLS switching also uses UDP for neighbor discovery but relies on the label distribution protocol for label exchange.
After configuring the routers to perform neighbor discovery and label/tag exchange the commands to verify that the process has completed successfully are as follows:
Neighbor Verification
TAG
P1# show tag-switching tdp-neighbor
MPLS
P1# show mpls ldp neighbor
The output of these commands will show the neighbor identity (in the form of an IP address) and state of the neighbor. Key inputs to look for are peer identity and the state of the peer. The peer state should be OPER (for operational).
Other Important Commands
Verification that MPLS is enabled on the appropriate interfaces
TAG
P1# show tag-switching interfaces
MPLS Verification
P1# show mpls Interface
Verify that the appropriate interfaces display
Verification of Tag/Label Distribution & Bindings
TAG
P1# show tag-switching tdp discovery
P1# show tag-switching tdp bindings
MPLS Verification
P1# show mpls ldp discovery
P1# show mpls ldp bindings
- Verify that TDP or LDP label discovery sources (e.g. where were labels learned)
- Verify that IP routes have labels bound to them
That is it for peer establishment, label exchange and IP to label bindings. The next article will focus on the commands required to configure virtual routing and forwarding tables in order to provide MPLS VPN services.
About the author:
Robbie Harrell (CCIE#3873) is the National Practice Lead for Advanced Infrastructure Solutions for SBC Communications. He has over 10 years of experience providing strategic, business, and technical consulting services to clients. Robbie resides in Atlanta, and is a graduate of Clemson University. His background includes positions as a Principal Architect at International Network Services, Lucent, Frontway and Callisma.
