MPLS provides ways to perform traffic engineering, to guarantee bandwidth or set paths, and to enable Virtual Private Networks (VPNs). It specifies ways that Layer-3 traffic can be mapped to connection-oriented Layer-2 transport protocols, such as ATM. It adds a label containing specific routing information to each IP packet that directs traffic through explicitly defined paths, and it allows managers to implement policies that assign labels to various CoS. This enables the managers to offer different CoS to different traffic types or from different customers. The policies could send traffic over a path that is not necessarily the path with the lowest routing metric. With MPLS, service providers can provide VPN services and scale those services for many customers.
In a MPLS environment, forwarding is based on the lookup of the MLPS label and not the Layer-3 IP address. Labels are inserted at the edge of the MPLS network and removed when the packet leaves the MPLS network. The MPLS network applies services and forwards packets that are based on the information contained in the label.
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MPLS Label
In a packet environment, the MPLS label is inserted between the Layer-2 header and the Layer-3 header in a Layer-2 frame. This applies for Packet over Synchronous Optical Network (SONET) (PoS), Ethernet, Frame Relay, and label over ATM. In ATM networks with label switching, the label is mapped into the virtual path identifier/virtual channel identifier (VPI/ VCI) fields of the ATM header. The MPLS label field is 32 bits in length, with the actual label (tag) being 20 bits.
MPLS Label Switch Routers (LSRs)
All routers within a MPLS network are LSRs, which forward based on the label and not based on routing protocols. If the MPLS network uses ATM, the LSRs are called ATM LSRs. The Edge Label Switch Router (Edge LSR) is responsible for adding or removing the label to the packet.
Figure 11-3 shows a diagram of these routers. Figure 11-3 MPLS LSRs
Figure 11-3 shows a diagram of these routers. Figure 11-3 MPLS LSRs
MPLS VPN Router Types
MPLS VPN architectures use the following four router types:
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• P router—The service provider’s internal core routers. These routers do not need to maintain VPN routes.
• C router—The customer’s internal routers. They do not connect to the provider. These routers do not maintain VPN routes.
• CE router—The edge routers on the customer side that connect to the service provider. These routers do not maintain VPN routes.
• PE router—The edge routers on the service provider side that connect with the customer’s CE routers. PE routers maintain VPN routes for the VPNs associated with the connected interfaces.
Figure 11-4 shows a diagram of these routers in an MPLS VPN network.
Figure 11-4 MPLS VPN Routers
Figure 11-4 MPLS VPN Routers
Enabling MPLS
To enable MPLS, Cisco Express Forwarding (CEF) must first be enabled on all routers running MPLS. You can accomplish this with the ip cef global command.
To enable MPLS on interfaces, use the mpls ip command, which replaces the tag-switching ip command. This command enables Cisco’s proprietary tag distribution protocol (TDP). The mpls ip command is configured on every interface of the LSR. A simple configuration of a LSR in a MPLS network is shown in Example 11-12.
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Example 11-12 Simple MPLS Router Configuration hostname mplsrouterl i ip cef i interface Loopback0 ip address 192.168.16.1 255.255.255.255
i interface Serial0/0 encapsulation frame-relay i interface Serial0/0.1 point-to-point ip address 192.168.20.5 255.255.255.252 mpls ip frame-relay interface-dlci 123
i interface Serial0/0.2 point-to-point ip address 192.168.20.9 255.255.255.252 mpls ip frame-relay interface-dlci 124
i interface Serial0/0.3 point-to-point ip address 192.168.20.14 255.255.255.252 mpls ip frame-relay interface-dlci 125
i router ospf network 192.168.16.0 0.0.0.255 area 1 network 192.168.20.0 0.0.0.255 area 1
i ip classless i end
This book covers MPLS at a high level. For more information, consult Cisco’s web site at www.cisco.com/warp/public/732/Tech/mpls/mpls_learnabout.shtml, RFC #2547, or the Cisco Press publication MPLS and VPN Architectures.
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