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Voice, IP, and ATM MPLS Features

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: May 17, 2002.

Cisco WAN Switches with MPLS Support

Figure 22-9 shows an example of an MPLS network.

Figure 22-9 MPLS Network Example

Based on the network routing protocol, a path (or route) is set up to all reachable IP networks. Simple label-switching tables identifying the labels and the destination interface are set up on all intermediate nodes. The nodes at the edge of the MPLS network attach the appropriate labels to the IP packets. The nodes in the network core simply switch the packets from one interface to the next based on the label. For example, if a packet with label 2A comes in on interface 01, it is switched to interface 06, and the label is changed to 15.

Label switching is similar to switching cells in an ATM network: Instead of a VPI and a VCI, there is a label. Because of this similarity, using an MPLS-enabled ATM network in the core is the ideal solution, especially in cases where ATM networks are already deployed.

An LSR is a core device that switches labeled packets according to predefined switching tables. An LSR can be a switch or a router. BPX 8650 switches (a BPX switch plus a 7200-series router) and MGX 8850-PXM45 switches (with RPM installed) are examples of LSRs.

An Edge LSR (ELSR) or Label Edge Router (LER) is an edge device that performs initial packet processing and classification and applies the first label. An ELSR can be either a router or a switch with built-in routing. The RPM installed in an MGX switch is an example of an ELSR.

A Label Switch Controller (LSC) is an MPLS router that controls the operation of an ATM switch in such a way that the two function together as a single ATM-LSR. An LSR comprises a switch and an LSC. A 7200 series router is an LSC for the BPX switch; an RPM is an LSC for the MGX 8850-PXM45 switch.

MPLS Features Supported on the MGX Switch

This section provides a high-level description of the MPLS features supported on the RPM installed in an MGX switch running Release 1.1.x firmware.

The RPM supports the following MPLS features:

  • The RPM as an ELSR or LER

  • Support for MPLS VPNs

  • Support for QoS

  • Support for MGX switches (with PXM1) interworking with MGX 8850-PXM45 switches via MPLS Virtual Switch Interface (VSI) virtual trunks

  • Support for MGX switches (with PXM1) interworking with BPX 8650 switches via MPLS VSI virtual trunks

The RPM as an Edge Label Switch Router

The RPM installed in an MGX switch is an ELSR or LER. As an ELSR, the RPM is responsible for initial packet processing and classification. It also applies the first label to the packets.

MPLS VPN Support on RPM

MPLS VPN support on RPM is described in greater detail in the upcoming section, "MPLS and Virtual Private Networks Using the Route Processor Module."

The VPN offers private connectivity over a public infrastructure. Each customer is assigned a VPN ID. You can think of the VPN ID as a prefix or extension of the IP address. For example, if a destination IP address is and the VPN ID is 243, the packet destination is effectively, uniquely identifying the destination. Even if another customer is using the same IP address, the VPN ID is different, making the packet address different as well. In the MPLS network, the VPN identifier and the destination IP address are associated with a label.

The VPN is also isolated by the routing protocol (BGP) so that routing updates are exchanged only between members of the same VPN. The router at one customer's site does not even know about another customer's routers.

Figure 22-10 shows an example of VPNs in the MPLS network. Company A and Company B are connected to the same public network; however, they each have a VPN that behaves like a private network.

Figure 22-10 Example of VPNs in the MPLS Network

MPLS QoS Support on RPM

Customers have different types of traffic that they want to send to the service provider. MPLS allows the service provider to assign a different label to each traffic class. This label determines how traffic will be handled in the network.

The MPLS QoS solution does not map different traffic classes into different virtual circuits, as is done in ATM. MPLS simply applies a different label. These labels move the packet from end to end, maybe over a different path. Each switch along the way has a label that tells it the packet's CoS.

MGX PXM1 MPLS Interworking with MGX 8850-PXM45 and BPX Switches

This feature allows RPMs as ELSRs in PXM1-based platforms to interwork with the MGX 8850-PXM45 or BPX 8650 LSC function. The MPLS VSI virtual trunks feature enables the definition of one virtual path (VP) tunnel per ELSR on a PXM1-based platform to be mapped to one VSI virtual trunk under the control of the LSC on the attached backbone switch.

Figure 22-11 shows the MPLS interworking feature for MGX 8850-PXM45 switches.

Figure 22-11 The MPLS Interworking Feature

MPLS Networks Using the MGX Switch

This section describes how the MGX switch is placed in an MPLS network. It shows how the MGX switches with the BPX ATM MPLS core, the MGX 8850-PXM45 ATM MPLS core, and the IP MPLS core.

MGX Switches with the BPX ATM MPLS Core

This ATM MPLS architecture uses BPX 8650 ATM LSRs. The ELSR or LERs in this network are RPMs installed in MGX 8230/8250/8850-PXM1 switches.

The MGX switches are connected to the BPX switches by T3, E3, OC-3/STM1, or OC-12/STM4 ATM links.

Figure 22-12 shows an ATM MPLS core with MGX switches with RPMs as ELSRs.

Figure 22-12 ATM MPLS Core with MGX Switches

Cisco MGX 8230, 8250, and 8850-PXM1 switches do not yet support LSCs. This means that all MPLS traffic to RPMs must be carried inside a PVC or a PVP. PVCs are used with packet-based MPLS. If an MGX switch is used with ATM MPLS, the connections to the RPMs must be PVPs, which are used as MPLS VP tunnels.

Two options exist for the PVP connections, as shown in Figure 22-13. In the first option, at least two separate ATM lines are used between the MGX switch and the BPX switch. One line is the feeder trunk, which carries all but MPLS traffic, and the second line carries PVPs from the RPMs to the BPX switch. The second line is a UNI or NNI port on the MGX switch, with several VSI virtual trunk endpoints on the BPX switch.

Figure 22-13 PVP Connections

The second line is required because the BPX switch does not support virtual trunk endpoints on interfaces that are configured as feeder trunks. Another option involves using three or four lines with the PVPs distributed across them, increasing the available bandwidth. In this topology, additional lines are configured in the same fashion as the second UNI or NNI (nonfeeder trunk) link.

If two or three UNI or NNI lines are used to carry MPLS PVPs, these can be connected from one MGX switch to two or three different BPX nodes. Non-MPLS traffic from the MGX switch is still carried on the feeder trunk to a single BPX switch.

It is possible to carry all traffic (MPLS and non-MPLS) on the feeder trunk between the MGX and BPX switches. This option requires loopback cables on the BPX node and additional connections between the BPX feeder trunk and UNI or NNI ports.

MGX Switches with the MGX 8850-PXM45 ATM MPLS Core

Using an MGX 8850-PXM45 ATM MPLS core for MGX switches is similar to using a BPX core. The main difference is that the LSC for the MGX 8850-PXM45 switch is an RPM card module rather than an external router.

Figure 22-14 shows MGX 8850-PXM45 switches and MGX PXM1 switches in an ATM MPLS network.

Figure 22-14 Cisco Switches in an ATM MPLS Network

Like a BPX ATM MPLS core, you must set up a second line between the MGX PXM1 and the MGX 8850-PXM45 switches to carry the PVPs from the RPM ELSR.

MGX Switches with the IP MPLS Core

It is possible to set up the MGX switches without an ATM MPLS core. In this case, PVCs are required between RPMs in the network that need to pass traffic between them. The ATM backbone can be BPX, MGX 8850-PXM45, or other-vendor switches, but it is not an MPLS-enabled network. The ATM backbone network simply transports ATM cells from one RPM to another without any knowledge of MPLS.

Figure 22-15 shows an example of MGX switches and an IP MPLS core.

Figure 22-15 Cisco Switches in an IP MPLS Core

Refer to this Web site for more information on the benefits of MPLS: http://www.ciscopress.com/book.cfm?series=1&book=168.

16. Setting Up MPLS on the MGX Switch | Next Section Previous Section

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