Designing and Dimensioning ATM MPLS Points of Presence

There are a number of ways to connect a customer edge (CE) router to an ATM MPLS network. These functions can be implemented on various types of equipment and can be combined with access equipment in various ways. They are shown in Figure 5-1.

Figure 5-1. Connecting a CE to an ATM MPLS Network

The three ways of connecting a CE to an ATM MPLS network are explained in the following sections.

ATM MPLS Network with Cell-Based Edge LSRs

The simplest ATM MPLS network structure is shown in part a) of Figure 5-1. CEs are connected directly to router-based ATM eLSRs, typically a Cisco 7200 or 7500 series. The edge Label Switch Routers (LSRs) are connected by ATM links to the core devices, which are ATM LSRs. The ATM LSRs may be BPX-8650 IP + ATM switches, MGX-8850 with PXM-45, MGX-8950, LS-1010, and other ATM switches.

Mixed Cell and Frame-Based MPLS Edge LSRs

You can have a network with a mixture of ATM MPLS and frame-based MPLS. A simple example of this is shown in part b) of Figure 5-1. In a network such as this, some links run packet-based MPLS, and some links run ATM MPLS. A cell-based backbone and a frame-based backbone may also exist. The devices that interface between packet-based MPLS and ATM MPLS are the same routers that act as ATM MPLS edge LSRs as well as packet-based LSRsanything from a Cisco 3600 up to a Cisco 12000.

MPLS Edge LSRs with Access Devices

ATM MPLS networks with router-based edge LSRs may also use access devices, as shown in part c) of Figure 5-1. The access devices can either be separate pieces of equipment or MGX-8230 or MGX-8250 access concentrators. This happens when access is required through a device that does not support MPLS services. There are some common situations in which this is required:

• Access is required to IP, Frame Relay, and ATM services through a single access device. The most common example when the access device doesn't support MPLS services is through the use of the MGX 8220. The access concentrator can support MPLS services such as the MGX-8230 or MGX-8250 access concentrator through the use of route processor module (RPM) cards. In this case, the access device can support legacy services in two different ways:

  - Normal Frame Relay or ATM services This refers to Frame Relay or ATM point-to-point PVC mesh services.
  
  

  - IP-enabled Frame Relay or ATM services MGX-8230 and MGX-8250 multiservice switching concentrators can IP-enable legacy services through the use of an RPM card in a point-to-cloud or point-to-network fashion. In this case, the CEs are connected through T1/E1 or fractional T1/E1 links to the MGX multiservice shelf, and the MGX cross-connects those PVCs to an RPM card to support connectionless IP VPN services.
  
  

• Higher densities of low-bandwidth access lines can be better supported by way of an access device than by simply using edge LSRs.

• Both point-to-point PVCs and MPLS VPN services are required by the customer network (C network).

ATM Label Switch Routers

There are in essence six main considerations when choosing ATM LSRs:

• LSR processing power and switching capacity

• Types and speeds of links

• Number of links

• Number of connections to be supported

• Whether VC-merge is required

• Redundancy and reliability requirements