PoP Design

Point of presence (PoP) design constraints include the choice of access line type and equipment for the network.

The location of PoPs is largely determined by user traffic and the location of population centers. Proper capacity planning must be performed prior to PoP design. The capacity-planning phase should make proper estimates and approximations for future traffic growth based on the existing customer base and anticipated customer growth and corresponding traffic growth. Oversubscription should not be factored in for baseline traffic planning.

Single ATM Edge LSR

A single Edge LSR PoP design, illustrated in Figure 9-9, is used if the node can support the number and types of access lines required by customer CE routers in the PoP location. The single Edge LSR is susceptible to scalability issues. These issues can be avoided if the ATM Edge LSR is co-located with the ATM LSR and additional Edge LSRs can be added to ATM ports on the core ATM LSR.

Figure 9-9. Single ATM Edge LSR

Multiple Edge LSRs and an ATM LSR

If multiple access lines are to be supported at a PoP, the PoP might require more than one Edge LSR at that location. Alternatively, different types of Edge LSRs might be required due to different types of access lines that might have to be supported. It is strongly recommended that you co-locate an ATM LSR if there are several Edge LSRs in a PoP.

This topology is shown in Figure 9-10.

Figure 9-10. Multiple Edge LSRs

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Figure 9-10. Multiple Edge LSRs

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The ATM LSR locally switches traffic going between different Edge LSRs in the PoP. It can also concentrate traffic going from the PoP onto a single set of ATM MPLS links. Routing scalability is improved, because only one IP routing protocol peering is required from the ATM LSR to other points in the MPLS network. Without the ATM LSR, separate peering would be required from all Edge LSRs.

Redundant pairs of links could be used between the Edge LSRs and the ATM LSR, depending on reliability requirements.

Access Concentrator, Edge LSR, and an ATM LSR

Access concentrators such as the MGX 8220 can be used in addition to Edge LSRs and an ATM LSR for concentrating user traffic and backhauling to an Edge LSR. An example of this type of PoP uses MGX 8220 access shelves; 6400, 7200, or 7500 Edge LSRs; and a BPX 8650 as a core ATM LSR. This topology is shown in Figure 9-11. IP traffic from access concentrators is carried in ATM PVCs to Edge LSRs. These may be carried through the same BPX 8650 that acts as the ATM LSR, because the BPX is an IP+ATM switch. As shown in Figure 9-11, LSR PE1 must have at least two ATM interfaces—one for access PVCs from the access concentrator, and one for ATM MPLS traffic.

Figure 9-11. Multiple Edge LSRs

Figure 9-11. Multiple Edge LSRs

There may also be Edge LSRs in the PoP that do not handle access PVCs at all and that have only directly connected access lines. The LSC could have such a configuration if provisioned to terminate CE routers.

NOTE

The LSC in the BPX 8650 can act as an Edge LSR simultaneously while performing its LSC function. However, use of an LSC as an Edge LSR is not recommended for providers who consider the separation of MPLS control functions from data forwarding functions vital.

The number of Edge LSRs required in the PoP depends on the total number of access lines and the total bandwidth of the access lines, calculated from the average utilization. For example, if the sum of the access lines' bandwidths were 1 Gbps, the utilization might not exceed 500 Mbps. The capacity of a 6400, 7200, or 7500 router running MPLS Edge functions is roughly the same as its ordinary IP capacity using Cisco Enhanced Forwarding (CEF). For example, a 7200 router with an NPE 200 processor can support close to 200 Mbps of MPLS Edge traffic at normal IP packet sizes.

If additional Edge QoS functions such as MQC policing, metering, and traffic shaping are used, performance might be affected. In these circumstances, the routers' performance should be verified for the particular combination of features to be used.

Multiservice Access Concentrator as an Edge LSR

Multiservice access concentrators such as the MGX 8850 and Cisco 6400 can integrate the access, Edge LSR, and ATM LSR functions described in the previous example into a single device, as illustrated in Figure 9-12. It consists of a multiservice access concentrator with various types of Frame Relay and ATM access lines, circuit emulation lines, and one or more Edge LSRs. Each Edge LSR is an RPM card in the case of the MGX

8850 or a node route processor (NRP) card in the case of the 6400. The number of RPMs or NRPs acting as Edge LSRs depends on the total number of access lines and the total bandwidth of the access lines, downrated according to the utilization. For example, if the sum of the access lines' bandwidth were 1 Gbps, the utilization might not exceed 500 Mbps.

Figure 9-12. Multiservice Access Concentrator

Figure 9-12. Multiservice Access Concentrator

Frigidaire Stove Wiring Diagram

The RPM with an NPE150 processor can support an MPLS edge function for up to 700 access lines. It can support up to 150 Mbps of MPLS edge traffic at normal IP packet sizes. However, the performance should be verified with the particular combination of edge functions such as QoS to be used in addition to MPLS edge functions. The NRP capabilities are similar to those of the RPM. The 6400 and MGX 8850 also have IP+ATM capability. In the MGX 8850, one of the RPM cards acts as a label switch controller. It may perform both LSC functions and Edge LSR functions simultaneously, if desired. The use of an RPM for simultaneous LSC and Edge LSR is not recommended for providers who consider the separation of MPLS control functions from data forwarding functions vital. In the Cisco 6400, the main node switch processor also acts as an LSC.

Single LSR PoP

A single LSR PoP site usually performs an ATM switching role to terminate customers using ATM router CEs. Such a site consists of a single ATM LSR, as shown in Figure 9-13, or possibly a redundant pair of ATM LSRs. The ATM LSR is typically a BPX 8650. The CE ATM router can terminate directly on the LSC using a TDM circuit or get switched over an ATM PVC provisioned in the BPX and terminate on an ATM Edge LSR or the LSC itself. However, true scalability can be achieved only by using ATM and a router-based LSR.

Figure 9-13. Single LSR PoP

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