Cisco 6100 ATM Addressing Guidelines

This appendix provides guidelines for network providers for setting up Cisco 6100 ATM virtual path connection (VPC) and virtual circuit connection (VCC) addresses. The information here assumes a working knowledge of ATM switching from a permanent virtual circuit (PVC) provisioning perspective.

When you determine the optimal Cisco 6100 ATM address space within a network provider's ATM network, consider the following:

ATM UNI 3.1 specification restrictions
Cisco 6100 VPC and VCC connection capacity
Number of subscribers per Cisco 6100 and number of connections per subscriber
Evolution to SVCs
Subtend connection mapping
Symmetry for API design

The following sections describe each of these factors.

B.1 ATM UNI Version 3.1 Specification Restrictions

The following address utilization constraints are imposed by the ATM Forum's UNI Version 3.1 specification:

Up to 65536 virtual circuit identifiers (VCIs) per physical UNI are permitted
VCIs can be mapped in up to as many as 256 VPs
VCIs 0-31 on each VP are reserved for future use and are typically not used for user traffic

Note Each ATM network element vendor compromises on the full VPC and VCC space as defined by the ATM Forum.

Forum specifications are designed for a scalable evolution and do not take into consideration the price/performance considerations of memory technology at given points in time.

B.2 Cisco 6100 VPC and VCC Connection Capacity

The following ATM address space type (VPC and VCC) and ranges are supported in Release 1.x and 2.x of Cisco 6100 system software:

VPI 1 supports up to 1600 VCIs, from VCI 0-1599
VPI 0, 227 each support up to 400 VCIs, from VCI 0-399
The remaining 228 VPs (allowed within the ATM UNI Version 3.1 standard) are usable for VPC switching only. Within the Cisco 6100, VPCs are intended for use when mapping subtended Cisco 6100 connections through a higher level Cisco 6100 to the edge switch connection.

The above address space type (VPC and VCC) and ranges are further conditioned into actual capacities per the following constraints. Each successive constraint is explained in detail to explain understanding for the difference between theoretical and actual maximums.

Theoretical maximum network-side VPI and VCI address space = 1600 + 27(400) = 12,400.

: This represents the maximum number of VPI and VCI addresses that can be switched across the host Cisco 6100, i.e., the Cisco 6100 that is physically connected to the ATM bearer service before to any ATM UNI specification or subtending constraints.

Theoretical maximum network-side VPI and VCI address space, while observing VCI 0-31 reservation guidelines = 1568 + 27(368) = 11504.

: This represents the maximum number of VPI and VCI addresses that you can switch across the host Cisco 6100, taking into consideration the ATM UNI Version 3.1 specification of reserving VCIs 0-31 on each VPI. This maximum does not take into consideration subtending port constraints.
: Due to the need for contiguous use of address space within the Cisco 6100's ATM chipset, if VCI 0-31 is reserved for each VP then 1600-32=1568 VCIs can be mapped across VPI 1, and 400-32=368 VCIs can be mapped across VPIs 0, and 2-27.

Theoretical maximum subscriber-side VPI or VCI address space (assuming 6 subtended Cisco 6100s, and utilizing VPC switching from subtended Cisco 6100s) = 7 LRs x 400 subs x 4 VCCs = 11200.

: This represents the maximum number of VPI or VCI addresses that would be required (out of the above possible 11,504) assuming a full subtended tree of six Cisco 6100s and one host Cisco 6100, with 4 VCCs allocated per subscriber. Note that due to a VCC address space constraint on the subtend port (discussed below), this connection space model assumes VPC switching is utilized across subtend ports.

The actual VPI or VCI address space maximum depends on whether VCIs traverse intermediate Cisco 6100s, on their way to the ATM bearer service, as VCCs or across virtual paths (VPCs). You can address a maximum of 4000 VCCs can be addressed to any one subtend port. This means that in a full subtend tree, the host Cisco 6100 can only be assigned 8000 VCCs. So the maximum number of VCCs that can be supported on a fully subtended Cisco 6100 network is 1600 + 8000 = 9600. If VPCs are used to traverse intermediate Cisco 6100s on their way to the ATM bearer service, then 7 x 400 subs x 4 VCCs per sub = 11200 VCs can be supported.

: Note also that in the initial version of the subtend port, the dual port DS3 subtend host module, each port's VPCs must be unique. This is because the subtend slot occupies one port on the ATM chipset's 66 port architecture, and must therefore treat each ATM cell as a uniquely addressed cell in order to preserve subscriber connection integrity.

B.3 Number of Subscribers and Connections per Subscriber

Each network provider must determine expected subscriber demand per Wire Center. Demand can be met in a variety of Cisco 6100 configuration methodologies ranging from a dedicated (64 subscribers per Cisco 6100) to fully concentrated (400 subscribers per Cisco 6100).

The network provider should also consider the number of PVCs required per subscriber.

B.4 Evolution to SVCs

The network provider should consider the point in time when due to either network design goals or the need from sheer volume connection deployment, SVCs begin to supersede PVCs as a means of connection establishment. One of the biggest drivers for SVC is to remove the need for service providers to discretely map thousands or even millions of PVC segments to enable end to end ATM connections across an ATM bearer service. SVCs also reduces the strain on the connection space limits imposed by various network elements, as connection space is only required for active connections.

Incumbent PVCs should be able to be preserved with the advent of SVCs. The signaling stack manages selection of an available VCC for each user call. Customers shifting from PVC service to SVC service would have their PVCs deleted by the network provider. Reservation of VCCs VPIn/VCI 0-31 protects appropriate space per VP for signaling purposes. SVC functionality is not available yet, and therefore, Cisco makes no guarantee that some PVC address space reprovisioning might not be required. A Cisco goal is to minimize this. Utilities are being investigated to remap these PVCs in an automated fashion.

B.5 Subtend Connection Mapping

To decide whether or not you should map subtended connections through to the ATM bearer service network as VPCs or VCCs, consider the following:

The greater the desire to minimize connection configuration time, the more attractive VP switching becomes. However, VPC switching has the disadvantage of not offering support for early packet discard (EPD).
If you want fully concentrated (400 subs x 4 VCCs per Cisco 6100) Cisco 6100s and a fully subtended network (7 Cisco 6100s per edge switch connection) is desired, set up VPC switching will be required to support all subscriber-side connections. Otherwise, the subtend address space VCC capacity may be exceeded.
VCC switching for subtended Cisco 6100s can be supported, up to the maximums defined in the "Cisco 6100 VPC/VCC Connection Capacity" section on page B-2.
Use the subtending hierarchy is recommended to be three levels deep at most (total of seven Cisco 6100s in a fully configured subnetwork). This is not a hard limit, but this limit provides reasonable statistical bandwidth per subscriber. Even if bandwidth or connection space did not present subscriber density limitations, the network provider should address the potiential impact on customers potentially impacted by a single failure (at the bearer service network edge).

B.6 Symmetry for API Design

For support of automated provisioning through an EMS/API and subtending, it is somewhat desirable to deploy Cisco 6100s using a standard "template." This template allows each Cisco 6100 to power up in a state in which all the Cisco 6100 requires to provide service is for a Cisco 605 or 675 to be attached to an incoming line, and an ATM connection to be made through the network at the edge switch attached to the NI.

To simplify API design, assume the following:

Identically configure each Cisco 6100.
Ensure the configuration supports the Cisco 6100 location anywhere in a subtended hierarchy.
Ensure the configuration supports the Cisco 6100 outside of a subtended hierachy.
VCIs=0-31 is not used in any VP
There are 1600 available VCIs in VPI=1
There are 400 available VCIs in each of the other VPs (ignoring UNI Version 3.1 constraints for simplicity, a few less if VCI 0-31 is reserved)
There are 256 VPs available on the network interface (per the UNI Version 3.1 standard)

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