Table of Contents
Cisco 6100 ATM Addressing Guidelines
The purpose of this appendix is to provide guideline recommendations to 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.
There are a number of factors that need to be taken into consideration when determining the optimal Cisco 6100 ATM address space within a network provider's ATM network. These include:
- ATM UNI 3.1 specification restrictions
- Cisco 6100 VPC/VCC connection capacity
- Number of subscribers per Cisco 6100 and number of connections per subscriber
- Evolution to SVCs
- ATM bearer service address mapping to FireRunner
- Subtend connection mapping
- Symmetry for API design
The relevance of each of these points is discussed in detail herein.
The following address utilization constraints are imposed by the ATM Forum's UNI 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.
The following ATM address space type (VPC/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 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/VCC) and ranges are further conditioned into actual capacities per the following constraints. Each successive constraint is explained in detail to provide understanding for the difference between theoretical and actual maximums.
- Theoretical maximum network-side VPI/VCI address space = 1600 + 27(400) = 12,400.
- This represents the maximum number of VPI/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 prior to any ATM UNI specification or subtending constraints.
- Theoretical maximum network-side VPI/VCI address space, while observing VCI 0-31 reservation guidelines = 1568 + 27(368) = 11504.
- This represents the maximum number of VPI/VCI addresses that can be switched across the host Cisco 6100, taking into consideration the ATM UNI 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 utilization 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/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/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/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). 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 have 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.
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.
The network provider should consider the point in time when due to either network design goals or the need for 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. Obviously, the goal will be to minimize this. Utilities may also be investigated that would remap these PVCs in an automated fashion.
Choosing an optimal connection mapping strategy across an ATM bearer service into FireRunner or other broadband service termination devices depends on several factors. First, if the bearer service network is treated as a sub-network for automated provisioning purposes, then mapping of Cisco 6100 connections to specific VPs becomes somewhat arbitrary, as each VCC will be re-mapped at the bearer network ingress point.
Whether or not to map subtended connections through to the ATM bearer service network as VPCs or VCCs is really a function of 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 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, VPC switching will be required to support all subscriber-side connections, as the subtend address space VCC capacity would 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 above.
- The subtending hierarchy is recommended to be three levels deep at most (total of seven
Cisco 6100s in a fully configured subnetwork). While this is not necessarily a hard limit, it is recommended that it be no deeper than this in order to provide reasonable statistical bandwidth per subscriber. Even if bandwidth or connection space did not present subscriber density limitations, the network provider would most likely want to address the issue of the number of customers potentially impacted by a single failure (at the bearer service network edge).
For support of automated provisioning through an EMS/API and subtending, it is somewhat desirable to deploy Cisco 6100s with a standard "template". This template will allow each
Cisco 6100 to come up in a state where all it 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. API design can be simplified with the following assumptions:
- Each Cisco 6100 must be identically configured
- The configuration must support the Cisco 6100's location anywhere in a subtended hierarchy
- The configuration must support the Cisco 6100 outside of a subtended hierarchy
- VCIs=0-31 will not be 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 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 3.1 standard)
Posted: Fri Apr 23 14:30:54 PDT 1999
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