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Internetwork Design Guide
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About this Document
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Introduction
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Internetworking Design Basics
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Designing Large-Scale IP Internetworks
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Designing SRB Internetworks
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Designing SDLC, SDLLC, and QLLC Internetworks
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Designing APPN Internetworks
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Designing DLSw+ Internetworks
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Designing ATM Internetworks
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Designing Packet Service Internetworks
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Designing DDR Internetworks
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Designing ISDN Internetworks
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Designing Switched LAN Internetworks
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Designing Internetworks for Multimedia
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RIP and OSPF Redistribution
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Dial-on-Demand Routing
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Increasing Security on IP Networks
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Integrating Enhanced IGRP into Existing Networks
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Reducing SAP Traffic in Novell IPX Networks
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UDP Broadcast Flooding
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STUN for Front-End Processors
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Using ISDN Effectively in Multiprotocol Networks
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Using HSRP for Fault-Tolerant IP Routing
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LAN Switching
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Multicasting in IP and AppleTalk Networks
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Scaling Dial-on-Demand Routing
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Large-Scale H.323 Network Design for Service Providers
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Appendix A: Subnetting an IP Address Space
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Appendix B: IBM Serial Link Implementation Notes
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Appendix C: SNA Host Configuration for SRB Networks
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Appendix D: SNA Host Configuration for SDLC Networks
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Appendix E: Broadcasts in Switched LAN Internetworks
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Appendix F: References and Recommended Reading
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Table Of Contents
Different Types of ATM Switches
Workgroup and Campus ATM Switches
General Operation on an ATM Network
Scaling an ELAN—Spanning-Tree Protocol Issues
Resiliency in LANE 1.0 Networks
Behavior of SSRP with the Well Known LECS Address
Behavior of SSRP in Network Partitions
Redundant ATM Port Card for the Catalyst 5000
Improved Network Price/Performance Scalability
Industry-Leading Investment Protection
Increases Portability and Guarantees an Infrastructure
Stratm-Based Cisco WAN Products
Cisco/StrataCom BXM Switch Modules
Cisco/StrataCom IGX Family of Switches
Configuring the AIP for ATM Signaling
Catalyst 5000 as an ATM Edge Device
Catalyst 3000 as an ATM Edge Device
Designing ATM Internetworks
This chapter describes current Asynchronous Transfer Mode (ATM) technologies that network designers can use in their networks today. It also makes recommendations for designing non-ATM networks so that those networks can take advantage of ATM in the future without sacrificing current investments in cable.
This chapter focuses on the following topics:
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ATM overview
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ATM Defined
ATM is an evolving technology designed for the high-speed transfer of voice, video, and data through public and private networks in a cost-effective manner. ATM is based on the efforts of Study Group XVIII of the International Telecommunication Union Telecommunication Standardization Sector (ITU-T, formerly the Consultative Committee for International Telegraph and Telephone [CCITT]) and the American National Standards Institute (ANSI) to apply very large-scale integration (VLSI) technology to the transfer of data within public networks. Officially, the ATM layer of the Broadband Integrated Services Digital Network (BISDN) model is defined by CCITT I.361.
Current efforts to bring ATM technology to private networks and to guarantee interoperability between private and public networks is being done by the ATM Forum, which was jointly founded by Cisco Systems, NET/ADAPTIVE, Northern Telecom, and Sprint in 1991.
Role of ATM in Internetworks
Today, 90 percent of computing power resides on desktops, and that power is growing exponentially. Distributed applications are increasingly bandwidth-hungry, and the emergence of the Internet is driving most LAN architectures to the limit. Voice communications have increased significantly with increasing reliance on centralized voice mail systems for verbal communications. The internetwork is the critical tool for information flow. Internetworks are being pressured to cost less yet support the emerging applications and higher number of users with increased performance.
To date, local and wide-area communications have remained logically separate. In the LAN, bandwidth is free and connectivity is limited only by hardware and implementation cost. The LAN has carried data only. In the WAN, bandwidth has been the overriding cost, and such delay-sensitive traffic as voice has remained separate from data. New applications and the economics of supporting them, however, are forcing these conventions to change.
The Internet is the first source of multimedia to the desktop and immediately breaks the rules. Such Internet applications as voice and real-time video require better, more predictable LAN and WAN performance. In addition, the Internet also necessitates that the WAN recognize the traffic in the LAN stream, thereby driving LAN/WAN integration.
Multiservice Networks
ATM has emerged as one of the technologies for integrating LANs and WANs. ATM can support any traffic type in separate or mixed streams, delay-sensitive traffic, and nondelay-sensitive traffic, as shown in Figure 8-1.
Figure 8-1 ATM support of various traffic types.
ATM can also scale from low to high speeds. It has been adopted by all the industry's equipment vendors, from LAN to private branch exchange (PBX). With ATM, network designers can integrate LANs and WANs, support emerging applications with economy in the enterprise, and support legacy protocols with added efficiency.
TDM Network Migration
In addition to using ATM to combine multiple networks into one multiservice network, network designers are deploying ATM technology to migrate from TDM networks for the following reasons:
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Reduced WAN Bandwidth Cost
The Cisco line of ATM switches provide additional bandwidth through the use of voice compression, silence compression, repetitive pattern suppression, and dynamic bandwidth allocation. The Cisco implementation of ATM combines the strengths of TDM—whose fixed time slots are used by telephone companies to deliver voice without distortion—with the strengths of packet-switching data networks—whose variable size data units are used by computer networks, such as the Internet, to deliver data efficiently.
While building on the strengths of TDM, ATM avoids the weaknesses of TDM (which wastes bandwidth by transmitting the fixed time slots even when no one is speaking) and PSDNs (which cannot accommodate time-sensitive traffic, such as voice and video, because PSDNs are designed for transmitting bursty data). By using fixed-size cells, ATM combines the isochronicity of TDM with the efficiency of PSDN.
Improved Performance
ATM offers improved performance through performance guarantees and robust WAN traffic management that support the following capabilities:
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Reduced Downtime
ATM offers high reliability, thereby reducing downtime. This high reliability is available because of the following ATM capabilities:
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Integrated Solutions
The trend in internetworking is to provide network designers greater flexibility in solving multiple internetworking problems without creating multiple networks or writing off existing data communications investments. Routers can provide a reliable, secure network and act as a barrier against inadvertent broadcast storms in the local networks. Switches, which can be divided into two main categories—LAN switches and WAN switches—can be deployed at the workgroup, campus backbone, or WAN level, as shown in Figure 8-2.
Figure 8-2 The role of ATM switches in an internetwork.
Underlying and integrating all Cisco products is the Cisco IOS software.The Cisco IOS software enables disparate groups, diverse devices, and multiple protocols all to be integrated into a highly reliable and scalable network.
Different Types of ATM Switches
Even though all ATM switches perform cell relay, ATM switches differ markedly in the following ways:
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Just as there are routers and LAN switches available at various price/performance points with different levels of functionality, ATM switches can be segmented into the following four distinct types that reflect the needs of particular applications and markets:
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As Figure 8-2 shows, Cisco offers a complete range of ATM switches.
Workgroup and Campus ATM Switches
Workgroup ATM switches are characterized by having Ethernet switch ports and an ATM uplink to connect to a campus ATM switch. An example of a workgroup ATM switch is the Cisco Catalyst 5000.
The Catalyst 5500 switch provides high-performance switching between workstations, servers, switches, and routers in wiring closet, workgroup, and campus backbone environments.
The Catalyst 5500 LAN is a 13-slot switch. Slot 1 is reserved for the supervisor engine module, which provides switching, local and remote management, and dual Fast Ethernet uplinks. Slot 2 is available for a second, redundant supervisor engine, or any of the other supported modules. Slots 3-12 support any of the supported modules.
Slot 13 can be populated only with a LightStream 1010 ATM Switch Processor (ASP). If an ASP is present in slot 13, slots 9-12 support any of the standard LightStream 1010 ATM switch port adapter modules (PAMs).
The Catalyst 5500 has a 3.6-Gbps media-independent switch fabric and a 5-Gbps cell-switch fabric. The backplane provides the connection between power supplies, supervisor engine, interface modules, and backbone module. The 3.6-Gbps media-independent fabric supports Ethernet, Fast Ethernet, FDDI/CDDI, ATM LAN Emulation, and RSM modules. The 5-Gbps cell-based fabric supports a LightStream 1010 ASP module and ATM PAMs.
Campus ATM switches are generally used for small-scale ATM backbones (for instance, to link ATM routers or LAN switches). This use of ATM switches can alleviate current backbone congestion while enabling the deployment of such new services as virtual LANs (VLANs). Campus switches need to support a wide variety of both local backbone and WAN types but be price/performance optimized for the local backbone function. In this class of switches, ATM routing capabilities that allow multiple switches to be tied together is very important. Congestion control mechanisms for optimizing backbone performance is also important. The LightStream 1010 family of ATM switches is an example of a campus ATM switch. For more information on deploying workgroup and campus ATM switches in your internetwork, see Chapter 12, "Designing Switched LAN Internetworks."
Enterprise ATM Switches
Enterprise ATM switches are sophisticated multiservice devices that are designed to form the core backbones of large, enterprise networks. They are intended to complement the role played by today's high-end multiprotocol routers. Enterprise ATM switches are used to interconnect campus ATM switches. Enterprise-class switches, however, can act not only as ATM backbones but can serve as the single point of integration for all of the disparate services and technology found in enterprise backbones today. By integrating all of these services onto a common platform and a common ATM transport infrastructure, network designers can gain greater manageability and eliminate the need for multiple overlay networks.
Cisco's BPX/AXIS is a powerful broadband ATM switch designed to meet the demanding, high-traffic needs of a large private enterprise or public service provider. This chapter focuses on this category of ATM switches.
Multiservice Access Switches
Beyond private networks, ATM platforms will also be widely deployed by service providers both as customer premises equipment (CPE) and within public networks. Such equipment will be used to support multiple MAN and WAN services—for instance, Frame Relay switching, LAN interconnect, or public ATM services—on a common ATM infrastructure. Enterprise ATM switches will often be used in these public network applications because of their emphasis on high availability and redundancy, their support of multiple interfaces, and capability to integrate voice and data.
ATM Overview
This section discusses the following ATM concepts:
Structure of an ATM Network
ATM is based on the concept of two end-point devices communicating by means of intermediate switches. As Figure 8-3 shows, an ATM network is made up of a series of switches and end-point devices. The end-point devices can be ATM-attached end stations, ATM-attached servers, or ATM-attached routers.
Figure 8-3 Components of an ATM network.
As Figure 8-3 shows, there are two types of interfaces in an ATM network:
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The UNI connection is made up of an end-point device and a private or public ATM switch. The NNI is the connection between two ATM switches. The UNI and NNI connections can be carried by different physical connections.
In addition to the UNI and NNI protocols, the ATM Forum has defined a set of LAN Emulation (LANE) standards and a Private Network to Network Interface (PNNI) Phase 0 protocol. LANE is a technology network designers can use to internetwork legacy LANs such as Ethernet and Token Ring with ATM-attached devices. Most LANE networks consist of multiple ATM switches and typically employ the PNNI protocol.
The full PNNI 1.0 specification was released by the ATM Forum in May 1996. It enables extremely scalable, full function, dynamic multi-vendor ATM networks by providing both PNNI routing and PNNI signaling. PNNI is based on UNI 3.0 signaling and static routes. The section "Role of LANE" later in this chapter discusses ATM LANE networks in detail.
General Operation on an ATM Network
Because ATM is connection-oriented, a connection must be established between two end points before any data transfer can occur. This connection is accomplished through a signaling protocol as shown in Figure 8-4.
Figure 8-4 Establishing a connection in an ATM network.
As Figure 8-4 shows, for Router A to connect to Router B the following must occur:
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This request contains the ATM address of the Router B as well as any QoS parameters required for the connection.
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If any switch along the path cannot accommodate the requested QoS parameters, the request is rejected and a rejection message is sent back to Router A.
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ATM Functional Layers
Just as the Open System Interconnection (OSI) reference model describes how two computers communicate over a network, the ATM protocol model describes how two end systems communicate through ATM switches. The ATM protocol model consists of the following three functional layers:
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As Figure 8-5 shows, these three layers correspond roughly to Layer 1 and parts of Layer 2 (such as error control and data framing) of the OSI reference model.
Figure 8-5 Relationship of ATM functional layers to the OSI reference model.
Physical Layer
The ATM physical layer controls transmission and receipt of bits on the physical medium. It also keeps track of ATM cell boundaries and packages cells into the appropriate type of frame for the physical medium being used. The ATM physical layer is divided into two parts:
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Physical Medium Sublayer
The physical medium sublayer is responsible for sending and receiving a continuous flow of bits with associated timing information to synchronize transmission and reception. Because it includes only physical-medium-dependent functions, its specification depends on the physical medium used. ATM can use any physical medium capable of carrying ATM cells. Some existing standards that can carry ATM cells are SONET (Synchronous Optical Network)/SDH, DS-3/E3, 100-Mbps local fiber (Fiber Distributed Data Interface [FDDI] physical layer), and 155-Mbps local fiber (Fiber Channel physical layer). Various proposals for use over twisted-pair wire are also under consideration.
Transmission Convergence Sublayer
The transmission convergence sublayer is responsible for the following:
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ATM Layer
The ATM layer establishes virtual connections and passes ATM cells through the ATM network. To do this, it uses the information contained in the header of each ATM cell. The ATM layer is responsible for performing the following four basic functions:
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ATM Adaptation Layer (AAL)
The AAL translates between the larger service data units (SDUs) (for example, video streams and data packets) of upper-layer processes and ATM cells. Specifically, the AAL receives packets from upper-level protocols (such as AppleTalk, Internet Protocols [IP], and NetWare) and breaks them into the 48-byte segments that form the payload field of an ATM cell. Several ATM adaptation layers are currently specified. Table 8-1 summarizes the characteristics of each AAL.
Table 8-1 ATM Adapter Layers
AAL1
AAL1 prepares a cell for transmission. The payload data consists of a synchronous sample (for example, one byte of data generated at a sampling rate of 125 microseconds). The sequence number field (SN) and sequence number protection (SNP) fields provide the information that the receiving AAL1 needs to verify that it has received the cells in the correct order. The rest of the payload field is filled with enough single bytes to equal 48 bytes.
AAL1 is appropriate for transporting telephone traffic and uncompressed video traffic. It requires timing synchronization between the source and destination and, for that reason, depends on a media that supports clocking, such as SONET. The standards for supporting clock recovery are currently being defined.
AAL3/4
AAL3/4 was designed for network service providers and is closely aligned with Switched Multimegabit Data Service (SMDS). AAL3/4 is used to transmit SMDS packets over an ATM network. The convergence sublayer (CS) creates a protocol data unit (PDU) by prepending a Beginning/End Tag header to the frame and appending a length field as a trailer as shown in Figure 8-6.
Figure 8-6 AAL3/4 cell preparation.
The segmentation and reassembly (SAR) sublayer fragments the PDU and prepends to each PDU fragment a header consisting of the following fields:
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The SAR sublayer also appends a CRC-10 trailer to each PDU fragment. The completed SAR PDU becomes the payload field of an ATM cell to which the ATM layer prepends the standard ATM header.
AAL5
AAL5 prepares a cell for transmission as shown in Figure 8-7.
Figure 8-7 AAL5 cell preparation.
First, the convergence sublayer of AAL5 appends a variable-length pad and an 8-byte trailer to a frame. The pad is long enough to ensure that the resulting PDU falls on the 48-byte boundary of the ATM cell. The trailer includes the length of the frame and a 32-bit CRC computed across the entire PDU, which allows AAL5 at the destination to detect bit errors and lost cells or cells that are out of sequence.
Next, the segmentation and reassembly segments the CS PDU into 48-byte blocks. Then the ATM layer places each block into the payload field of an ATM cell. For all cells except the last cell, a bit in the PT field is set to zero to indicate that the cell is not the last cell in a series that represents a single frame. For the last cell, the bit in the PT field is set to one. When the cell arrives at its destination, the ATM layer extracts the payload field from the cell; the SAR sublayer reassembles the CS PDU; and the CS uses the CRC and the length field to verify that the frame has been transmitted and reassembled correctly.
AAL5 is the adaptation layer used to transfer most non-SMDS data, such as classical IP over ATM and local-area network (LAN) emulation.
ATM Addressing
The ATM Forum has adapted the subnetwork model of addressing in which the ATM layer is responsible for mapping network-layer addresses to ATM addresses. Several ATM address formats have been developed. Public ATM networks typically use E.164 numbers, which are also used by Narrowband ISDN (N-ISDN) networks.
Figure 8-8 shows the format of private network ATM addresses. The three formats are Data Country Code (DCC), International Code Designator (ICD), and Network Service Access Point (NSAP) encapsulated E.164 addresses.
Figure 8-8 ATM address formats.
Fields of an ATM Address
The fields of an ATM address are as follows:
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The ATM address formats are modeled on ISO NSAP addresses, but they identify subnetwork point of attachment (SNPA) addresses. Incorporating the MAC address into the ATM address makes it easy to map ATM addresses into existing LANs.
ATM Media
The ATM Forum has defined multiple standards for encoding ATM over various types of media. Table 8-2 lists the framing type and data rates for the various media, including unshielded twisted-pair (UTP) and shielded twisted-pair (STP) cable.
Table 8-2 ATM Physical Rates
Because the FDDI chipset standard, TAXI 4B/5B, was readily available, the ATM Forum encouraged initial ATM development efforts by endorsing TAXI 4B/5B as one of the first ATM media encoding standards. Today, however, the most common fiber interface is STS3c/STM.
There are two standards for running ATM over copper cable: UTP-3 and UTP-5. The UTP-5 specification supports 155 Mbps with NRZI encoding, while the UTP-3 specification supports 51 Mbps with CAP-16 encoding. CAP-16 is more difficult to implement, so, while it may be cheaper to wire with UTP-3 cable, workstation cards designed for CAP-16-based UTP-3 may be more expensive and will offer less bandwidth.
Because ATM is designed to run over fiber and copper cable, investments in these media today will maintain their value when networks migrate to full ATM implementations as ATM technology matures.
ATM Data Exchange Interface
To make ATM functionality available as soon as possible, the ATM Forum developed a standard known as the ATM Data Exchange Interface (DXI). Network designers can use DXI to provide UNI support between Cisco routers and ATM networks, as shown in Figure 8-9.
Figure 8-9 ATM DXI topology.
The ATM data service unit (ADSU) receives data from the router in ATM DXI format over a High-Speed Serial Interface (HSSI). The DSU converts the data into ATM cells and transfers them to the ATM network over a DS-3/E3 line.
ATM DXI is available in several modes:
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On the router, data from upper-layer protocols is encapsulated into ATM DXI frame format. Figure 8-10 shows the format of a Mode 1a ATM DXI frame.
Figure 8-10 ATM DXI frame format.
In Figure 8-11, a router configured as a data terminal equipment (DTE) device is connected to an ADSU. The ADSU is configured as a data communications equipment (DCE) device. The router sends ATM DXI frames to the ADSU, which converts the frames to ATM cells by processing them through the AAL5 CS and the SAR sublayer. The ATM layer attaches the header, and the cells are sent out the ATM UNI interface.
Figure 8-11 ATM DXI Mode 1a and Mode 1b protocol architecture for AAL5.
ATM DXI addressing consists of a DFA, which is equivalent to a Frame Relay data link connection identifier (DLCI). The DSU maps the DFA into appropriate VPI and VCI values in the ATM cell. Figure 8-12 shows how the DSU performs address mapping.
Figure 8-12 ATM DXI address mapping.
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Role of LANE
The ATM Forum has defined a standard for LANE. LANE is a technology that network designers can deploy to internetwork their legacy LANs (for example, Ethernet and Token Ring LANs), with ATM-attached devices. LANE uses MAC encapsulation (OSI Layer 2) because this approach supports the largest number of existing OSI Layer 3 protocols. The end result is that all devices attached to an emulated LAN (ELAN) appear to be on one bridged segment. In this way, AppleTalk, IPX, and other protocols should have similar performance characteristics as in a traditional bridged environment.
In ATM LANE environments, the ATM switch handles traffic that belongs to the same ELAN, and routers handle inter-ELAN traffic. Figure 8-13 shows an example of an ATM LANE network.
Figure 8-13 Components of an ATM LANE network.
As Figure 8-13 shows, network designers can use the LANE technology to interconnect legacy LANs to any of the following types of ATM-attached devices:
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LANE Components
LANE components include the following:
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The following is an example of this database.
finance
47.0091.8100.0000.0800.200c.1001. 0800.200c.1001.01
marketing
47.0091.8100.0000.0800.200c.1001. 0800.200c.1001.02
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The BUS also acts as a multicast server. LANE is defined on ATM adaptation layer 5 (AAL5), which specifies a simple trailer to be appended to a frame before it is broken into ATM cells. The problem is that there is no way to differentiate between ATM cells from different senders when multiplexed on a virtual channel. It is assumed that cells received will be in sequence, and when the End of Message (EOM) cell arrives, you should just have to reassemble all of the cells that have already arrived.
The BUS takes the sequence of cells on each Multicast Send VCC and reassembles them into frames. When a full frame is received, it is queued for sending to all of the LECs on the Multicast Forward VCC. This way, all the cells from a particular data frame can be guaranteed to be sent in order and not interleaved with cells from any other data frames on the point-to-multipoint VCC.
Note that because LANE is defined at OSI Layer 2, the LECS is the only security checkpoint available. Once it has been told where to find the LES and it has successfully joined the ELAN, the LEC is free to send any traffic (whether malicious or not) into the bridged ELAN. The only place for any OSI Layer 3 security filters is in the router that routes this ELAN to other ELANs. Therefore, the larger the ELAN, the greater the exposure to security violations.
How LANE Works
An ELAN provides Layer 2 communication between all users on an ELAN. One or more ELANs can run on the same ATM network. However, each ELAN is independent of the others and users on separate ELANs cannot communicate directly. Communication between ELANs is possible only through routers or bridges.
Because an ELAN provides Layer 2 communication, it can be equated to a broadcast domain. VLANs can also be thought of as broadcast domains. This makes it possible to map an ELAN to a VLAN on Layer 2 switches with different VLAN multiplexing technologies such as Inter-Switch Link (ISL) or 802.10. In addition, IP subnets and IPX networks that are defined on Layer 3-capable devices such as routers frequently map into broadcast domains (barring secondary addressing). This makes it possible to assign an IP subnetwork or an IP network to an ELAN.
An ELAN is controlled by a single LES/BUS pair and the mapping of an ELAN to its LES ATM address is defined in the LECS database. ELANs consists of multiple LECs and can be Ethernet or Token Ring but not both at the same time.
In order for ELAN to operate properly, the LECs on that ELAN need to be operational. Each LEC goes through a boot up sequence that is described in the following sections.
LANE Operation
In a typical LANE operation, the LEC must first find the LECS to discover which ELAN it should join. Specifically, the LEC is looking for the ATM address of the LECS that serves the desired ELAN.
Finding the LECS
To find the ATM address of the LECS, the LEC does the following:
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Contacting the LECS
The LEC creates a signaling packet with the ATM address of the LECS. It signals a Configure Direct VCC and then issues an LE_CONFIGURE_REQUEST on that VCC. The information in this request is compared with the data in the LECS database. The source ATM address is most commonly used to place a LEC into a specific ELAN. If a matching entry is found, a successful LE_CONFIGURE_RESPONSE is returned with the ATM address of the LES that serves the desired ELAN.
Configuring the LECS database
You can configure the LECS database in any of the following three ways:
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The following is an example of LEC-to-ELAN mapping at the LEC:
lane database test-1name finance server-atm-address 47.0091.8100.0000.0800.200c.1001. 0800.200c.1001.01name marketing server-atm-address 47.0091.8100.0000.0800.200c.1001. 0800.200c.1001.02default-name finance•
With this method, the LECs are identified by their ATM addresses or MAC addresses. Because wildcarding of ATM address prefixes is also supported, it is useful to make such relationships as "Assign any LEC joining with a prefix of A to ELAN X." The following is an example of LEC-to-ELAN mapping in the LECS database:
lane database test-2name finance server-atm-address 47.0091.8100.0000.0800.200c.1001. 0800.200c.1001.01name marketing server-atm-address 47.0091.8100.0000.0800.200c.1001. 0800.200c.1001.02default-name financeclient-atm-address 47.0091.8100.0000.08... name financeclient-atm-address 47.0091.8100.0000.09... name marketingmac-address 00c0.0000.0100 name financemac-address 00c0.1111.2222 name marketing•
Joining the LES
After the LEC has discovered the ATM address of the desired LES, it drops the connection to the LECS, creates a signaling packet with the ATM address of the LES, and signals a Control Direct VCC. Upon successful VCC setup, the LES sends an LE_JOIN_REQUEST. This request contains the LEC ATM address as well as a MAC address that the LEC wants to register with the ELAN. This information is maintained so that no two LECs can register the same MAC or ATM addresses.
Upon receipt of the LE_JOIN_REQUEST, the LES checks with the LECS via its own open connection with the LECS and verifies the request, thus confirming the client's membership. Upon successful verification, the LES adds the LEC as a leaf of its point-to-multipoint Control Distribute VCC. Finally, the LES issues the LEC a successful LE_JOIN_RESPONSE that contains a LANE client ID (LECID), which is an identifier that is unique to the new client. This ID is used by the LEC to filter its own broadcasts from the BUS. Figure 8-14 shows examples of LES connections.
Figure 8-14 LAN emulation server (LES) connections.
Finding the BUS
After the LEC has successfully joined the LES, its first task is to find the ATM address of the BUS and join the broadcast group. The LEC creates an LE_ARP_REQUEST packet with the MAC address 0xFFFFFFFF. This special LE_ARP packet is sent on the Control Direct VCC to the LES. The LES recognizes that the LEC is looking for the BUS, responds with the ATM address of the BUS, and forwards that response on the Control Distribute VCC.
Joining the BUS
When the LEC has the ATM address of the BUS, its next action is to create a signaling packet with that address and signal a Multicast Send VCC. Upon receipt of the signaling request, the BUS adds the LEC as a leaf on its point-to-multipoint Multicast Forward VCC. At this time, the LEC has become a member of the ELAN. Figure 8-15 shows examples of BUS connections.
Figure 8-15 BUS connections.
Address Resolution
The real value of LANE is the ATM forwarding path that it provides for unicast traffic between LECs. When a LEC has a data packet to send to an unknown destination, it issues an LE_ARP_REQUEST to the LES on the Control Direct VCC. The LES forwards the request on the Control Distribute VCC, so all LEC stations hear it. In parallel, the unicast data packets are sent to the BUS, to be forwarded to all endpoints. This "flooding" is not the optimal path for unicast traffic, and this transmission path is rate-controlled to 10 packets per second (per the LANE standard). Unicast packets continue using the BUS until the LE_ARP_REQUEST has been resolved.
If bridging or switching devices with LEC software participate in the ELAN, they translate and forward the ARP on their LAN interfaces. One of the LECs should issue an LE_ARP_RESPONSE and send it to the LES, which forwards it to the Control Distribute VCC so that all LECs can learn the new MAC-to-ATM address binding.
When the requesting LEC receives the LE_ARP_RESPONSE, it has the ATM address of the LEC that represents the MAC address being sought. The LEC should now signal the other LEC directly and set up a Data Direct VCC that will be used for unicast data between the LECs.
While waiting for LE_ARP resolution, the LEC forwards unicasts to the BUS. With LE_ARP resolution, a new "optimal" path becomes available. If the LEC switches immediately to the new path, it runs the risk of packets arriving out of order. To guard against this situation, the LANE standard provides a flush packet.
When the Data Direct VCC becomes available, the LEC generates a flush packet and sends it to the BUS. When the LEC receives its own flush packet on the Multicast Forward VCC, it knows that all previously sent unicasts must have already been forwarded. It is now safe to begin using the Data Direct VCC. Figure 8-16 shows an example of a fully connected ELAN.
Figure 8-16 Fully connected ELAN.
LANE Implementation
As Table 8-3 indicates, the LANE functionality (the LECS, LEC, LES, and BUS) can be implemented in different Cisco devices.
Table 8-3 Cisco LANE Implementation
These functions will be defined on ATM physical interfaces and subinterfaces. A subinterface can be defined as a logical interface and is a part of a physical interface such as an Optical Carrier 3 (OC-3) fiber. ATM interfaces on the Cisco routers and the ATM module on the Catalyst 5000 switch can be logically divided into up to 255 logical subinterfaces. On the Catalyst 3000 switch, although the same Cisco IOS Software code is used, the subinterface concept does not apply. The LEC can be configured using the menu-driven interface.
This section examines the implementation of ATM LANE networks and covers the following topics:
LANE Design Considerations
The following are some general LANE design considerations:
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PNNI in LANE Networks
Network designers can deploy PNNI as a Layer 2 routing protocol for bandwidth management, traffic distribution, and path redundancy for LANE networks. PNNI is an ATM routing protocol used for routing call setups and is implemented in the ATM switches. Most LANE networks consist of multiple ATM switches and typically employ the PNNI protocol.
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The LightStream 1010 ATM switch supports some PNNI-related features that can be useful in scaling LANE networks:
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Figure 8-17 shows how the LightStream 1010 switch supports load balancing.
Figure 8-17 Load balancing calls across multiple paths and multiple links.
As Figure 8-17 shows, load balancing of calls is enabled by default on the LightStream 1010 switch. Background routing, however, is not enabled by default. Background routing can be thought of as routing of call setups using a path from a precomputed route database. The background routing process computes a list of all possible paths to all destinations across all the service categories (for example, constant bit rate [CBR], virtual bit rate-real time [VBR-RT], virtual bit rate and nonreal time [VBR-NRT] and available bit rate-unspecified bit rate [ABR-UBR]).
When a call is placed from Point A to Point B, PNNI picks a cached routed from the background route table instead of computing a route on demand. This eases the CPU load and provides a faster rate of processing the call setups.
Background routing can be useful in networks that have a stable topology with respect to QoS. It is, however, not very effective in networks that have rapidly changing topologies (for example, Internet Service Providers [ISP] networks or carrier networks). Campus LANE networks can use this feature effectively because all the SVCs in the network belong to the UBR or ABR category. To enable this feature, use the following command:
atm router pnninode 1 level 56bg-routesThe current implementation of PNNI on the LightStream 1010 switch is full, ATM Forum-PNNI Version 1 compliant. The LightStream default PNNI image license supports a single level of hierarchy, where multiple peer groups can be interconnected by IISP or by other switches that support full PNNI hierarchy; extra PNNI image license will support multiple levels of routing hierarchy.
The PNNI protocols have been designed to scale across all sizes of ATM networks, from small campus networks of a handful of switches, to the possible global ATM Internet of millions of switches. This level of scalability is greater than that of any existing routing protocol, and requires very significant complexity in the PNNI protocol. Specifically, such scalability mandates the support of multiple levels of routing hierarchy based upon the use of prefixes of the 20-byte ATM address space. The lowest level of the PNNI routing hierarchy consists of a single peer group within which all switches flood all reachability and QoS metrics to one another. This is analogous, for instance, to a single area in the OSPF protocol.
Subsequently, multiple peer groups at one level of the hierarchy are aggregated into higher-level peer groups, within which each lower-level peer group is represented by a single peer group leader, and so on iteratively up the PNNI hierarchy. Each level of the hierarchy is identified by a prefix of the ATM address space, implying that PNNI could theoretically contain over 100 levels of routing hierarchy. However, a handful of levels would be adequate for any conceivable network. The price to be paid for such scalability is the need for highly complex mechanisms for supporting and bringing up the multiple levels of hierarchy and for electing the peer group leaders within each peer group at each level.
Scaling an ELAN—Spanning-Tree Protocol Issues
Spanning-Tree Protocol is implemented in Layer 2 switches/bridges to prevent temporary loops in networks with redundant links. Because a LEC essentially bridges Ethernet/Token Ring traffic over an ATM backbone, the Spanning-Tree Bridge Protocol Data Units (BPDUs) are transmitted over the entire ELAN. The ATM network appears as a shared Ethernet/Token Ring network to the spanning-tree process at the edge of the Layer 2 switches.
The spanning-tree topology of a LANE-based network is substantially simpler than a pure frame-switched network that employs the Spanning-Tree Protocol. It follows that spanning-tree convergence times, which can be a major issue in large frame-switched networks, can be less of an issue in LANE networks. Note that Spanning Tree must reconverge if there are failures at the edge devices or inside the ATM network. If there is a need to tune the convergence time to a lower or higher value, the forward delay parameter can be used.
LANE Redundancy
Although LANE allows network designers to connect their legacy LANs to an ATM network, LANE Version 1.0 does not define mechanisms for building redundancy and fault tolerance into the LANE services. Consequently, this makes the LANE services a single point of failure. Moreover, router redundancy and path/link redundancy are also issues that the network designer needs to consider.
Network designers can use the following techniques to build fault-tolerant and resilient LANE networks:
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The following subsections examine these various mechanisms and highlights design rules and issues to consider while implementing redundant LANE networks. It begins with a discussion on SSRP that was developed to provide redundant LANE services.
Although many vendors have implemented redundant LANE services of some fashion, they violate the LANE 1.0 specification and therefore are not interoperable with other third-party implementations. SSRP, however, does not violate the LANE 1.0 specification and is interoperable with third-party LEC implementations, which is important when implementing an interoperable ATM network.
The discussion on SSRP is followed by a description of HSRP over LANE, which provides a mechanism for building router redundancy. Following this is a discussion on the Spanning-Tree Protocol and other product-specific features that can be used to build link and path redundancy into edge devices.
Issues in a LANE 1.0 Network
The main issue with a LANE 1.0 network is that only one set of LANE service components can be accessed by a LEC at any given time. This results in the following limitations:
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A failure in any of these service components has the following impact on network operation:
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It is clear that these issues can be limiting to networks where resiliency and robustness is a requirement and might even be a deciding factor in your design of whether to implement LANE-based ATM networks. In addition, there are other design considerations such as the placement of the LANE service components within an ATM network that can have implications on the overall robustness of the LANE environment.
Resiliency in LANE 1.0 Networks
Increasing the resiliency of a LANE-based network essentially includes delivering increased robustness in the LANE service components such as the LECS, LES, and BUS. Such robustness is provided by SSRP through a primary-secondary combination of the LANE services. For LECS redundancy, one primary LECS is backed up by multiple secondary LECSs. LES/BUS redundancy is also handled in a similar fashion where one primary LES/BUS pair is backed up by multiple secondaries. Note that the LES/BUS functions are always co-located in a Cisco implementation and the pair is handled as one unit with respect to redundancy.
LECS Redundancy
In the LANE 1.0 specification, the first step for a LEC during initialization is to connect with the LECS to obtain the LES ATM address for the ELAN it wants to join. In order for the LEC to connect to the LECS, multiple mechanisms are defined. The first mechanism that a LEC should use is to query the ATM switch it is attached to for the LECS address. This address discovery process is done using the ILMI protocol on VPI, VCI - 0, 16.
The following is an example of the configuration command to add a LECS address to a LightStream 1010 switch:
atm lecs-address <LECS NSAP address> <index>With SSRP, multiple LECS addresses are configured into the ATM switches. An LEC, which requests the LECS address from the ATM switch, gets the entire table of LECS addresses in response. The behavior of the LEC should be to attempt to connect to the highest ranking LECS address. If this fails, it should try the next one in the list and so on until it connects to the LECS.
Whereas the LEC always tries to connect to the highest ranking LECS available, SSRP ensures that there is only a single primary that responds to the Configure Request queries coming from the LEC. The establishment of a primary LECS and placing the others in backup goes to the heart of SSRP. The following describes the mechanism used by SSRP to establish a primary LECS. Upon initialization, a LECS obtains the LECS address table from the switch. The LECS then tries to connect to all the LECSs that are below itself in rank. The rank is derived from the index entry in the LECS address table.
If a LECS has a connection (VCC) from a LECS whose rank is higher than its own, it is in backup mode. The highest ranking LECS does not have any other LECS that connect to it from above and assumes the role of the primary LECS.
Figure 8-18 shows the procedure of a backup taking over in the case of a failed primary LECS. The LANE network shown in Figure 8-18 has four LECS entities (LECS A, B, C, and D). All the ATM switches in the network are configured with the same LECS address table. After startup, LECS A obtains the LECS address table from the ATM switch it is attached to and finds that it has three LECSs below itself and therefore tries to connect to LECS B, C, and D. LECS B connects to LECS C and LECS D, and LECS C connects to LECS D. There is a downward establishment of VCCs. Because LECS A does not have any VCCs from above, it becomes the primary LECS.
Figure 8-18 LECS redundancy.
During normal network operation, LECS A responds to all the configure requests and the backup LECS (LECS B, C and D) do not respond to any queries. If for some reason the primary LECS (LECS A) fails due to such conditions as a box failure, LECS B loses its VCC from LECS A as do the other LECS.
At this point, LECS B does not have any VCCs from above and therefore is now the highest ranking available LECS in the network. LECS B now becomes the primary LECS. LECS C and LECS D still have connections from higher ranking LECSs and therefore continue to operate in backup mode as shown in Step 2b of Figure 8-18.
LES/BUS Redundancy
The LES/BUS redundancy portion of SSRP supports the configuration of multiple LES/BUS pairs that work in a primary-secondary fashion. However, the mechanisms used here are different from those used for the LECS redundancy described in the preceding section.
Multiple LES/BUS pairs for a given ELAN are first configured into the LECS database. Within this database, each LES/BUS pair is assigned a priority. After initialization, each LES/BUS opens a VCC with the primary LECS using the LECS address discovery mechanism. The LES/BUS pair with the highest priority that has an open VCC to the LECS is assigned as the primary LES/BUS by the primary LECS.
SSRP Usage Guidelines
There are no theoretical limits on the number of LECSs that can be configured using SSRP, however a recommended number is two (one primary plus one backup) or three LECSs (one primary plus two backups). Any more redundancy should be implemented only after very careful consideration because it will add a significant amount of complexity to the network. This added complexity can result in a substantial increase in the amount of time required to manage and troubleshoot such networks.
SSRP Configuration Guidelines
To support the LECS redundancy scheme, you must adhere to the following configuration rules. Failure to do so will result in improper operation of SSRP and a malfunctioning network.
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SSRP Interoperability Notes
SSRP can be used with independent third-party LECs if they use ILMI for LECS address discovery and can appropriately handle multiple LECS addresses returned by the ATM switch. For example, the LEC should step through connecting to the list of LECS addresses returned by the ATM switch. The first LECS that responds to the configuration request is the master LECS.
Behavior of SSRP with the Well Known LECS Address
SSRP also works with LECS Well Known Address (47.0079....) defined in the LANE 1.0 specification. The Cisco LECS can listen on multiple ATM addresses at the same time. Therefore, it can listen on the Well Known Address and the auto-configured ATM address, which can be displayed using the show lane default command.
When the LECS is enabled to listen on the Well Known Address, it registers the Well Known Address with the ATM switch so that the ATM switches can advertise routes to the Well Known Address and route any call setups requests to the correct place.
Under SSRP, there are multiple LECSs in the network. If each LECS registers the Well Known Address to the ATM switches that it is connected to, call setups are routed to different places in the network. Consequently, under SSRP you must configure an autoconfigured address so that the negotiation of the master first takes place and then the master registers the Well Known Address with the ATM switch. If the master fails, the Well Known Address moves with the master LECS. The PNNI code on the LightStream 1010 switch takes care of advertising the new route to the Well Known Address when there is a change of LECS mastership. Therefore, third-party LECs that use only the Well Known Address can also interoperate with SSRP. SSRP is the only redundancy scheme that can be used with almost any LEC in the industry.
To implement SSRP with the Well Known Address, use the following steps:
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Behavior of SSRP in Network Partitions
In the event of network partitions where two separate ATM clouds are formed due to an interconnecting link or switch failure, each cloud has its own set of LANE services if SSRP is configured to handle network partitions.
When configuring SSRP, use the following guidelines to accommodate the possibility of network partition:
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