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Cisco Systems Data-Link Switching

Table Of Contents

Cisco Systems Data-Link Switching Plus

Enhanced Scalability through Peer Groups

Enhanced Availability and Load Balancing

Enhanced Performance through Flexible Transport Options

Media Conversion Flexibility

Reverse LAN to SDLC

Qualified Logical Link Control (QLLC)

Frame Relay via RFC 1490

Why Use DLSw+?

Choosing between DLSw+ and RSRB

Application Example

DLSw+ Using TCP/IP Encapsulation

DLSw+ Interoperability

Planned Product Availability


Cisco Systems Data-Link Switching Plus


Data-Link Switching Plus (DLSw+), the third generation of DLSw, supports any-to-any networks with thousands of routers while at the same time providing full standards compliance.

DLSw+ offers significantly greater functionality than the two earlier generations. Request for Comments (RFC) 1434 was the first generation of data-link switching. The new DLSw standard, which represents the second generation of data-link switching, was completed in October 1994 and obsoletes RFC 1434. The standard includes key additions such as flow control and prioritization. DLSw+ is the third generation, providing full compliance with the DLSw standard while at the same time providing the only means to scale DLSw to any-to-any networks with thousands of routers. DLSw+ minimizes router processing requirements, reduces broadcast traffic, and simplifies configuration.

The following table compares the functionality of all three generations of data-link switching: RFC 1434, DLSw, and DLSw+.

Table 1. DLSw+ Functionality Comparison

 
Feature
RFC 1434
DLSw
DLSw+
Transport

TCP

X

X

X

 

Direct HDLC/LLC

   

X

 

Direct RFC 1400

   

X

 

FST

   

X

Media Conversion

SDLC to LLC2

X

X

X

 

LLC2 to SDLC

   

X

 

QLLC <-> LLC2

   

X

 

QLLC to SDLC

   

X

Performance

Flow Control

 

X

X

 

Custom Queuing

   

X

 

Prioritization

 

X

X

 

Load Balancing

   

X

Scalability

Caching

   

X

 

RIF Reduction

X

X

X

 

Peer Groups

   

X

 

On-Demand Peers

   

X

Availability

Backup Peers

   

X

 

Local Termination

X

X

X

Management

Standard1 MIB

 

X

X

Interoperability

Standards-Based

 

X

X

1 *Separate RFC


Enhanced Scalability through Peer Groups

A key limitation of the DLSw standard is that it requires permanent TCP connectivity between any pair of communicating routers. For any-to-any networks, this flat network design does not scale beyond approximately 100 routers. There are three key reasons. First of all, typical branch office routers do not have the memory or processor capability to support more than 100 TCP connections. In addition, the replication of explorer traffic over each TCP connection creates congestion on access links and introduces processing delays. Finally, configuration management is difficult, because every new router must be configured in every existing router.

DLSw+ overcomes the limitations imposed by the flat DLSw design by allowing large, meshed networks to be broken up into smaller groups, called Peer Groups, which are logically connected by "Border Peers."

DLSw+ routers do not require permanent TCP connections to all routers in an internetwork or even within a group. DLSw+ routers use a concept known as "On-Demand Peers" to dynamically find partners, establish connections for the duration of session activity, and then deactivate the connection. This not only addresses the limitation of concurrent TCP connections, it also addresses the configuration management issue, since On-Demand Peers require no configuration.

To reduce traffic caused by duplication of explorer frames on access links, DLSw+ access routers only generate a single explorer for each new destination, and Border Peers assume the task of explorer forwarding. Because no duplicate explorers are sent over any links, processing is simplified and traffic is reduced.

Besides using the peer concept to reduce broadcast traffic, DLSw+ always attempts to find a resource on a local ring before sending any "search" frames, which further reduces unnecessary WAN traffic. In addition, if a DLSw+ router receives multiple search requests for the same resource, it only sends the first one, and when it finds the resource, it responds to all the requests.

Figure 1. Scalability with DLSw+

Enhanced Availability and Load Balancing

DLSw+ improves availability and performance in a network with backup paths and backup routers. If more than one DLSw+ partner can reach a given destination, DLSw+ stores both a preferred partner and one or more capable backup partners. If a peer connection fails, another connection can be quickly reestablished without waiting to find alternate paths. This capability vastly improves network availability and reduces downtime.

If preferred, DLSw+ can be configured to load balance among multiple capable partners to enhance performance and improve availability.

These features are especially attractive when used in a network with multiple Network Control Programs (NCPs) that have the same Token Ring MAC address (known as a duplicate Token Ring interface coupler [TIC] configuration), because they can either allow traffic to be split between NCPs (load-balancing mode) or speed up recovery (backup mode).

Enhanced Performance through Flexible Transport Options

DLSw+ provides the same flexible transport options for enhanced performance that have made Cisco's remote source-route bridging (RSRB) the most popular means of integrating SNA with multiprotocol LANs. Cisco's unique Internetwork Operating System (IOS) offers alternatives to full TCP encapsulation while concurrently providing scalability features (such as NetBIOS name caching, hop-count reduction, and source-route bridge explorer reduction). DLSw+ supports full TCP/IP encapsulation with local acknowledgment, which is the best solution for SNA transport over low-speed (56-kbps) or highly congested lines. This is the only transport option defined by the data-link switching standard, and hence, must also be used when interoperability with other vendors' routers is required.

For environments with higher-speed lines, however, the processing and WAN overhead of TCP may be unnecessary. In these cases, higher performance can be achieved with either Fast-Sequenced Transport (FST) or direct encapsulation in HDLC, Frame Relay (RFC 1490), or FDDI. Cisco's FST is a unique IP encapsulation technique that allows dynamic rerouting around failures without the overhead of TCP. An innovative sequencing technique ensures that data arrives in sequence. DLSw+ also supports direct encapsulation, which provides the fastest throughput and is recommended when rerouting is unnecessary, such as when there is only a point-to-point line, a bridged LAN, or a Frame Relay network between the peers. Cisco's flexible transport options allow efficient network design regardless of line speeds and carrier options.

Media Conversion Flexibility

The data-link switching standard defines how to transport SNA between like LANs and how to convert between remote SDLC to local Token Ring. DLSw+ provides significantly more options.

Reverse LAN to SDLC

Many enterprises have selected Ethernet for their remote offices because of its lower cost. In these enterprises, there is a requirement for remote Ethernet to be converted to SDLC to allow attachment to a front-end processor (FEP). DLSw+ provides this reverse LAN-to-SDLC conversion. DLSw+ also supports remote Ethernet to local Token Ring conversion.

Qualified Logical Link Control (QLLC)

Cisco's QLLC can be used at a central site to convert remote QLLC traffic to Token Ring. This feature eliminates the requirement for running NCP Packet Switching Interface (NPSI) in the FEP to handle SNA traffic, reducing cost, simplifying configuration, and improving performance.

Cisco's QLLC conversion also allows remote routers to convert SDLC or LAN traffic into QLLC for transport directly to a FEP running NPSI. This saves money, because no X.25 hardware or software is required at the remote sites, and no router is required at the central site for SNA traffic. In addition, it simplifies deployment of LANs in an environment where X.25 is the most cost-effective WAN solution.

Frame Relay via RFC 1490

DLSw+ also supports SDLC or LAN traffic conversion to Frame Relay, according to RFC 1490, minimizing the WAN bandwidth and reducing the processing required when local acknowledgment is not required.

Why Use DLSw+?

DLSw+ offers the ideal way to consolidate SNA, NetBIOS, and multiprotocol LAN internetworks. It allows consolidation onto a single physical backbone, and with Cisco's custom queuing, it allows efficient utilization of bandwidth to ensure, for example, that mission-critical traffic is never delayed by electronic mail or large file transfers. In addition, it improves availability of SNA sessions by nondisruptively rerouting around link failures, nondisruptively returning to the primary path when the link recovers, and eliminating session-level timeouts during periods of peak congestion. It is a low-cost solution that utilizes WAN bandwidth far better than either bridging or time division multiplexing (TDM). DLSw+ uses a minimal amount of router memory and processor cycles, and hence can run effectively even in low-end routers with minimal memory configurations.

Choosing between DLSw+ and RSRB

RSRB provides excellent functionality for existing customers with hierarchical SNA networks or moderate-sized NetBIOS or APPN networks. For some SNA environments and for large NetBIOS or APPN environments, DLSw+ addresses requirements not addressed by RSRB. These requirements include interoperability with other routers, scalability for any-to-any networks consisting of thousands of routers, local acknowledgment over Ethernet, and SDLC-to-LAN conversion for PU 2.1 traffic.

The table that follows illustrates the differences between RSRB and DLSw+.

Table 2. DLSw+ Compared with RSRB

 
Feature
RSRB
DLSw+
Transport

TCP

X

X

 

Direct HDLC/LLC

X

X

 

Direct RFC 1490

1Q95

1H95

 

FST

X

X

Conversion

SDLC to LLC2

PU 2.0

PU 2.0/2.1

 

LLC2 to SDLC

X

X

 

QLLC <-> LLC2

X

1H95

 

QLLC to SDLC

X

1H95

 

Local Acknowledgment/ Ethernet

 

X

Performance

Flow Control

X

X

 

Custom Queuing

X

X

 

Prioritization

X

X

Scalability

Caching

X

X

 

RIF Reduction

Virtual Ring

RIF Term

 

Peer Groups

 

X

 

On-Demand Peers

 

X

Management

MIB

1H95

X1

Interoperability

Standards-Based

 

X

1 Standard MIB with separate RFC


Application Example

DLSw+ Using TCP/IP Encapsulation

Figure 2 shows an example of connecting retail outlets to a distribution center over low-speed (19.2-kbps) lines. In this example, the following DLSw+ features ap ply:

TCP/IP encapsulation ensures that delays in transmission do not cause SNA sessions to be dropped.

DLSw+ Border Peers eliminate duplicate broadcasts on low-speed access links.

On-Demand Peers allow use of any-to-any applications, such as Lotus Notes, between stores, without requiring preconfiguration or excessive numbers of TCP connections.

DLSw+ Interoperability

DLSw+ will interoperate with other vendors' implementations of the DLSw standard. A capabilities exchange between peers ensures that only features common to both peers are supported over a given connection. Cisco plans to test its DLSw+ implementation with other vendors' DLSw implementations as soon as they are available.

DLSw+ routers can concurrently interoperate with RSRB routers, DLSw standard compliant routers, and DLSw+ routers.

Planned Product Availability

DLSw+ is available in the IBM Base option for the Cisco AccessPro PC Card, Cisco 2500, and Cisco 4000 Series. SDLC and QLLC conversion require the IOS Enterprise Software Set. DLSw+ comes with the bridging feature of the AGS+ and Cisco 7000 Series. QLLC conversion and RFC 1490 support require the packet-switching feature.

DLSw+ will be orderable in January 1995 for a 1Q95 delivery. The initial release will include support for TCP, FST, and HDLC encapsulation. Support for RFC 1490 and QLLC will be available in 1H95 for DLSw+.

Figure 2. DLSw+ in Retail Application