Catalyst 3550 Multilayer Switch Software Configuration Guide, 12.1(8)EA1
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Table Of Contents



Management Options

Management Interface Options

Advantages of Using CMS and Clustering Switches

Network Configuration Examples

Design Concepts

Small to Medium-Sized Network Using Mixed Switches

Large Network Using Only Catalyst 3550 Switches

Multidwelling Network Using Catalyst 3550 Switches


This chapter provides these topics about the Catalyst 3550 multilayer switch software:


Management Options

Network Configuration Examples


The Catalyst 3550 software supports the hardware listed in the release notes. These sections describe the features supported in this release.

Note Layer 3 (routing) features require that you have the enhanced multilayer software image installed on your switch. All Catalyst 3550 Gigabit Ethernet switches ship with the enhanced multilayer software image (EMI) installed. Catalyst 3550 Fast Ethernet switches can be shipped with either the standard multilayer software image (SMI) or EMI pre-installed. You can order the Enhanced Multilayer Software Image Upgrade kit to upgrade Catalyst 3550 Fast Ethernet switches from running the SMI to the EMI.

Table 1-1 Features 

Ease of Use and Ease of Deployment

Cluster Management Suite (CMS) software for simplifying switch and switch cluster management through a web browser, such as Netscape Communicator or Microsoft Internet Explorer, from anywhere in your intranet

Switch clustering technology used with CMS, for

Unified configuration, monitoring, authentication, and software upgrade of multiple switches (refer to the release notes for a list of eligible cluster members).

Automatic discovery of candidate switches and creation of clusters of up to 16 switches that can be managed through a single IP address.

Extended discovery of cluster candidates that are not directly connected to the command switch.

Hot Standby Router Protocol (HSRP) for command-switch redundancy (requires the enhanced multilayer software image).

Note See the "Advantages of Using CMS and Clustering Switches" section. Refer to the release notes for the CMS, cluster hardware, software, and browser requirements.


Autosensing of port speed and autonegotiation of duplex mode on all switch ports for optimizing bandwidth

IEEE 802.3X flow control on all Ethernet ports

EtherChannel for enhanced fault tolerance and for providing up to 8 Gbps (Gigabit EtherChannel) or 800 Mbps (Fast EtherChannel) full duplex of bandwidth between switches, routers, and servers

Port Aggregation Protocol (PAgP) for automatic creation of EtherChannel links

Per-port storm control for preventing broadcast, multicast, and unicast storms

Port blocking on forwarding unknown unicast and multicast traffic

Cisco Group Management Protocol (CGMP) server support and Internet Group Management Protocol (IGMP) snooping for IGMP versions 1 and 2:

(For CGMP devices) CGMP for limiting multicast traffic to specified end stations and reducing overall network traffic

(For IGMP devices) IGMP snooping for limiting flooding of multicast traffic

Multicast VLAN registration (MVR) to continuously send multicast streams in a multicast VLAN while isolating the streams from subscriber VLANs for bandwidth and security reasons

IGMP filtering for controlling the set of multicast groups to which hosts on a switch port can belong

System Database Management (SDM) templates for allocating system resources to maximize support for user-selected features


Dynamic Host Configuration Protocol (DHCP) for automating configuration of switch information (such as IP address, default gateway, host name, and Domain Name System [DNS] and Trivial File Transfer Protocol [TFTP] server names)

Directed unicast requests to a DNS server for identifying a switch through its IP address and its corresponding host name and to a TFTP server for administering software upgrades from a TFTP server

Address Resolution Protocol (ARP) for identifying a switch through its IP address and its corresponding Media Access Control (MAC) address

Cisco Discovery Protocol (CDP) versions 1 and 2 for network topology discovery and mapping between the switch and other Cisco devices on the network

Network Time Protocol (NTP) for providing a consistent timestamp to all switches from an external source

Cisco IOS File System (IFS) for providing a single interface to all file systems that the switch uses

In-band management access through CMS

In-band management access through up to 16 simultaneous Telnet connections for multiple command-line interface (CLI)-based sessions over the network

In-band management access through Simple Network Management Protocol (SNMP) versions 1 and 2c get and set requests

Out-of-band management access through the switch console port to a directly attached terminal or to a remote terminal through a serial connection or a modem

Note For additional descriptions of the management interfaces, see the "Management Options" section.


Hot Standby Router Protocol (HSRP) for command switch and Layer 3 router redundancy

UniDirectional Link Detection (UDLD) on all Ethernet ports for detecting and disabling unidirectional links on fiber-optic interfaces caused by incorrect fiber-optic wiring or port faults

IEEE 802.1D Spanning Tree Protocol (STP) for redundant backbone connections and loop-free networks. STP has these features:

Per-VLAN Spanning Tree (PVST) for balancing load across VLANs

Port Fast mode for eliminating forward delay by enabling a port to immediately change from a blocking state to a forwarding state

UplinkFast, cross-stack UplinkFast, and BackboneFast for fast convergence after a spanning-tree topology change and for achieving load balancing between redundant uplinks, including Gigabit uplinks and cross-stack Gigabit uplinks

STP root guard for preventing switches outside the network core from becoming the STP root

Note The switch supports up to 128 spanning-tree instances.

VLAN Support

Support for up to 1005 VLANs for assigning users to VLANs associated with appropriate network resources, traffic patterns, and bandwidth

VLAN Query Protocol (VQP) for dynamic VLAN membership

Inter-Switch Link (ISL) and IEEE 802.1Q trunking encapsulation on all ports for network moves, adds, and changes; management and control of broadcast and multicast traffic; and network security by establishing VLAN groups for high-security users and network resources

Dynamic Trunking Protocol (DTP) for negotiating trunking on a link between two devices and for negotiating the type of trunking encapsulation (802.1Q or ISL) to be used

VLAN Trunking Protocol (VTP) and VTP pruning for reducing network traffic by restricting flooded traffic to links destined for stations receiving the traffic


Password-protected access (read-only and read-write access) to management interfaces (CMS and CLI) for protection against unauthorized configuration changes

Multilevel security for a choice of security level, notification, and resulting actions

Static MAC addressing for ensuring security

Protected port option for restricting the forwarding of traffic to designated ports on the same switch

Port security option for limiting and identifying MAC addresses of the stations allowed to access the port

Bridge Protocol Data Unit (BPDU) Guard for shutting down a Port Fast-configured port when an invalid configuration occurs

Standard and extended IP access control lists (ACLs) for defining security policies on routed interfaces

VLAN ACLs (VLAN maps) for providing intra-VLAN security by filtering traffic based on information in the MAC, IP, and TCP/User Datagram Protocol (UDP) headers

Source and destination MAC-based ACLs for filtering non-IP traffic

IEEE 802.1X port-based authentication to prevent unauthorized devices (clients) from gaining access to the network

Terminal Access Controller Access Control System Plus (TACACS+), a proprietary feature for managing network security through a TACACS server

Remote Authentication Dial-In User Service (RADIUS), which provides detailed accounting information and flexible administrative control over authentication and authorization processes

Quality of Service and Class of Service

IP type-of-service/Differentiated Services Code Point (IP TOS/DSCP) and 802.1P class of service (CoS) marking priorities on a per-port basis for protecting the performance of mission-critical applications

IP TOS/DSCP and 802.1P CoS marking based on flow-based packet classification (classification based on information in the MAC, IP, and TCP/UDP headers) for high-performance quality of service at the network edge, allowing for differentiated service levels for different types of network traffic and for prioritizing mission-critical traffic in the network


Traffic-policing policies on the switch port for managing how much of the port bandwidth should be allocated to a specific traffic flow

Aggregate policing for policing traffic flows in aggregate to restrict specific applications or traffic flows to metered, predefined rates

Up to 128 policers on ingress Gigabit-capable Ethernet ports
Up to eight policers on ingress 10/100 ports
Up to eight policers per egress port (aggregate policers only)


Out-of-profile markdown for packets that exceed bandwidth utilization limits

Egress Policing and Scheduling of Egress Queues

Four egress queues on all switch ports. These queues can either be configured with the Weighted Round Robin (WRR) scheduling algorithm or configured with one queue as a strict priority queue and the other three queues for WRR. The strict priority queue must be empty before the other three queues are serviced. You can use the strict priority queue for mission-critical and time-sensitive traffic.

Tail drop and Weight Random Early Detection (WRED) techniques for avoiding congestion on Gigabit Ethernet ports; tail drop for congestion avoidance on Fast Ethernet ports

Layer 3 Support

Note These features requires the enhanced multilayer software image.

Hot Standby Router Protocol (HSRP) for Layer 3 router redundancy

IP routing protocols for load balancing and for constructing scalable, routed backbones:

Routing Information Protocol (RIP) versions 1 and 2

Open Shortest Path First (OSPF)

Interior Gateway Routing Protocol (IGRP) and Enhanced IGRP (EIGRP)

IP routing between VLANs (inter-VLAN routing) for full Layer 3 routing between two or more VLANs, allowing each VLAN to maintain its own autonomous data-link domain

Fallback bridging for forwarding non-IP traffic between two or more VLANs

Static IP routing for manually building a routing table of network path information

Equal-cost routing for load balancing and redundancy

Internet Control Message Protocol (ICMP) and ICMP Router Discovery Protocol (IRDP) for using router advertisement and router solicitation messages to discover the addresses of routers on directly attached subnets

Protocol-Independent Multicast (PIM) for multicast routing within the network, allowing for devices in the network to receive the multicast feed requested and for switches not participating in the multicast to be pruned. Includes support for PIM sparse mode (PIM-SM), PIM dense mode (PIM-DM), and PIM sparse-dense mode.

Distance Vector Multicast Routing Protocol (DVMRP) tunnelling for interconnecting two multicast-enabled networks across non-multicast networks

DHCP relay for forwarding UDP broadcasts, including IP address requests, from DHCP clients


Switch LEDs that provide port- and switch-level status

Switch Port Analyzer (SPAN) for traffic monitoring on any port or VLAN

Four groups (history, statistics, alarms, and events) of embedded remote monitoring (RMON) agents for network monitoring and traffic analysis

Syslog facility for logging system messages about authentication or authorization errors, resource issues, and time-out events

MAC address notification for tracking users on a network by storing the MAC addresses that the switch has learned or removed

Management Options

The Catalyst 3550 switch is designed for plug-and-play operation: you need to configure only basic IP information for the switch and connect it to the other devices in your network. If you have specific network needs, you can configure and monitor the switch—on an individual basis or as part of a switch cluster—through its various management interfaces.

Management Interface Options

You can configure and monitor individual switches and switch clusters by using these interfaces:

CMS—CMS is a graphical user interface that can be launched from anywhere in your network through a web browser such as Netscape Communicator or Microsoft Internet Explorer. CMS is already installed on the switch. Using CMS, you can configure and monitor a standalone switch, a specific cluster member, or an entire switch cluster. You can also display network topologies to gather link information and to display switch images to modify switch and port settings.

For more information about CMS, see "Getting Started with CMS."

CLI—The switch IOS CLI software is enhanced to support desktop- and multilayer-switching features. You can configure and monitor the switch and switch cluster members from the CLI. You can access the CLI either by connecting your management station directly to the switch console port or by using Telnet from a remote management station.

For more information about the CLI, see "Using the Command-Line Interface."

SNMP—SNMP provides a means to monitor and control the switch and switch cluster members. You can manage switch configuration settings, performance, security, and collect statistics by using SNMP management applications such as CiscoWorks2000 LAN Management Suite (LMS) and HP OpenView.

You can manage the switch from an SNMP-compatible management station that is running platforms such as HP OpenView or SunNet Manager. The switch supports a comprehensive set of MIB extensions and four RMON groups.

For more information about using SNMP, see "Configuring SNMP."

Advantages of Using CMS and Clustering Switches

Using CMS and switch clusters can simplify and minimize your configuration and monitoring tasks. You can use Cisco switch clustering technology to manage up to 16 interconnected, supported Catalyst switches through one IP address. This can conserve IP addresses if you have a limited number of them. CMS is the easiest interface to use and makes switch and switch cluster management accessible to authorized users from any PC on your network.

By using switch clusters and CMS, you can

Manage and monitor interconnected Catalyst switches (refer to the release notes for a list of supported switches), regardless of their geographic proximity and interconnection media, including Ethernet, Fast Ethernet, Fast EtherChannel, Cisco GigaStack Gigabit Interface Converter (GBIC), Gigabit Ethernet, and Gigabit EtherChannel connections.

Accomplish multiple configuration tasks from a single CMS window without needing to remember CLI commands to accomplish specific tasks.

Apply actions from CMS to multiple ports and multiple switches at the same time. Here are some examples of configuring and managing multiple ports and switches:

Port configuration such as speed and duplex settings

Port and console port security settings

NTP, STP, VLAN, and QoS configurations

Inventory and statistic reporting and link- and switch-level monitoring and troubleshooting

Group software upgrades

View a topology of interconnected devices to identify existing switch clusters and eligible switches that can join a cluster. You can also use the topology to quickly identify link information between switches.

Monitor real-time status of a switch or multiple switches from the LEDs on the front-panel images. The system, redundant power system (RPS), and port LED colors on the images are similar to those used on the physical LEDs.

Use an interactive mode that takes you step-by-step through configuring complex features such as VLANs, ACLs, and QoS.

Use a wizard that prompts you to provide only minimal required information to configure complex features such as QoS priorities for video traffic, priority levels for data applications, and security.

For more information about CMS, see "Getting Started with CMS." For more information about switch clusters, see "Clustering Switches."

Network Configuration Examples

This section provides network configuration concepts and includes examples of using the switch in different network topologies.

Design Concepts

As your network users compete for network bandwidth, it takes longer to send and receive data. When you configure your network, consider the bandwidth required by your network users and the relative priority of the network applications they use.

Table 1-2 describes what can cause network performance to degrade and how you can configure your network to increase the bandwidth available to your network users.

Table 1-2 Increasing Network Performance

Network Demands
Suggested Design Methods

Too many users on a single network segment and a growing number of users accessing the Internet

Create smaller network segments so that fewer users share the bandwidth, and use VLANs and IP subnets to place the network resources in the same logical network as the users who access those resources most.

Use full-duplex operation between the switch and its connected workstations.

Increased power of new PCs, workstations, and servers

High bandwidth demand from networked applications (such as e-mail with large attached files) and from bandwidth-intensive applications (such as multimedia)

Connect global resources—such as servers and routers to which the network users require equal access—directly to the high-speed switch ports so that they have their own high-speed segment.

Use the EtherChannel feature between the switch and its connected servers and routers.

Bandwidth alone is not the only consideration when designing your network. As your network traffic profiles evolve, consider providing network services that can support applications for voice and data integration, multimedia integration, application prioritization, and security. Table 1-3 describes some network demands and how you can meet those demands.

Table 1-3 Providing Network Services

Network Demands
Suggested Design Methods

Efficient bandwidth usage for multimedia applications and guaranteed bandwidth for critical applications

Use IGMP snooping to efficiently forward multimedia and multicast traffic.

Use other QoS mechanisms such as packet classification, marking, scheduling, and congestion avoidance to classify traffic with the appropriate priority level, thereby providing maximum flexibility and support for mission-critical, unicast, and multicast and multimedia applications.

Use optional IP multicast routing to design networks better suited for multicast traffic.

Use MVR to continuously send multicast streams in a multicast VLAN, but to isolate the streams from subscriber VLANs for bandwidth and security reasons.

High demand on network redundancy to provide always on mission-critical applications

Use HSRP for router redundancy.

Use VLAN trunks, cross-stack UplinkFast, and BackboneFast for traffic-load balancing on the uplink ports so that the uplink port with a lower relative port cost is selected to carry the VLAN traffic.

An evolving demand for IP telephony

Use QoS to prioritize applications such as IP telephony during congestion and to help control both delay and jitter within the network.

Use switches that support at least two queues per port to prioritize voice and data traffic as either high- or low-priority, based on 802.1P/Q.

Use voice VLAN IDs (VVIDs) on the Catalyst 2900 XL and 3500 XL switches to provide separate VLANs for voice traffic.

A growing demand for using existing infrastructure to transport data and voice from a home or office to the Internet or an intranet at higher speeds

Use the Catalyst 2900 LRE XL switches to provide up to 15 Mb of IP connectivity over existing infrastructure, such as existing telephone lines.

Note Long-Reach Ethernet (LRE) is the technology used in the Catalyst 2900 LRE XL switches. Refer to the Catalyst 2900 XL and 3500 XL documentation set about these switches and the LRE technology.

Figure 1-1 shows three configuration examples of using Catalyst switches to create the following:

Cost-effective wiring closet—A cost-effective way to connect many users to the wiring closet is to connect a Catalyst switch cluster of up to nine Catalyst 3550 XL switches (or with a mix of Catalyst 3550, Catalyst 2950, Catalyst 3500 XL, and Catalyst 2900 XL switches) through GigaStack GBIC connections. To preserve switch connectivity if one switch in the stack fails, connect the bottom switch to the top switch to create a GigaStack loopback, and enable cross-stack UplinkFast on the cross-stack Gigabit uplinks.

You can have redundant uplink connections, using Gigabit GBIC modules, from the GigaStack cluster to a Gigabit backbone switch such as the Catalyst 3550-12T or Catalyst 3550-12G switch. You can also create backup paths by using Fast Ethernet, Gigabit, or EtherChannel links. If one of the redundant connections fails, the other can serve as a backup path. You can configure the Catalyst 3550-12T or Catalyst 3550-12G switch as a switch cluster manager to manage stack members through a single IP address. The Catalyst 3550-12T or Catalyst 3550-12G switch can be connected to a Gigabit server through a 1000BASE-T connection.

High-performance workgroup—For high-speed access to network resources, you can use Catalyst 3550 switches in the access layer to provide Gigabit Ethernet to the desktop. To prevent congestion, use QoS DSCP marking priorities on these switches. For high-speed IP forwarding at the distribution layer, connect the Catalyst 3550 switches in the access layer to a Gigabit multilayer switch (such as the Catalyst 3550 multilayer switch) in the backbone.

Each switch in this configuration provides users with a dedicated 1-Gbps connection to network resources in the backbone. Compare this with the switches in a GigaStack configuration, where the 1-Gbps connection is shared among the switches in the stack. Using these Gigabit GBIC modules also provides flexibility in media and distance options:

1000BASE-SX GBIC: fiber-optic connections of up to 1804 ft (550 m)

1000BASE-LX/LH GBIC: fiber-optic connections of up to 32,808 ft (10 km)

1000BASE-ZX GBIC: fiber-optic connections of up to 328,084 ft (100 km)

1000BASE-T GBIC: copper connections of up to 328 ft (100 m)

Redundant Gigabit backbone—Using HSRP, you can create backup paths between two Catalyst 3550 multilayer switches to enhance network reliability and load balancing for different VLANs and subnets. Using HSRP also provides faster network convergence if any network failure occurs. You can connect the Catalyst switches, again in a star configuration, to two Catalyst 3550 multilayer backbone switches. If one of the backbone switches fails, the second backbone switch preserves connectivity between the switches and network resources.

Figure 1-1 Example Configurations

Small to Medium-Sized Network Using Mixed Switches

Figure 1-2 shows a configuration for a network of up to 500 employees. This network uses Catalyst 3550 multilayer switches to aggregate up to ten wiring closets through high-speed uplinks. For network reliability and load balancing, this network includes two routers and two Catalyst 3550 multilayer switches, all with HSRP enabled. This ensures connectivity to the Internet, WAN, and mission-critical network resources if one of the routers or Catalyst 3550 multilayer switches fails.

The wiring closets have a mix of switches such as the Catalyst 3550, Catalyst 3500 XL, Catalyst 2950, Catalyst 2900 XL, Catalyst 2820, and Catalyst 1900 switches. These switches are connected to workstations, Cisco IP Phones, and local servers. You can cluster these switches into multiple clusters, as shown, or into a single cluster. You can manage a cluster through the IP address of its primary and secondary command switches, regardless of the geographic location of the cluster members.

This network uses VLANs to segment the network logically into well-defined broadcast groups and for security management. Data and multimedia traffic are configured on the same VLAN.

When an end station in one VLAN needs to communicate with an end station in another VLAN, a router or multilayer switch routes the traffic to the appropriate destination VLAN. In this network, the Catalyst 3550 multilayer switches provide inter-VLAN routing. VLAN access control lists (VLAN maps) on the Catalyst 3550 switches provide intra-VLAN security and prevent unauthorized users from accessing critical pieces of the network.

In addition to inter-VLAN routing, the Catalyst 3550 multilayer switches provide QoS mechanisms such as DSCP priorities to prioritize the different types of network traffic and to deliver high-priority traffic in a predictable manner. If congestion occurs, QoS drops low-priority traffic to allow delivery of high-priority traffic.

With the Catalyst 3550 multilayer switches providing inter-VLAN routing and other network services, the routers focus on firewall services, Network Address Translation (NAT) services, voice-over-IP (VoIP) gateway services, and WAN and Internet access.

Figure 1-2 Catalyst 3550 Switches in a Collapsed Backbone Configuration

Large Network Using Only Catalyst 3550 Switches

Switches in the wiring closet have traditionally been Layer 2-only devices, but as network traffic profiles evolve, switches in the wiring closet are increasingly employing multilayer services such as multicast management and traffic classification. Figure 1-3 shows a configuration for a network exclusively using Catalyst 3550 multilayer switches in the wiring closets and a Catalyst 6000 switch in the backbone to aggregate up to ten wiring closets.

In the wiring closet, each Catalyst 3550 switch has IGMP snooping enabled to efficiently forward multimedia and multicast traffic. QoS ACLs that either drop or mark nonconforming traffic based on bandwidth limits are also configured on each switch. VLAN maps provide intra-VLAN security and prevent unauthorized users from accessing critical pieces of the network. QoS features can limit bandwidth on a per-port or per-user basis. The switch ports are configured as either trusted or untrusted. You can configure a trusted port to trust the CoS value, the DSCP value, or the IP precedence. If you configure the port as untrusted, you can use an ACL to mark the frame in accordance with the network policy.

Within each wiring closet is a Catalyst 3550 multilayer switch for inter-VLAN routing. These switches provide proxy ARP services to determine IP and MAC address mapping, thereby removing this task from the routers and lessening this type of traffic on the WAN links. These switches also have redundant uplink connections to the backbone switches, with each uplink port configured as a trusted routed uplink to provide faster convergence in case of an uplink failure.

The routers and Catalyst 6000 multilayer backbone switches have HSRP enabled for load balancing and redundant connectivity to guarantee mission-critical traffic.

Figure 1-3 Catalyst 3550 Switches in Wiring Closets in a Backbone Configuration

Multidwelling Network Using Catalyst 3550 Switches

A growing segment of residential and commercial customers are requiring high-speed access to Ethernet metropolitan-area networks (MANs). Figure 1-4 shows a configuration for a Gigabit Ethernet MAN ring using Catalyst 3550 multilayer switches as aggregation switches in the mini-point-of-presence (POP) location. These switches are connected through 1000BASE-X GBIC ports.

The resident switches can be Catalyst 3550 switches, providing customers with high-speed connections to the MAN. Catalyst 2912-LRE or 2924-LRE XL Layer 2-only switches also can be used as residential switches for customers requiring connectivity through existing phone lines. The Catalyst 2912-LRE or 2924-LRE XL switches can then connect to another residential switch or to an aggregation switch. For more information about the LRE switches, refer to the Catalyst 2900 Series XL Hardware Installation Guide.

All ports on the residential Catalyst 3550 switches (and Catalyst 2912-LRE XL or 2924-LRE XL switches if they are included) are configured as 802.1Q trunks with protected port and STP root guard features enabled. The protected port feature provides security and isolation between ports on the switch, ensuring that subscribers cannot view packets destined for other subscribers. STP root guard prevents unauthorized devices from becoming the STP root switch. All ports have IGMP snooping or CGMP enabled for multicast traffic management. ACLs on the uplink ports to the aggregating Catalyst 3550 multilayer switches provide security and bandwidth management.

The aggregating switches and routers provide services such as those described in the previous examples, "Small to Medium-Sized Network Using Mixed Switches" and "Large Network Using Only Catalyst 3550 Switches."

Figure 1-4 Catalyst 3550 Switches in a MAN Configuration