Networking

Configure IT Quick: Configure your Cisco Catalyst 5000 RSM for time-sensitive applications

Learn how to use the Cisco Catalyst for applications that need careful timing


To meet the needs of time-sensitive applications such as voice over IP (VoIP), many network administrators have replaced routers with Layer 2 switching technologies. Layer 2 switches—for example, the Catalyst 5000—can provide high-speed data transmission with low overhead.

However, what the network administrator soon realizes is that building a network using only switches creates a large flat network consisting of one broadcast domain. As a network grows larger, broadcast traffic increases.

Left unchecked, broadcast traffic can consume a large amount of bandwidth. As a rule of thumb, broadcasts should not exceed 20 percent of network traffic. When broadcast traffic reaches this threshold, the network should be segmented into multiple broadcasts domains known as VLANs (for more information on VLANs, check out “Microsegment your network with a virtual LAN”).

Separate VLANs, separate devices
Unfortunately, while VLANs solve broadcast problems, two devices on separate VLANS cannot communicate with each other without a router. Therefore, we have now gone full circle and must reintroduce a router into the network. There are two methods for performing inter-VLAN routing:
  1. Connect a router to a switch and establish trunking between the two devices. Trunking is a method used by a switch for sharing VLAN information with other routers and switches. This method is sometimes referred to as a “router-on-a-stick.”
  2. The Route Switch Module (RSM) provides the second method of inter-VLAN routing. Cisco created the RSM to overcome a problem created by the router-on-a-stick method. Because trunking uses an external routing device connected to a Fast Ethernet switch port, speed and integration become an issue.

Overview of the RSM
The Route Switch Module is a full-featured Layer 3 routing device based on the Cisco 7500 series router. Essentially, the RSM is a Cisco 7500 series router on a line card that is installed into an open slot of the Catalyst 5000. Although the Cisco IOS used by the RSM is configured the same way, do not use a 7500 series router IOS image on an RSM. The RSM uses its own IOS image sets. By using the high-speed backplane of the Catalyst switch, the RSM provides greater speeds and integration than the router-on-a-stick method of inter-VLAN routing.

Route once, switch many
The RSM uses the theory of “route once, switch many.” The theory involves routing the first packet destined for a Layer 3 address and switching subsequent packets destined for the same Layer 3 address. To perform “route once, switch many,” the route information and MAC address of the first Layer 3 packet is stored in cache. This cached information is used to switch subsequent packets to the destination address.

RSM configuration
The RSM module can be configured from the console port or by accessing the RSM from the switch. To access the RSM module from the switch, you must use the sessionmod-num command (where mod-num equals the slot number into which the RSM is installed). When the session command is issued from the Catalyst 5000 user prompt, a Telnet session is opened across the Catalyst’s backplane. For example, to access the RSM installed in slot three, use the following command:
Switch> session 3

When configuring the RSM, the native Cisco IOS commands are used. Additionally, like all Cisco routers, the RSM uses interfaces. However, instead of using interfaces such as Ethernet0, the RSM uses virtual interfaces that correspond to VLANs. For example, interface vlan3 could be used to represent VLAN 3.

Use the RSM to create high-speed Layer 3 switched networks
Combining the Catalyst 5000 with a Route Switch Module can create a high-speed Layer 3 switched network. And because the RSM runs the same IOS as the Cisco 7500 series routers, configuration is made easier and network administrators don’t have to learn new commands. The Catalyst 5000 switch with an RSM is a good solution for inter-VLAN routing and time-sensitive applications.

Warren Heaton Jr., MCSE+I, CCNP, CCDP, is the Cisco Program Manager for A Technological Advantage in Louisville, KY.


To meet the needs of time-sensitive applications such as voice over IP (VoIP), many network administrators have replaced routers with Layer 2 switching technologies. Layer 2 switches—for example, the Catalyst 5000—can provide high-speed data transmission with low overhead.

However, what the network administrator soon realizes is that building a network using only switches creates a large flat network consisting of one broadcast domain. As a network grows larger, broadcast traffic increases.

Left unchecked, broadcast traffic can consume a large amount of bandwidth. As a rule of thumb, broadcasts should not exceed 20 percent of network traffic. When broadcast traffic reaches this threshold, the network should be segmented into multiple broadcasts domains known as VLANs (for more information on VLANs, check out “Microsegment your network with a virtual LAN”).

Separate VLANs, separate devices
Unfortunately, while VLANs solve broadcast problems, two devices on separate VLANS cannot communicate with each other without a router. Therefore, we have now gone full circle and must reintroduce a router into the network. There are two methods for performing inter-VLAN routing:
  1. Connect a router to a switch and establish trunking between the two devices. Trunking is a method used by a switch for sharing VLAN information with other routers and switches. This method is sometimes referred to as a “router-on-a-stick.”
  2. The Route Switch Module (RSM) provides the second method of inter-VLAN routing. Cisco created the RSM to overcome a problem created by the router-on-a-stick method. Because trunking uses an external routing device connected to a Fast Ethernet switch port, speed and integration become an issue.

Overview of the RSM
The Route Switch Module is a full-featured Layer 3 routing device based on the Cisco 7500 series router. Essentially, the RSM is a Cisco 7500 series router on a line card that is installed into an open slot of the Catalyst 5000. Although the Cisco IOS used by the RSM is configured the same way, do not use a 7500 series router IOS image on an RSM. The RSM uses its own IOS image sets. By using the high-speed backplane of the Catalyst switch, the RSM provides greater speeds and integration than the router-on-a-stick method of inter-VLAN routing.

Route once, switch many
The RSM uses the theory of “route once, switch many.” The theory involves routing the first packet destined for a Layer 3 address and switching subsequent packets destined for the same Layer 3 address. To perform “route once, switch many,” the route information and MAC address of the first Layer 3 packet is stored in cache. This cached information is used to switch subsequent packets to the destination address.

RSM configuration
The RSM module can be configured from the console port or by accessing the RSM from the switch. To access the RSM module from the switch, you must use the sessionmod-num command (where mod-num equals the slot number into which the RSM is installed). When the session command is issued from the Catalyst 5000 user prompt, a Telnet session is opened across the Catalyst’s backplane. For example, to access the RSM installed in slot three, use the following command:
Switch> session 3

When configuring the RSM, the native Cisco IOS commands are used. Additionally, like all Cisco routers, the RSM uses interfaces. However, instead of using interfaces such as Ethernet0, the RSM uses virtual interfaces that correspond to VLANs. For example, interface vlan3 could be used to represent VLAN 3.

Use the RSM to create high-speed Layer 3 switched networks
Combining the Catalyst 5000 with a Route Switch Module can create a high-speed Layer 3 switched network. And because the RSM runs the same IOS as the Cisco 7500 series routers, configuration is made easier and network administrators don’t have to learn new commands. The Catalyst 5000 switch with an RSM is a good solution for inter-VLAN routing and time-sensitive applications.

Warren Heaton Jr., MCSE+I, CCNP, CCDP, is the Cisco Program Manager for A Technological Advantage in Louisville, KY.

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