In the case of the router-on-a-stick design, traffic flows to the router within the source VLAN where it is routed into the destination VLAN. This created an out and back flow to the router. Technically, the Catalyst 5000 RSM uses a very similar flow, but with one important difference: the stick becomes the Catalyst 5000 backplane (the high-speed switching path used inside the Catalyst chassis). This difference provides two key benefits:

  • Speed
  • Integration

Because the RSM directly connects to the Catalyst 5000 backplane, it allows the router to be much more tightly integrated into the Catalyst switching mechanics. Not only can this ease configuration tasks, it can provide intelligent communication between the Layer 2 and Layer 3 portions of the network (several examples are discussed later in the chapter). Also, because it provides a faster link than a single Fast Ethernet ISL interface, the performance can be greater. In general, the RSM provides 125,000–175,000 pps for IP and approximately 100,000 pps for other protocols.

  • Tip
    If necessary, more than one RSM can be used in a single Catalyst chassis for additional throughput.

RSM Configuration

One of the appealing benefits of the RSM is its familiarity. From a hardware perspective, it is almost identical to an RSP2 (the second version of the Route Switch Processor) from a Cisco 7500. It has the same CPU and contains the same console and auxiliary ports for out-of-band configuration. It has its own flash, dynamic random-access memory (DRAM), and nonvolatile random-access memory (NVRAM). And, because it runs the full IOS, RSM is configured almost exactly like any Cisco router.

  • Tip
    Although the IOS is identical from a configuration standpoint, do not try to use a 7500 router image on an RSM—the RSM uses its own image sets. Under Cisco’s current naming convention, RSM images begin with the characters c5rsm.

The most obvious modification is a set of dual direct memory access (DMA) connections to the Catalyst 5000 backplane. (The backplane connection remains 1.2 Gbps even in 3.6 Gbps devices such as the Catalyst 5500.) The status of these two connections is indicated by the Channel 0 and Channel 1 LEDs on the front panel. Each channel provides 200 Mbps of throughput for a total of 400 Mbps.

Because the RSM runs the full IOS and contains its own image and memory, it shares some of the same configuration aspects as the LANE module discussed in Chapter 9. To configure the RSM, you need to enter the session slot command. For example, to configure an RSM located in slot 3, you enter session 3. This instantly transports you from the Catalyst world of set and show commands to the router world of config t. The full range of IOS help and command-line editing features are available. The RSM also requires you to save your configuration changes to NVRAM using the copy run start command.

  • Tip
    Don’t forget to save your RSM configurations with copy run start or write mem. Unlike the Catalyst Supervisor, the RSM does not automatically save configuration changes.

Although the session command is the most common way to configure an RSM, the console and auxiliary ports can be useful in certain situations. Many organizations use the auxiliary port to connect a modem to the Catalyst. This is especially useful for Supervisors that do not contain an Aux port (or, in the case of the Catalyst 5000 Supervisor III, where the Aux port is not enabled).

  • Tip
    The session command opens a Telnet session across the Catalyst’s backplane. The destination address is 127.0.0.slot_number + 1. For example, slot 3 uses Some versions (but unfortunately not all) of the RSM code allow you to enter telnet to Telnet from the RSM to the Supervisor in slot 1 (or for slot 2). This can very useful when accessing the box from a modem connected to the RSM’s auxiliary port. If the code on your RSM does not permit the use of the 127.0.0.X addresses, use normal IP addresses assigned to both SC0 and an RSM interface. However, this obviously requires a valid configuration on the Supervisor before you remotely dial into the RSM.

Just as the auxiliary port is useful for connecting a modem to the Catalyst, the RSM’s console port is useful for password recovery operations.

  • Tip
    RSM password recovery is identical to normal Cisco router password recovery. See the IOS “System Management” documentation for more details.

RSM Interfaces

The RSM uses interfaces just as any Cisco router does. However, instead of using the usual Ethernet0 and Fast Ethernet1/0, the RSM uses virtual interfaces that correspond to VLANs. For example, interface vlan 1 and interface vlan 2 can be used to create interfaces for VLANs 1 and 2, respectively. These virtual interfaces are automatically linked to all ports configured in that VLAN on the Catalyst Supervisor. This creates a very flexible and intuitive routing platform. Simply use the set vlanvlan_number port_list command on the Supervisor to make VLAN assignments at will, and the RSM automatically reflects these changes.

  • Tip
    RSMs do not use subinterfaces for VLAN configuration. Instead, the RSM uses virtual VLAN interfaces (that function as major interfaces). In fact, these VLAN interfaces currently do not support subinterfaces.

Except for the earliest versions of RSM code, RSM virtual interfaces only become active if the Supervisor detects active ports that have been assigned to that VLAN. For example, if VLAN 3 has no ports currently active, a show interface vlan 3 command on the RSM shows in the interface in the down state. If a device in VLAN 3 boots, the RSM’s VLAN 3 interface enters the up state. This value-added feature further reflects the tight integration between the Supervisor and RSM and is useful for avoiding black hole routing situations.

  • Tip
    You cannot activate an RSM interface until the corresponding VLAN has one or more active ports.

This black hole prevention feature can be controlled through the use of the set rsmautostate [ enable| disable ] Supervisor command. In modern Catalyst images, this feature is enabled by default.

The RSM contains no VLANs by default. The VLAN virtual interfaces are created as interface vlan commands are first entered. Each VLAN interface can then be configured with the addressing and other parameters associated with that VLAN. For example, the code sample in Example 11-3 creates three interfaces that correspond to three VLANs.

Example 11-3 RSM Configuration

As with the earlier examples, Vlan1 is only used for IP traffic. Vlan2 adds support for IPX routing, and Vlan3 is running IP, IPX, and AppleTalk services.

  • Tip
    RSM interfaces are in a shutdown state when they are first created. Don’t forget to use the no shutdown command to enable them.

Troubleshooting with the RSM

The usefulness of an RSM can go way beyond being a tightly integrated router—it can be a very powerful troubleshooting tool. Because the RSM uses the full IOS, you have access to all of the debug and show commands normally present on any Cisco router. Extended ping and trace can be very useful when the more limited capabilities of the Supervisor’s tools have failed to reveal the problem. For example, Example 11-4 shows some of the extended ping options.

Example 11-4 Using the RSM for Extended pings

Example 11-4 illustrates the use of the repeat count, datagram size, and data pattern options. Respectively, these can be useful when trying to create a sustained stream of rapid-fire pings, to probe for maximum transmission unit (MTU) problems, and to detect ones-density problems on serial links.

However, the most powerful troubleshooting advantage to the RSM is debug. For example, debug ip icmp or debug ip packet[access-list-number] can be extremely useful when trying to track down the reason why some ping operation mysteriously fails.

The usual caveats about debug output volumes apply, though. Be very careful, especially when using commands such as debug ip packet in production networks. It is almost always advisable to use the access-list-number parameter to very specifically limit the amount of output. Also, because the RSM automatically sends debug output to a connection made via the session command, the terminal monitor command is not necessary.

  • Tip
    To make it easier to enter commands while the router is generating debug or informational output, use the logging synchronous line command. For the RSM, it is most useful to enter this under line vty 0 4. However, this command can be useful on all of Cisco’s router platforms running 10.2+ code (in which case it should also be applied to line con 0 and line aux 0).

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