Cisco Integrated Service Router QOS with Switch Module

The Cisco Integrated Service Router (ISR) product line has the option to include switch ports using either a fixed-port configuration on low-end models, or switch modules on higher-end ISR routers. The newer second generation (G2) routers support gigabit switch ports.

To route packets between these switch ports and built-in or added “routed” ports, IOS supports a switch feature called “Switched Virtual Interface” (SVI). The SVI is entered into your configuration with an interface name corresponding to a switch vlan number. For the default VLAN (1) this would be: interface Vlan1. You can assign an IP to each SVI and then the router will forward packets to and from the associated VLAN in the same fashion as a standard routed interface.

In addition to routing, you can apply some very basic QOS to each SVI. The specific QOS features allowed for an ISR SVI are listed in the Cisco article: “Switch Virtual Interface for Cisco Integrated Services Routers.” NOTE that ONLY a basic policing rate for the SVI is supported, along with the ability to mark classes of traffic (DSCP or IP Precedence). Some advanced QOS features NOT permitted on an SVI include:

  • Class-Based and other forms of Traffic Shaping are NOT Supported on an SVI
  • Class-Based Weighted Fair Queueing (CBWFQ) is NOT Supported on an SVI
  • Low-Latency Queue (LLQ) / Strict Priority Queue is NOT Supported on an SVI

Sample commands for the basic rate-limit feature include the following. Please see the IOS Quality of Service … Command Reference: rate-limit and IOS Quality of Service … Configuration Guide: Policing for details on rate-limit burst calculation, etc.

  • interface Vlan1
    • rate-limit ...
  • show interfaces rate-limit

For these standard ISR routers, you can use more advanced QOS features ONLY on “routed” ports (non-switch ports). For your VLAN SVI “logical” ports, you will need to limit your QOS config to a single rate limit for all traffic forwarded to or from the VLAN. See the Committed Access Rate (CAR) example at the bottom of the SVI ISR article for hints on how to configure basic policing for your VLAN traffic.

In addition to my notes above, you may also find the following Cisco Support Community Discussion of interest: “Cisco QOS is not supporting […] SVI interface.” The question posted shows frustration over the lack of assumed functionality on a VLAN SVI – QOS features which are likely only permitted on true routed (non-switch) interfaces.

Another thing I would like to mention – these limitations are specific to the ISR product line. You may find more QOS functionality available for use with SVI logical ports on Catalyst Switches, Nexus Routers, and other related Cisco products. See the official product documentation or contact your Cisco support or sales representatives if you have questions about specific features being available for use on specific network devices.

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About notesbytom

Keeping technology notes on WordPress.com to free up my mind to solve new problems rather than figuring out the same ones repeatedly :-).
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