338 Chapter 9: QoS Support on the Catalyst 6500 MSFC and FlexWAN
• Congestion Avoidance
• Summary
This chapter focuses on these QoS features available to software-switched traffic on the
Catalyst 6500 and FlexWAN traffic flows. It also discusses the mechanisms used to ensure
reliable delivery of traffic throughout the entire network via the use of the FlexWAN and
MSFC. Prior to reading Chapter 9, it is strongly recommended to review Chapter 2 and
Chapter 5, “Introduction to the Modular QoS Command-Line Interface,” to obtain the
appropriate background.
MSFC and FlexWAN Architectural Overview
This section expands on some of the concepts presented in Chapter 8 within the section
titled “Catalyst 6500 Architectural Overview.” As discussed in Chapter 8, the MSFC, in
conjunction with the PFC, is responsible for Layer 3 forwarding within the Catalyst 6500.
With a Supervisor I Engine, the first packet in a flow is software switched by the MSFC.
When the first packet is forwarded, the forwarding decision made by the MSFC is also
programmed into hardware ASICs on the supervisor engine. This process is referred to as
hardware-based multilayer switching (MLS). When the initial packet is forwarded in
software by the MSFC, and the MLS flow is completed in hardware, all subsequent packets
are switched by the PFC. With a Supervisor II Engine, the MSFC is not primarily respon-
sible for forwarding packets. The MSFC builds the Layer 3 forwarding information, which
is passed via an out-of-band channel to the PFC on the supervisor engine. The MSFC builds
a Cisco Express Forwarding (CEF) table, which is copied directly into hardware on the
PFC. Copying the CEF table to the PFC permits all forwarding decisions to be made in
hardware. In the event an entry does not exist in the PFC for an arriving packet, it is then
software switched by the MSFC. The switching process on the Supervisor II Engine is
referred to as CEF-based Layer 3 switching.
NOTE For additional information on MLS-based switching on the Supervisor I Engine, refer to
the following technical document at Cisco.com:
“Configuring IP Unicast Layer 3 Switching on Supervisor Engine I”
For additional information on CEF-based switching on the Supervisor II Engine, refer to
the following technical document at Cisco.com:
“Configuring CEF for PFC2”
In addition to building the Layer 3 forwarding information, the MSFC is also responsible
for applying configurations to the FlexWAN module. In addition, the MSFC switches and
encapsulates non-IP traffic for both ingress and egress packets traversing the FlexWAN.
MSFC and FlexWAN Architectural Overview 339
The FlexWAN module binds to the active or designated MSFC within the 6500. As stated
in the introduction, the FlexWAN extends the Catalyst 6500’s reachability to the MAN and
WAN. The FlexWAN is a single-slot module that can be integrated into the Catalyst 6500.
The physical appearance of the FlexWAN module can be compared to a Versatile Interface
Processor (VIP) for a 7500. Similar to the VIP, the FlexWAN has two bays, which accom-
modate two modular WAN port adapters. From the MSFC, the FlexWAN port adapters are
configured identically to the way Cisco 7200 or 7500 port adapters are configured. The
FlexWAN contains two VIPs capable of supporting one port adapter for each VIP. The
FlexWAN port adapters are configurable strictly from the MSFC and not recognized by the
switch in Hybrid mode. If running the Cisco Native IOS, the FlexWAN module interfaces
are configured from the command line similar to any other available interface. Table 9-3 depicts
the 7200/7500 WAN port adapters supported by the FlexWAN.
Unlike the VIP, which only incorporates a single processor and memory to control both
bays, the FlexWAN module is comprised of two VIPs. Therefore, the FlexWAN module
services the two installed port adapters with a dedicated processor and memory for each
bay. Each processor performs the encapsulation and QoS functions independently for its
assigned slot. Also the MSFC communicates with each processor on the FlexWAN
independently through the Ethernet out-of-band channel (EOBC). However, only system
control information is exchanged via the EOBC. Routing and forwarding decisions are still
maintained by the MSFC and central PFC. As a result, the FlexWAN uses the central data
bus (D-bus) and results bus (R-bus) for forwarding and receiving data packets. This even
applies for packets being forwarded out a different subinterface on the same port adapter.
The D-bus, R-bus, and EOBC are introduced in Chapter 8.
For more detailed information regarding the architecture of the Catalyst 6500, consult the
following technical document at Cisco.com:
“Catalyst 6000 and 6500 Series Architecture”
Hardware and Software Requirements
As described in Chapter 8, the administrator has two software options available to support
operations on the Catalyst 6500. The first option requires the administrator to load two
separate software versions. One version supports the supervisor module, and the other
version supports the MSFC. This configuration is referred to as Hybrid mode. The alter-
native arrangement requires only one version of software to be positioned on the platform.
This one version sustains both the supervisor and the MSFC. This version of software is
referred to as Cisco Native IOS.
For additional information on naming conventions and differentiating the various software
versions for the Catalyst 6500, refer to the section titled “Identifying the Catalyst Software”
in Chapter 8.
340 Chapter 9: QoS Support on the Catalyst 6500 MSFC and FlexWAN
The MSFC is supported in both Native IOS and Hybrid. The only hardware requirement for
supporting an MSFC is that the supervisor engine must incorporate a PFC. Table 9-1
depicts the minimum versions of software required for both Native IOS and Hybrid to
support an MSFC with the respective supervisor engine.
The FlexWAN requires both a PFC and an MSFC to be installed within the 6500. As stated
previously, the FlexWAN is associated with the designated MSFC. In the presence of a
redundant MSFC configuration, the FlexWAN interfaces appear only on the designated
MSFC. As a result, a failover to the backup MSFC must occur for the FlexWAN interfaces
to appear on the redundant MSFC. When the interfaces are available on the alternate
MSFC, they can be configured and saved to memory. If a failover occurs in the future, the
saved configuration on the redundant MSFC is used. Similar to the MSFC, the FlexWAN
module is supported in either Hybrid or Native IOS. Table 9-2 provides the minimum
software versions for both Native IOS and Hybrid to support the FlexWAN module.
When selecting software to support the FlexWAN module, it is necessary to choose a
version with a “v” listed in the feature set field. This is identical to the requirement for
supporting a VIP on a 7500. If the intent is to load the Enterprise feature set with support
for Secure Shell (SSH), Triple Data Encryption Standard (3DES), and the FlexWAN
module, the feature set field would appear as jk2sv. This requirement applies to IOS
versions released prior to 12.1(5a)E. For 12.1(5a)E and all subsequent releases, FlexWAN
support is incorporated into each software version. The exception to this rule begins with
IOS Release 12.1(13)E. Starting with this software release, the “LAN-only” feature set is
introduced. This specific feature set does not incorporate support for the FlexWAN module.
However, this exception only applies to Native IOS; the Hybrid Software version maintains
support for the FlexWAN module.
Table 9-1 Minimum Software Versions for MSFC Support
Sup I
MSFC I
Sup I
MSFC II
Sup II
MSFC II
Native IOS 12.0(7)XE 12.1(2)E 12.1(8a)E
Hybrid
CatOS: 5.3(1)CSX
Cisco IOS: 12.0(3)XE1
CatOS: 5.4(3)
Cisco IOS: 12.1(2)E
CatOS: 6.1(1)
Cisco IOS: 12.1(3a)E1
Table 9-2 Minimum Software Versions for FlexWAN Support
Sup IA
MSFC I with
FlexWAN
Sup IA
MSFC II with
FlexWAN
Sup II
MSFC II with
FlexWAN
Native IOS 12.1(5a)E1 12.1(5a)E1 12.1(8a)E
Hybrid CatOS: 5.4(2)
Cisco IOS:
12.1(1)EX1 and 12.1(1)E
CatOS: 5.4(2)
Cisco IOS:
12.1(2)E
CatOS: 6.1(1)
Cisco IOS:
12.1(3a)E1
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