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Cisco ISP Software and Router Management

Chapter Description

This chapter covers general features that ISPs should consider for their routers and network implementations. Most are good design practices and don't leverage particular unique Cisco IOS Software features, but each demonstrates how IOS Software can aid the smooth operation of an ISP's business.

From the Book

Cisco ISP Essentials

Cisco ISP Essentials


Cisco Express Forwarding

CEF is now the recommended forwarding/switching path for Cisco routers in an ISP environment. CEF adds increased performance, scalability, and resilience, and enables new functionality over the older optimum switching. Details on the operation and functionality of CEF are now covered in detail by the IOS Software documentation and in several whitepapers describing CEF (see references in the "Technical Reference and Recommended Reading" section at the end of this book).

Implementation is simple with either of the following commands (depending on the platform):

ip cef

ip cef-distributed

The key issue for ISPs is ensuring that CEF is turned on. On most platforms, CEF is not turned on by default, so ISP engineers need to ensure that CEF is turned on. Table 2-1 provides a list of the default CEF configurations for various Cisco platforms. ISPs should check their configuration scripts to ensure that CEF/dCEF is turned on, especially for the 7200-based edge platforms such as uBR, 6400, and 5800 NAS.

Table 2-1 Default Configuration for CEF on Various Platforms

On This Platform. . .

The Default Is. . .


CEF is not enabled.


CEF is not enabled.

7000 series with RSP 7000

CEF is not enabled.


CEF is not enabled.


CEF is enabled, but distributed CEF is not.

7600 OSR

CEF is enabled.

12000 GSR

Distributed CEF is enabled.

CEF will be discussed in more depth in Chapter 4, "Security." One of the best security tools available for an ISP is Unicast Reverse Path Forwarding. This requires CEF to be activated on the router because the reverse path check is dependent on the FIB table, which is part of the CEF process.