Chapter 17. Bridging: Miscellaneous Topics

In the previous chapters, we saw how bridging and the STP are implemented, and how they fit into the network stack. In this chapter, we conclude the bridging part of the book with a description of how the subsystem interacts with the user-space commands that configure bridging. I will not describe the commands themselves, because administration is outside the scope of this book.

We will also look at the various files exported in the /sys directory that can be used to tune bridging. The chapter concludes with a detailed description of the data structures introduced in Chapter 16.

User-Space Configuration Tools

Bridging can be configured with brctl, a utility you can download at With brctl, you can create bridge devices, enslave NICs to bridge devices, and configure bridge parameters and bridge port parameters for the STP.

brctl uses the ioctl interface to talk to the kernel unless the libsysfs library is installed, in which case the sysfs interface becomes the preferred choice. The libsysfs library, which can be downloaded at, provides all the necessary primitives to access and modify the content of the variables exported in /sys. See the section "Tuning via /sys Filesystem.”

In the section "Data Frames Versus BPDUs" in Chapter 16, we introduced ebtables. The user-space configuration tool can be downloaded at We will not look at ...

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