Wednesday, July 21, 2010

My top 2 articles on the internals of broadcom-based routers

Embedded devices are energy efficient, and their limited memory and storage present some satisfying challenges. How have I missed this for all these years, I don't know, but it is time to play catch up. For a few weeks, I've been spending some of my free time reading on Openwrt and the WRT54GL and other devices of similar design.

Of course, flashing a custom firmware can be daunting enough for an average user, but an average geek like myself will want to know how exactly these devices work. There is a published book on the subject but upon reading the table of contents, I decided against buying it as I deemed it not technical enough.

So, I assumed there was information somewhere for people like me, info that didn't require me to read source code. And there is, but you have to search for it.

And what are the top two articles I found on the subject? Here they are.

1. First of all, there is a three year-old post entitled "Everything you need to know about Broadcom hardware" here in the Openwrt forums:

It is extremely informative on the boot process of these devices and it finally clarified for me how the CFE bootloader works. I didn't understand the SquashFS + JFFS2 combination details, thanks to this post now I do. You also should read about union mounts if you're not familiar with the topic (I wasn't, HP-UX does not support this!).

2. Then you should read about how VLANs and network interfaces work on the Broadcom platform -- this article is from the old OpenWRT wiki but well done, too. It is specifically for the Asus WL-500g but being a Broadcom design I can only suspect other routers are very similar, if not identical:

This article explains why you have a bridge interface on your router. It especially shines in explaining how these routers isolate the interfaces cheaply using VLANs. In fact, there is only one twisted pair interface (eth0), the other one being the wireless antenna (eth2). I was under the impression that firewalls needed different and isolated interfaces, but the VLAN trick lets you do something similar on cheaper designs. And I guess it is good enough!

I might replace my m0n0wall PC soon in order to reclaim real estate in my utility room and save a few bucks on electricity. I just need a router, and disable the wireless radio. But I will NOT be purchasing a WRT54GL. Why? I'll tell you in a future post!


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