In 1998 Cisco entered the "hot" intrusion detection market buy buying WheelGroup and their NetSonar product. Seemingly unable or unwilling to understand/develop the product, it was finally killed in 2003, by which time Cisco had put some (not very good) IDS technology into their own core products.
Largely an irrelevance in the IDS world up until today.... they just decided to have another go at it..
"Cisco Systems Inc. said today it will acquire IDS, IPS and anti -malware specialist Sourcefire Inc.for $2.7 billion."
Cisco are NOT going to merge with Sourcefire, but this time around say they will leave it as a separate business unit, perhaps it will work better for them than NetSonar in 1998, perhaps it wont wither and die.
Incredible that Cisco could not successfully grow such capability in-house. Just goes to show that large companies cant achieve much!
It's pretty simple, Cisco is notorious for writing spectacularly shitty software, especially anything security related. They make decent enough hardware, but their software is attrocious.
So, they just periodically buy up vendors who have fairly good software in the market they want, and use it. Generally after a few revisions it turns into the same Cisco shitpile as their own home-grown stuff, but that's when they decide to go buy someone else.