2 posts tagged with "Routing Protocols"

More than just an interview question​

Over my years in networking I've sat on both sides of countless technical interviews. There's a familiar dance that occurs when discussing OSPF: the candidate confidently states "OSPF uses Dijkstra's algorithm for route calculation," and the interviewer will nod approvingly. Yet recently, I had a moment of clarity: in hundreds of these exchanges, I've never once asked a candidate to explain what that actually means, nor have I been asked to explain it myself. This perfunctory mention of Dijkstra has become almost ceremonial in our industry, a shibboleth that we repeat without truly engaging with its significance. Yet understanding this algorithm isn't just academic—it fundamentally shapes how OSPF operates, influences our network designs, and explains why certain design patterns have become best practices. When a link fails in your network and OSPF begins recalculating routes, there's significant computational overhead that many engineers never consider. This processing cost isn't just theoretical—it's the hidden force behind many of our design decisions, from area sizing to adjacency limits. Today, we'll bridge the gap between theory and practice, exploring how this fundamental algorithm shapes the way we deploy and scale OSPF networks, and why it matters for your day-to-day operations.

There used to be a great little website for route summarisation and it did it far more intelligently than Cisco kit does it. It looks like the site has dropped off the internet which is a shame but there is a handy python library called netaddr with has the same capabilities.

I have written a little wrapper for it which will regex the prefixes out of a ‘show ip bgp’ and then list the summary routes. You pass the output of ‘show ip bgp’ as a text file, it’s the only argument the script expects.