Hazard's stuff

12 Jan, 2013

OSPF: a protocol from hell, or Type 5 vs Type 7 LSAs

— Posted by hazard @ 2013-01-12 02:13
I've always thought that OSPF was one of the rotten tomatoes in the generally nicely-looking suite of core IP protocols. Its architecture is simply not suited well for real-life carrier networks. OSPF tries to impose its own rules on how the network should be built, instead of providing flexibility to adjust the protocol to the network. However, instead of throwing OSPF out and concentrating on a better IGP (e.g. something EIGRP-like), Internet/IETF community kept adding more and more band-aids to it. As a result, we've got a number of monstrous and complex specifications, which even vendors don't fully understand, resulting in bugs and incompatibilities between implementations, as well as lots of confusion to engineers.

A few days ago I got hit by another case which re-enforced my beliefs. I had to implement an OSPF network which had both Type 5 LSA E2 and Type 7 N2 routes for the same prefix. Moreover, there was a mix of IOS and JUNOS speakers in the same network. So, what is the route selection algorithm in that scenario? Google and you'll find at least three different answers. An incorrect answer from one of very reputable sources will say that E2 route will win over N2 no matter what the cost is. Another answer is that E2 route will be preferred if it has the same or lower metric than N2 route. And the third answer is the opposite: N2 route will win over E2 if it has the same or lower metric.

Well, to ease the pain of future generations, I'll say that the correct answer is that N2 route will win unless E2 has lower cost. That is, unless that future generation lives in a world where they have released an RFC to supersede RFC 3101, since things might change the same way as they did since RFC 1587, which apparently specified opposite behavior. If you're using Cisco IOS, it depends on which IOS you use, many IOSes will prefer E2, as they follow the behavior from RFC 1587, even though they were released much later than RFC 3101. Other IOSes will prefer N2.

I rest my case.

Long live OSPF, the reason for late-night maintenance headaches! As well as an additional source of revenue for network engineer certifications. :)