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Understanding OSPF Routing (part 2)
Smart Tinkering and Unnecessary Tinkering
October 29, 2008
This article will cover LSA types, packet types, and area types. First, however, we’d like to dispel a common misunderstanding about dynamic routing:
People have a tendency to tinker with traffic, even when they aren’t suffering from under-provisioned bandwidth. In OSPF, you cannot really influence the way traffic is routed, aside from adjusting a path’s metric. Some routers support making changes to weights, but this isn’t usually necessary. OSPF generally takes care of assigning weights, based on the speed of the interfaces on a router. You can also use ECMP (equal cost multipath) with OSPF, if you have two links to the same place and wish to load balance in a round-robin fashion. Don’t try tinkering with OSPF parameters; more likely than not, if you think you have a problem it’s a network design issue, and fixing that will accomplish your goals.
The LSA and Packets
Finding the shortest path on a weighted, directed graph is computationally hard, and takes considerable time, even on today’s routers. Thankfully Edsger W. Dijkstra made this better with his SPF algorithm, but it’s still tough. This is the main reason OSPF can’t be used on the Internet, and you don’t want to squirt your full BGP Internet routing table into OSPF. Every time a network is deleted or added, an SPF recalculation happens.
Try not to be confused by another “type.” OSPF has many, so be sure to pay attention to the “type” you’re referring to. LSAs can be either an update packet, or a request packet. These are the different types of LSAs that can be sent, and these are either Type 3 or 4 OSPF packets:
Notice that we have both a router and a network LSA. The reason a router LSA exists is because in the absence of a DR, there is no network LSA sent. The router LSA would include a list of all links to the other routers on a network. So OSPF can work in the absence of a DR or BDR, albeit with increased complexity due to the fact that the DR is no longer providing nice summaries.