Chapter 27 sections

CS 332Spring 2014

Link-state Routing

Q: Could you go over link-state routing (like OSPF)?A: Each router constructs a graph of the network in its memory. Each router broadcasts on its interfaces – “I’m on this network”. Routers then figure out they are directly connected. When they learn they are directly connected to other routers they share that information with neighbors. So, all routers learn the entire topology and can run Dijkstra’s algorithm to compute best paths.

OSPF vs. RIP

Q: How common is RIP vs. OSPF?

A: I don’t think most routers use RIP anymore. RIP wasn’t designed for CIDR addressing (although it was probably updated to handle it). RIP also has weird problems… “split horizon”, “count-to-infinity” and other stuff I’ve forgotten about. Most everyone uses OSPF, afaik.

Broken connections

Q: How does a routing protocol know that a connection is broken?

A: The application somehow gets info from the OS that a port/connection is down (register for a callback or polls the connection status often). Then, when it goes down it quickly recomputes routes and sends out a status update to other routers.

Routing Groups

Q: Are routing groups statically or dynamically configured?A: Statically – i.e., manually by a network manager.

Dijkstra’s Algorithm

Q: In OSPF, for Dijkstra’s algorithm, what does it use for link metrics? (i.e., length’s of edges between routers)

A: By default (afaik), it uses 1 for each connection. It can be configured by a network manager to have different metrics for each connection.

ASes and sub-ASes

Q: Can Autonomous Systems be sub-divided into sub-ASes?

A: Not that I know.

Routers not in an AS?

Q: Is there ever a situation where there is a functional group of internetworked routers that is not classified as an autonomous system?

A: Not every group of routers is an AS – or has an AS number assigned. As far as I can tell, Calvin does not have an AS assigned. I think you only need an AS number if you have multiple service providers connected via BGP.