Victor Norman
IS333
Spring 2015

• How are entries added to the forwarding table?
– via static configuration,
• configured by the administrator
• by adding an interface (using ifconfig , perhaps)
• by adding a route explicitly (using route add , perhaps)
• by DHCP
– via a dynamic routing protocol.

Q: Does the routing algorithm care how routes were added?
A: No.
Q: Do hosts usually have dynamic routes added?
A: No: or at least much less often than routers do.

Q: Does Calvin use static or dynamic routes?
A: Most everything at Calvin is switched, as you recall. So, most machines use DHCP to get their
IP addresses and default route. This would be considered a static route. Hosts, in general, do not run any routing protocol.
The (default) route to US Signal on Calvin’s main router is probably a static route.

Q: With static routing, can computers be added to the routing table?
A: We answered this when we did the dualhomed host lab: Yes. We can add host-specific routes to the routing table. (What is the mask for a host-specific route?)

Q: What is an example of an Interior Gateway
Protocol?
A: OSPF and RIP are two IGPs. (IGP is just a generic term for actual protocols run within an organization. Similar for EGP.)

• OSPF uses a link-state algorithm for computing best routes.
– Each router builds up a picture of the network in memory
– which routers connect to each other, and which routers connect to which networks.
– Each router then computes best ways for it to get to remote networks and adds routes to its routing table.
• RIP uses distance-vector.
– Each router tells its neighbors it is directly connected to
“its” networks. Then the neighbors tell their networks they are 1 hop from those networks, etc.
– Sounds good, but there are many subtle problems when links go up and down, get moved, etc…

Q: What does it mean that OSPF supports metrics? How does an administrator know anything about the cost of a route?
A: A hop across a network generally costs “1” – 1 hop. OSPF advertises connectivity by how many networks a packet has to traverse. But, an administrator can manually configure certain
“hops” to cost more than 1.

Q: Can you explain what an autonomous system is?
A: Yes! It is a group of networks/routers run by one organization. It is only really necessary if you connect to the Internet with multiple service providers, because BGP uses AS numbers to coalesce routes.

Q: What does the author mean when he states that routers “summarize” routing information before passing to another autonomous system?
A: Consider a router connected to 4 network:
153.106.0/24, 153.106.1/24, 153.106.2/24,
153.106.3/24. It advertises 4 networks to the world.
Or, it could advertise 1 network: 153.106.0/22.

Q: The book says that most hosts use static routes. When would be a case when a host would use dynamic routing?
A: I can’t really think of a case. Perhaps if a host is connected to two or more networks, it might run OSPF to figure how to most efficiently get out to the Internet…

Q: Other than simplicity, are there any advantages to static routing? Why would one explicitly choose it over dynamic routing?
A: There is no advantage in terms of forwarding traffic – a route is a route is a route. To have a host get dynamic routes, it would have to run OSPF or
RIP, which takes some CPU time, perhaps some configuration, etc. Much simpler to just get the route from DHCP and not mess with it.

Q: A routing table is made, right, when the system boots?
A: The routing table is there, but is empty, usually until the machine gets a DHCP response. Then, the interface is created and an entry goes in the routing table, and, the default route is added from the
DHCP response.
Q: Hosts use static and routers use dynamic?
A: Yes, for the most part, I’d say. Dynamic routes get updated dynamically, without human intervention. Static routes never change.

Q: When an administrator sets a route, is the route permanent or does it timeout?
A: The route is “static” – i.e., permanent. It will not be changed, updated, or replaced.

Q: For OSPF to work, do all routers have to run it?
A: Yes. If a router is running OSPF, it will communicate with other routers running OSPF and they will automatically figure how the network topology and how to set routes in the routing table so that there are no loops, etc.
Routers that don’t participate may screw that all up.

Q: What is link-state routing?
A: Routers figure out their neighbor routers. Then, they communicate with each other sharing who they are all connected to. Each router builds up a model of the network connectivity in memory, and then runs Dijkstra’s algorithm on it to figure out the best path to send packets to get them through the network. Whenever a change in the network happens, they all update their models and change their routes.

Q: How can OSPF import routes?
A: An edge router running BGP can learn the routes from the Internet, and then OSPF can
(somehow) import those routes and advertise them throughout the internal network. (Not usually necessary, though, I would think.)