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Chapter 27 Q and A
Victor Norman
IS333
Spring 2015
Static vs. Dynamic Routes
• 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.
Static vs. Dynamic
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.
Static vs. Dynamic Routes
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.
Computers added to routing table?
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?)
Interior Gateway Protocol
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.)
Link-state vs. Distance-vector
algorithms
• 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…
OSPF and metrics
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.
Autonomous System
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.
Summarizing 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.
Old Slides
Dynamic routes on hosts?
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…
Static vs. Dynamic routes
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.
Static vs. Dynamic routes (2)
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.
Route timeouts?
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.
OSPF and routers
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.
Link-state Algorithms
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.
Import routes into OSPF
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.)