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1. IP Routing

2. Routing
• The process that a router uses to forward packets
toward the destination network.
• A router makes this decision based on the destination
IP address
• If dynamic routing is used, routers have to learn
routes from other routers.
• If static routing is used, the administrator configures
this information into the router manually
– So, you might say that updates to a routing table
are made by the administrator.

3. Configuring Static Routes
with Outgoing Interface
Administrative
distance of 1 -
default
Outgoing
interface

4. Configuring Static Routes
with Next-hop IP Address
Administrative
distance of 1 -
default
Next hop
interface

5. Configuring Static Routes
• An administrator actually enters static routes into the routing
table.
• That makes them static route entries – because the router is
not “discovering” those routes.
• If for some reason that outgoing interface goes down or is not
available for some reason, then at that time the route will be
removed from the routing table.
show ip route shows the routing table.
The route would still be in the configuration (because it was
entered globally), but that route could now no longer be used by
the router because the interface it refers to is down for some
reason.

6. Administrative Distance
• What is the default for a outgoing interface?
• What is the default for the next-hop address?
• Defaults can always be changed.
• Just make it higher if you want it to be a
“backup” route (lower numbers are preferred)
ip route 192.168.2.0 255.255.255.0 192.188.4.1 120

7. Router A Router B Router C
S0 192.168.2.1/24
S1 192.168.2.2/24
S0 192.168.4.1/24
S1 192.168.4.2/24
192.168.1.0/24 192.168.3.0/24 192.168.5.0/24
What would you enter to configure a static route from Router
C to the LAN on Router A using outgoing interface?
The LAN on Router B from Router A using next-hop?

8. Static Default Routes
• A router should be configured with a special type of
static route – a default route.
• This default route routes packets with destinations
that do not match any of the other routes in the
routing table
• It is a “gateway of last resort” that allows the router to
forward “destination unknown” packets out a
particular interface
ip route 0.0.0.0 0.0.0.0 [next-hop-address |
outgoing interface]

9. Default Route on non-directly connected
networks

10. Verifying a Default Route
• To verify a default route, use the following
commands:
– show run to make sure you typed it in correctly
– show ip route to make sure the router placed it
in the routing table.

11. Examine the show ip route Command
[Administrative distance / Hop count]

12. Troubleshooting static route configuration
• Ping and traceroute should be used to test basic
connectivity.
• But before using ping and traceroute, it is a good
idea just to see if the physical interface is “up”.
show interfaces
show interface s0
show interface s1
show ip interface brief

13. Routed VS. Routing
• Routed protocols
are protocols that
are routed over an
Internetwork (IP,
AppleTalk, IPX)
• Routing protocols
use algorithms to
route routed
protocols through
the Internetwork
(RIP, IGRP, OSPF)
•Routed protocols are used BETWEEN routers to direct traffic
•Routing protocols allow routers to share information about
known networks with other networks (their purpose)

14. Routing Protocol and Autonomous
System
• An autonomous system (AS) is a collection of networks
under a common administration sharing a common routing
strategy.
• The goal of a routing protocol is to build and maintain the
routing table.
• A routers learns about routes to a network by:
– Gathering information from its own configuration
regarding directly connected networks.
– Other routers forward information about known
networks.
– Manually entered routes by a network administrator.

15. Convergence
• When all routers in an Internetwork are operating with the
same knowledge, the Internetwork is said to have
converged.
• Fast convergence is desirable because it reduces the
period of time in which routers would continue to make
incorrect routing decisions.
• In routers that use dynamic routing protocols, it is
important to have fast convergence because routers could
make incorrect forwarding decisions until the network has
fully converged.
• A network has converged when the routers in the network
are operating with consistent routing knowledge.

16. Router A Router B Router C
S0
S1
S0
S1
192.168.1.0/24 192.168.3.0/24 192.168.5.0/24
The routing table on these routers would be changed if
something happened to the links BETWEEN the routers (a
serial interface goes down for some reason). Anything that
happens to the Ethernet interfaces would not affect the
routing tables. Routing takes place between routers.
E0
E0 E0

17. Distance Vector Routing
Neighbor to
neighbor
ONLY

18. Distance Vector Routing

19. Topology Changes (distance vector)
Routing table updates
take place when the
topology changes.
Router to router
(neighbor tells
neighbor)
Distance vector – each
router sends its entire
routing table to
neighbor table

20. Link-state Routing
• Shortest Path First (SPF)
• Flood routing information about it’s OWN links
• Analyze incoming routing update messages
• If the message indicates that a network change has
occurred, the routing software recalculates routes
and sends out new routing update messages
• These messages permeate the network, stimulating
routers to rerun their algorithms and change their
routing tables accordingly.
• After the initial flood, it passes small event-
triggered updates to all other routers

21. In Order to Converge, a Router in a Link-
State network must:
• Remember its neighbor’s name, when it’s link is up or down
(status), and the cost of the path to that router.
• Create an LSP (link-state packet) that lists its neighbor’s
name and relative costs.
• Send the newly created LSP to all other routers participating
in the link-state network.
• Receive LSPs from other routers and update its own
database. (Particularly when there are changes in the
network)
• Build a complete map of the Internetwork’s topology from all
the LSPs received, then compute the best route to each
network destination.

22. Link-State Concerns
• Processor overhead
• Memory requirements (use more router resources
initially when flooding info)
• Bandwidth Consumption on initial flood
All of these are considerations when selecting a
routing protocol to be used over an enterprise
network.

23. Routing Decisions
• The router uses the
network portion of the
address to choose the
best path
• Router “switches” the
packet to the best port
for forwarding using
path determination