1. Computer Networks
Routing Algorithms

2. Static v.s. Dynamic Routing (Basics)
Static Routing Tables are entered manually
Strengths of Static Routing
Ease of use
Control
Efficiency
Weaknesses of Static Routing
Not Scalable
Not adaptable to link failures
Dynamic Routing Tables are created through the exchange of information between routers on the availability and status of the networks to which an individual router is connected to. Two Types
Distance Vector Protocols
RIP: Routing Information Protocol
Link State Protocols
OSPF: Open Shortest Path First

4. Choosing the Right Protocol
Interior Routing Protocols
Used within an autonomous system
Used within an area of administrative control
Exterior Routing Protocols
Used between autonomous systems
Used to peer with networks in which you have no administrative control

5. Choosing the Right Protocol (Cont.)
Interior Routing Protocols
Static
RIP
OSPF
EIGRP
ISIS
Exterior Routing Protocols
BGP
NOTE: This is not an exhaustive list of protocols available but merely a list of those commonly used.

6. Choosing the Right Protocol (Cont.)
Static Routing
May be suitable on small networks
Administration intensive as changes have to be made on each router

7. Choosing the Right Protocol (Cont.)
Dynamic Routing Protocol Types
Distance Vector
Routing Information Protocol(RIP)
Interior Gateway Routing Protocol(IGRP)
Enhanced Interior Gateway Routing Protocol(EIGRP)
Link State
Open Shortest Path First(OSPF)
Intermediate System to Intermediate System(ISIS)
Path Vector
Border Gateway Protocol(BGP)

8. Choosing the Right Protocol (Cont.)
Routing Information Protocol(RIP)
RFC 1058(RIPv1), 1988
Classful, no support for VLSM
No support for authentication
RFC 2453(RIPv2), 1998
Classless, support for CIDR
Support for authentication
Uses hop count as routing metric
Slow to converge
Not very scalable
Limited to 15 hops

9. Choosing the Right Protocol (Cont.)
Interior Gateway Routing Protocol(IGRP)
Invented by Cisco to overcome limitations of RIP
Allows for hop count up to 255
Allows for multiple route metrics
Bandwidth
Delay
Load
Reliability
Classful, no support for VLSM

10. Choosing the Right Protocol
Enhanced Interior Gateway Routing Protocol(EIGRP)
Replaced IGRP
Maintains a Topology table
Successors, feasible successors
Allows for multiple route metrics
Classless, support for CIDR
Very fast to converge
Maintains neighbor relationships
Diffusing Update Algorithm (DUAL)
Not as CPU intensive as OSPF

11. Notes Routing Information Protocol (RIP)
A distance vector protocol that has 2 versions
RIPv1 – a classful routing protocol
RIPv2 - a classless routing protocol
Enhanced Interior Gateway Routing Protocol (EIGRP)
A distance vector routing protocols that has some features of link state routing protocols
A Cisco proprietary routing protocol

12. Choosing the Right Protocol (Cont.)
Open Shortest Path First(OSPF)
RFC 2328(OSPFv2), 1998
Maintains neighbor relationships
Concept of Areas
Different areas can be used to control flooding of routing information
Classless, supports VLSM
Fast to converge
CPU Intensive Dijkstra Algorithm
Designing can be complicated

13. Choosing the Right Protocol (Cont.)
Intermediate System to Intermediate System(ISIS)
RFC 1142, 1990
Dijkstra Algorithm
Mainly used by large service providers
Does not use IP to carry routing information
Uses ISO addresses
Level Concept
Level 1 or Intra Area
Level 2 or Inter Area
Level 1/2 or Both
Classless, supports VLSM

14. Choosing the Right Protocol (Cont.)
Border Gateway Protocol(BGP)
RFC 4271(BGPv4), 2006
Peers manually defined
Used typically for multi-homing to ISP(s)
Very scalable
Makes decisions based upon AS Path
Lots of policy options
Very granular control

15. IP Packet Delivery
Two Processes are required to accomplish IP packet delivery:
Routing (Establish end-to-end paths)
discovering and selecting the path to the destination
layer-3 functionality
Forwarding
determine next hop

16. Routing Tables
Routing Tables are built up by the routing algorithms with components:
Destination Network Address: The network portion of the IP address for the destination network
Subnet Mask: used to distinguish the network address from the host address
The IP address of the next hop to which the interface forwards the IP packet for delivery
The Interface with which the route is associated

17. Forwarding Tables
After the routing lookup is completed and the next hop is determined, The IP packet is forwarded according to:
Local delivery model
destination and host are on the same local network
Remote delivery model
destination and host are on different networks

18. Routing Metrics
Used by dynamic routing protocols to establish preference for a particular route.
Support Route Diversity and Load Balancing
Most Common routing metrics:
Hop Count (minimum # of hops)
Bandwidth/Throughput (maximum throughput)
Load (actual usage)
Delay (shortest delay)
Cost

19. Dynamic Routing Protocols (1)
Distance Vector (DV) Protocols
based on the Bellman-Ford algorithm
Each router on the network compiles a list of the networks it can reach (in the form of a distance vector)
exchange this list with its neighboring routers only
Upon receiving vectors from each of its neighbors, the router computes its own distance to each neighbor.
for every network X, router finds that neighbor who is closer to X than to any other neighbor. Router updates its cost to X.

20. Example: Initial DistancesB E C D
Info at
node 7 ~ 1 2 8
Distance to node

21. Router E receives Router D Table1
Distance to node B C
Info at
node A B C D E 7 A 7 ~ ~ 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 ~ 2

22. Router E updates cost to Router CB E C D
Info at
node 7 ~ 1 2 8
Distance to node 4

23. Router A receives Router B TableD
Info at
node 7 ~ 1 2 8
Distance to node 4

24. Router A updates Cost to Router CB E C D
Info at
node 7 8 1 ~ 2
Distance to node 4

25. Router A receives Router E TableD
Info at
node 7 8 1 ~ 2
Distance to node 4

26. Router A updates Costs to Routers C&DB E C D
Info at
node 7 5 1 ~ 2 8
Distance to node 3 4

27. Final Distances A B C D Info at node 6 5 1 3 2 7 8 Distance to node E6 5 1 3 2 7 8
Distance to node E 4

28. Another Example (More Realistic)B C D E F G 1 ∞
Internal Information at each node ----->

29. Routing Tables With this information, routing table at A is -->
Cost
Next Hop B 1 C D ∞ - E F G
With this information, routing table at A is -->

30. Evolution of the table. Cost Next Hop B 1 C D 2 E F G
Each node sends a message to neighbors with a list of distances.
F --> A with G is at a distance 1
C --> A with D at distance 1.
Cost
Next Hop B 1 C D 2 E F G

31. Final Distance Matrix A B C D E F G 1 2 3