1. TODAY’S TENTATIVE AGENDA
5pm-515pm
1. Return graded project 1
2. Lecture 14: I know you are anxious and want to only focus on taking the exam – so I will briefly cover OSPF and post the details
515pm-630pm
3. Exam
Dr. Clincy

2. (How the routers’ tables are filled in)
Chapter 11
Unicast Routing
Protocols
(RIP, OSPF, BGP)
(How the routers’ tables are filled in)
Dr. Clincy

3. Popular Unicast Routing protocols
RIP (already covered) – Routing Information Protocol – treats each network the same (assigns the same cost for each network)
OSPF – Open Shortest Path First protocol – assigns a cost for passing through a network based on the type of service required – routes through the network can have different cost – each router would have several tables
BGP – Border Gateway Protocol – is an exterior routing protocol that uses a policy that defines what paths should be chosen
Dr. Clincy

4. OSPF: Open Shortest Path First
Similar to RIP however, divide autonomous system into areas
Routers with in an area floods the area with routing info – router sends to all it’s neighbors and each neighbor sends to all it’s neighbors and etc..
At the border of an area, special routers called area border routers are used to (1) summarize info about an area and (2) send info amongst areas
A special area called the backbone is used to tie together all of the areas – backbone is primary area and all other areas are secondary areas – backbone area uses backbone routers
Note: backbone router can also be an area border router
Each area has an ID (backbone’s Id is 0)
Explain Current Real World Practices relating to this topic
Dr. Clincy

5. Recall OSPF: Open Shortest Path First
The OSPF is similar to RIP however, it allows the admin the ability to assign a cost or metric to each route.
The metric can be based on a type of service (ie . Min delay, max throughput, etc..)
For OSPF, a router will have multiple routing tables – one for each TOS
Unlike RIP, sharing or updating is done when there is a change (not periodically)
For OSPF, the objective is for the routers to contain the full picture or topology of the Internet – by having this, the router can figure out the “shortest path” or “least cost” route between itself and each network
To do this, the Internet is represented by a graph – set of edges and nodes
Dr. Clincy

6. You Can Now Start Exam 2
Dr. Clincy

7. Defining edges or connections/links
Point-to-point link
Virtual link
Direct connection between two routers, no IP address needed
Bi-directional Edge
When link between two routers are broken, admin creates a new route across multiple routers
Transient link
Stub link
Represents the network
Connects to only one router – packets enter and leave through this same router
Dr. Clincy

8. Graphical representation of an internet
Dr. Clincy

9. OSPF uses Dijkstra’s Shortest Path Algorithm
Dr. Clincy

10. Dijkstra’s Algorithm Cont..
Dr. Clincy

11. Dijkstra’s Algorithm Continued
At this point, we would have the least cost path from A to all other nodes
Note: to find the least cost paths from E to all other nodes, Disjkstra’s algorithm has to run again
Dr. Clincy

12. Answer in Hyphen Format
A-D
A-D-N3
A-D-N3-F
A-D-N3-F-N5
A-N1
A-N1-B
A-N1-B-E
A-N1-B-E-N4
A-N1-C
A-N1-C-N2
Dr. Clincy

13. How do the OSPF packets travel across the network(s) ??????????????
A re-occurring theme
How do the OSPF packets travel across the network(s) ??????????????
Dr. Clincy

14. OSPF packets are encapsulated in IP datagrams.
Dr. Clincy

15. BGP: Border Gateway Protocol
BPG is an inter-autonomous system routing protocol that makes use of path vector routing
For BPG, each routing entry contains (1) destination network, (2) next router (hop) and (3) entire path to reach destination
Path is an ordered list of autonomous systems that the packet should travel through to make it to the destination
Dr. Clincy