1. Communication Networks NETW 501
Lecture 11
Interconnecting Networks: Routers
Course Instructor: Dr.-Ing. Maggie Mashaly
C3.220

2. Bridges vs. Routers Both store and forward devices
Routers: Network layer devices
Bridges: Datalink layer devices
Routers maintain routing tables, implement routing algorithms
Bridges maintain forwarding tables, implement filtering, learning and spanning tree algorithms
BRIDGES do well in SMALL Networks (few hundred hosts) while ROUTERS used in LARGE Networks (thousands of hosts)

3. Bridges Characteristics
Advantages:
Bridge operation is simpler requiring less processing bandwidth
Limitations:
Homogeneous link layer (e.g., all Ethernet) is desirable for transparent bridging
Topologies are restricted with bridges: a spanning tree must be built to avoid cycles
As a result of the spanning tree algorithm the root bridge may represent a bottleneck
Bridges do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a bridge)

4. Routers Characteristics
Goal:
Scalable interconnection of a large numbers of networks of different types
Advantages:
Arbitrary topologies can be supported, cycling is limited by TTL (time-to-live counters) and good routing protocols.
Provides firewall protection against broadcast storms
Disadvantages:
Require IP address configuration (not plug and play)
Require higher processing bandwidth

5. Router Functions Forwarding Routing Queuing Buffer Management
Move packet from input link to the appropriate output link
Purely local computation
Must go by very fast (executed for ever packet)
Routing
Keep track of network topology so you know where to forward packets
Queuing
Buffer (i.e., store) packets if router can’t forward it right now
Buffer Management
Drop packet if you run out of buffer space

6. Routing Algorithm Classification
Global or decentralized information?
Global
all routers have complete topology, link cost info
“link state” algorithms
Decentralized
router knows physically-connected neighbors, link costs to neighbors
iterative process of computation, exchange of info with neighbors
“distance vector” algorithms
Static or dynamic?
Static
routes change slowly over time
Dynamic
routes change more quickly
periodic update
in response to link cost changes

7. Routing Approaches Static Distance vector Link state
Type in the right answers and hope they are always true
Distance vector
Tell your neighbors what you know about everyone
Link state
Tell everyone what you know about your neighbors

8. Bellman-Ford Distance Vector Routing Algorithms
Assumption
Each router knows own address & cost to reach neighbors
Objective
Calculate routing table containing next-hop information for every destination at each router
Distributed Bellman-Ford algorithm
Each router maintains a vector of costs to all destinations
Initialize neighbors with known cost, others with infinity
Periodically send copy of distance vector to neighbors
On reception of a vector, If neighbor’s path to a destination is shorter, switch to it

9. Note: (a,b)=(next hop, total cost to destination)
Bellman-Ford Algorithm: Shortest Path to Node 6
Note: (a,b)=(next hop, total cost to destination)
Step (Iteration)
Node 1
Node 2
Node 3
Node 4
Node 5
Initial
(-1,∞) 1
(6,1)
(6,2) 2
(3,3)
(5,6) 3
(4,4) 4

10. Bellman-Ford Algorithm
Iteration
Node 1
Node 2
Node 3
Node 4
Node 5
Initial
(-1, ) 1 2 3
Table entry
@ node 1
for dest SJ
Table entry
@ node 3
for dest SJ 3 1 5 4 6 2
San
Jose

11. Bellman-Ford Algorithm
Iteration
Node 1
Node 2
Node 3
Node 4
Node 5
Initial
(-1, ) 1
(6,1)
(6,2) 2 3
D3=D6+1
n3=6
D6=0 1 3 1 5 4 6 2
San
Jose 2
D6=0
D5=D6+2
n5=6

12. Bellman-Ford Algorithm
Iteration
Node 1
Node 2
Node 3
Node 4
Node 5
Initial
(-1, ) 1
(6, 1)
(6,2) 2
(3,3)
(5,6) 3 3 1 3 1 5 4 6 2 3
San
Jose 6 2

13. Bellman-Ford Algorithm
Iteration
Node 1
Node 2
Node 3
Node 4
Node 5
Initial
(-1, ) 1
(6, 1)
(6,2) 2
(3,3)
(5,6) 3
(4,4) 1 3 3 1 5 4 6 2 3
San
Jose 6 4 2

14. Network disconnected; Loop created between nodes 3 and 4
Bellman-Ford Algorithm
Iteration
Node 1
Node 2
Node 3
Node 4
Node 5
Initial
(3,3)
(4,4)
(6, 1)
(6,2) 1
(4, 5) 2 3 1 5 3 3 1 5 4 6 2 3
San
Jose 4 2
Network disconnected; Loop created between nodes 3 and 4

15. Node 4 could have chosen 2 as next node because of tie
Bellman-Ford Algorithm
Iteration
Node 1
Node 2
Node 3
Node 4
Node 5
Initial
(3,3)
(4,4)
(6, 1)
(6,2) 1
(4, 5) 2
(3,7)
(5,5) 3 5 7 3 3 1 5 4 6 2 5 3
San
Jose 2 4
Node 4 could have chosen 2 as next node because of tie

16. Bellman-Ford Algorithm
Iteration
Node 1
Node 2
Node 3
Node 4
Node 5
Initial
(3,3)
(4,4)
(6, 1)
(6,2) 1
(4, 5) 2
(3,7)
(5,5) 3
(4,6)
(4, 7) 5 7 7 3 1 5 4 6 2 5
San
Jose 2 4 6
Node 2 could have chosen 5 as next node because of tie

17. Bellman-Ford Algorithm
Iteration
Node 1
Node 2
Node 3
Node 4
Node 5 1
(3,3)
(4,4)
(4, 5)
(6,2) 2
(3,7)
(2,5) 3
(4,6)
(4, 7)
(5,5) 4
(2,9) 7 7 9 3 5 4 6 2 1 5
San
Jose 6 2
Node 1 could have chose 3 as next node because of tie

18. Counting to Infinity Problem3 1 2 4 X
(a)
(b)
Nodes believe best path is through each other
(Destination is node 4)
Update
Node 1
Node 2
Node 3
Before break
(2,3)
(3,2)
(4, 1)
After break 1
(3,4) 2
(2,5) 3
(3,6) 4
(2,7) 5
(3,8) …

19. Problem: Bad News Travel Slowly
Solutions:
Split Horizon
Do not report route to a destination to the neighbor from which route was learned
Poisoned Reverse
Report route to a destination to the neighbor from which route was learned, but with infinite distance
Breaks erroneous direct loops immediately
Does not work on some indirect loops

20. Nodes believe best path is through each other
Split Horizon with Poison Reverse 3 1 2 4 X
(a)
(b)
Nodes believe best path is through each other
Update
Node 1
Node 2
Node 3
Before break
(2, 3)
(3, 2)
(4, 1)
After break
(-1, )
Node 2 advertizes its route to 4 to node 3 as having distance infinity; node 3 finds there is no route to 4 1
Node 1 advertizes its route to 4 to node 2 as having distance infinity; node 2 finds there is no route to 4 2
Node 1 finds there is no route to 4

21. Dijkstra Algorithm 3 1 5 4 6 2 Iteration N Node 2 Node 3 Node 4 Node 5
Initial
{1} 3 2 5 ∞ 1
{1,3} 4
{1,2,3} 7
{1,2,3,6}
{1,2,3,4,6}
{1,2,3,4,5,6}

22. References NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy
“Communication Networks 2nd Edition”, A. Leon-Garcia and I. Widjaja, McGraw Hill, 2013
“Computer Networks 4th Edition”, A. S. Tanenbaum, Pearson International