• Routers = {A,B,C,D,E,F}
• Links = {(A,B),(A,D),(B,C),(D,E),(D,C),(C,F),(E,F)}
• Cost of Link (example) = c(A,B) = 5 
• Cost can be related to bandwidth, throughput, congestion, delay...
• Cost of Path (A,B,C,F) (example)= c(A,B)+c(B,C)+c(C,F)

Network Graphs

What is the least-cost path between A and F?

Routing algorithms are responsible to answer the question

Dynamic Routing Protocol Overview

Routing within autonomous systems (group of networks under a single administration) are referred as interior gateway protocols, whereas routing between autonomous systems are referred as exterior gateway protocols.

RIP protocol was updated to RIPv2 to accommodate growth in the network environment
     RIPv2 does not scale to current larger network implementations 

Routing Protocols developed to meet the need of larger networks include:
     Open Shortest Path First (OSPF)
     Intermediate System-to-Intermediate System (IS-IS).
     Enhanced IGRP (EIGRP)

Border Gateway Protocol (BGP) is used between Internet service providers (ISPs)

The Bellman-Ford algorithm’s primary principle is that it starts with a single source and calculates the distance to each node. The distance is initially unknown and assumed to be infinite, but as time goes on, the algorithm relaxes those paths by identifying a few shorter paths. Hence it is said that Bellman-Ford is based on “Principle of Relaxation“.

Bellman-Ford equation

cost from neighbour v to destination y

cost to neighbour v

minimum taken over all neighbours v of x

• Each node constructs a one-dimensional array (a vector) containing the "distances" (costs) to all other nodes and distributes that vector to its immediate neighbours.

• The starting assumption for distance-vector routing is that each node knows the cost of the link to each of its directly connected neighbours.

• These costs may be provided when the router is configured by a network manager. A link that is down is assigned an infinite cost.

RIP (Routing Information Protocol)

Initial Distances Stored at Each Node

• Add one hop to the hop count for each advertised destination.

• Repeat the following steps for each advertised destination:

  • If (destination not in the routing table)

    • Add the advertised information to the table.

  • Else

    • If (next-hop field is the same)

      • Replace entry in the table with the advertised one.

    • Else

      • If (advertised hop count smaller than one in the table)

        • Replace entry in the routing table.

• Return

RIP Updating Algorithm

• Routers send their advertisements every 30 seconds;

• A router also sends an update message whenever an update from another router causes it to change its routing table.

• One point of interest is that it supports multiple address families, not just IP. RIP version 2 (RIPv2) also introduced the subnet masks, whereas RIP version 1 worked with the old classful addresses of IP.

• All link costs are equal to 1, just as in our example. Thus, it always tries to find the minimum hop route. Valid distances are 1 through 15, with 16 representing infinity. This also limits RIP to running on fairly small networks—those with no paths longer than 15 hops.

RIP (Routing Information Protocol)

Final Distances Stored at Each Node

Before Failure

AftEr Failure

AftEr A received Update from B

AftEr B Receives update from A

Finally

• When a route fails, distance vector protocols spread the news about the route failure by poisoning the route.

• Route poisoning involves advertising the failed route with a special metric value called Infinity (e.g., RIP defines infinity as 16).

• Routers consider routes advertised with an infinite metric as failed and avoid using them.

• This prevents routers from endlessly updating each other with incorrect information.

solution - Route Poisoning

• RIPv2 has some Link State characteristics

• RIPv2 supports classless routing

• RIPv2 supports Authentication

Main differences

• RIP has a limit of 15 hops. A network that spans more than 15 hops (15 routers) is considered unreachable.

• RIP cannot handle Variable Length Subnet Masks (VLSM). Given the shortage of IP addresses and the flexibility VLSM gives in the efficient assignment of IP addresses, this is considered a major flaw.

• Periodic broadcasts of the full routing table consume a large amount of bandwidth. This is a major problem with large networks especially on slow links and WAN clouds.

• RIP converge is slower than OSPF. In large networks convergence gets to be in the order of minutes.

• RIP routers go through a period of a hold-down and garbage collection and slowly time-out information that has not been received recently. This is inappropriate in large environments and could cause routing inconsistencies.

• RIP has no concept of network delays and link costs. Routing decisions are based on hop counts. The path with the lowest hop count to the destination is always preferred even if the longer path has a better aggregate link bandwidth and less delays.

• RIP networks are flat networks. There is no concept of areas or boundaries. With the introduction of classless routing and the intelligent use of aggregation and summarization, RIP networks have fallen behind.

RIP Limitations

Example of Building a route table for Link State Routing

Authentication of Routing Messages

Open Shortest Path First Protocol (OSPF)

Additional Hierarchy

Load Balancing

Multiple Cost Metrics

Send information from one node to many other nodes, and the entire network can thus be impacted by bad information from one node. For this reason authenticating routing messages has become very important.

OSPF introduces another layer of hierarchy into routing by allowing a domain to be partitioned into areas. A router within a domain only needs to know how to get to the right area reducing the amount of information in the routing table.

Allows multiple routes to the same place to be assigned the same cost and will cause traffic to be distributed evenly over those routes, thus making better use of the available network throughput.

For each link, multiple cost metrics for different Types of Services (ToS) (e.g., satellite link cost (more delay involved) set low for best effort ToS; high for real-time ToS)

Rip Configuration

Rip Configuration

Rip Configuration

• Do a simple design of 4 routers with two Hosts connected at each end. Find a suitable topology

• Do another network with 16 nodes to prove that RIP fails at 16 hops

Tasks