Routing principles/Issues.

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What is Routing?

Routing is the process of selecting the best or most suitable path for forwarding data packets from a source network to a destination network through one or more routers.
Routing is mainly performed at the Network Layer (Layer 3) of the OSI Model.

Routing Process

Simple Example

 Source Computer
       │
       ▼
    Router R1
       │
       ├────────► Router R2 ───────► Router R4
       │                              │
       └────────► Router R3 ──────────┘
                                      │
                                      ▼
                              Destination Computer

Here, the router has to decide which path should be used to reach the destination.

2. How Does Routing Work?

Suppose a computer with IP address 192.168.1.10 wants to send data to 192.168.4.20.

The router performs the following steps:

Receives the data packet.
Checks the Destination IP Address.
Looks for the destination network in its Routing Table.
Compares available routes using routing metrics.
Selects an appropriate route.
Sends the packet to the Next Hop.

Example Routing Table

Destination Network	Next Hop	Metric
192.168.1.0/24	Direct	0
192.168.2.0/24	Router R2	1
192.168.3.0/24	Router R3	2
192.168.4.0/24	Router R2	3

If the destination is 192.168.4.20, Router R1 can select Router R2 as the next hop.

3. Routing Principles

The major principles of routing are:

3.1 Correctness

The routing algorithm should select a path that can successfully deliver the packet to the correct destination.

Example:

Source → R1 → R2 → Destination
                 ✓
              Correct Path

A router should not forward packets through an unavailable or invalid route.

3.2 Simplicity

The routing mechanism should be simple and efficient.

A simple routing system:

Requires less processing
Uses fewer resources
Is easier to manage
Reduces configuration errors

3.3 Robustness

Robustness means that the routing system should continue to work even when some network components fail.

R2

/ \

Source → R1 R5 → Destination

\ /

R3

If the link through R2 fails, traffic can be redirected through R3.

3.4 Stability

A routing algorithm should not change routes unnecessarily or too frequently.

Frequent route changes may cause:

Routing instability
Extra routing traffic
Delays
Packet loss

A stable routing system provides more predictable network performance.

3.5 Fairness

Routing should provide fair access to network resources.
For example, one connection should not unnecessarily consume all the available capacity of a network path.

3.6 Optimality

The routing algorithm should select an appropriate or optimal path according to its routing metric.

For example:

Path 1:  A → R1 → R2 → B
         3 Hops

Path 2:  A → R3 → R4 → R5 → B
         4 Hops

If hop count is the selected metric, Path 1 may be preferred.
However, the shortest path is not always the fastest path because bandwidth, delay and congestion can also affect route selection.

4. Routing Metrics

A routing metric is a value used by a routing algorithm to compare different routes.
Common routing metrics include:

1. Hop Count

Number of routers a packet must pass through.

2. Bandwidth

Capacity of a network link.

3. Delay

Time required for a packet to travel through a network.

4. Cost

A numerical value assigned to a network path.

5. Reliability

Indicates how dependable a particular link or path is.

6. Load

Amount of traffic currently using a network path.

             Path A
       ┌─────────────────┐
       │                 │
Source ── R1 ── R2 ──────┴──► Destination
       │
       │    Path B
       └──── R3 ── R4 ─────────►

The routing algorithm uses its defined metric(s) to select a suitable route.

5. Routing Issues

Several important issues must be considered while designing a routing system.

5.1 Path Selection

The router must determine which path should be used from source to destination.

                 R2
                /  \
Source → R1 ───     ─── R5 → Destination
                \  /
                 R3

When multiple paths are available, the routing algorithm selects one according to its metrics and rules.

5.2 Routing Table Management

A router maintains a routing table containing information about available routes.

             ROUTER
                │
                ▼
       ┌─────────────────┐
       │  Routing Table  │
       ├─────────────────┤
       │ Destination     │
       │ Next Hop        │
       │ Metric          │
       │ Interface       │
       └─────────────────┘

The router uses this information to determine where the packet should be forwarded.

5.3 Routing Loops

A routing loop occurs when packets continuously travel between routers without reaching their destination.

┌─────────┐

▼ │

R1 ──────► R2

▲ │

│ ▼

└──────── R3

Routing loops can cause:

Wasted bandwidth
Increased network traffic
Packet loss
Network performance problems

5.4 Congestion

Congestion occurs when a network path receives more traffic than it can efficiently handle.

PC1 ──┐
PC2 ──┤
PC3 ──┼──► Router R1 ───► Router R2 ───► Destination
PC4 ──┤                         ▲
PC5 ──┘                         │
                           Heavy Traffic

Congestion can result in:

Increased delay
Packet loss
Reduced throughput
Poor network performance

5.5 Network Failure

Routers, switches, cables or links can fail.
A good routing system should detect the failure and, where possible, use an alternative path.

Normal:

A ── R1 ── R2 ── R3 ── B

After R2 Failure:

A ── R1 ── R4 ── R5 ── R3 ── B

5.6 Scalability

A routing system should work efficiently even when the network becomes very large.
For example, the Internet contains a huge number of networks and routers.

Therefore, routing protocols must efficiently handle:

Large routing tables
Many routers
Frequent network changes
Large amounts of routing information

5.7 Routing Overhead

Routers need to exchange routing information.

       Routing Updates
       ↔          ↔
      R1 ───────── R2
       │            │
       ↕            ↕
      R3 ───────── R4

This exchange consumes:

Bandwidth
CPU resources
Memory

Therefore, routing protocols should minimize unnecessary routing updates.

5.8 Security

Routing information can be manipulated by unauthorized users or malicious devices.

Possible security issues include:

Fake routing information
Unauthorized route changes
Traffic redirection
Routing-table manipulation

Therefore, secure routing mechanisms and authentication can be important in network design.

6. Static and Dynamic Routing

Static Routing	Dynamic Routing
Routes are manually configured	Routes are learned automatically
Suitable for small/simple networks	Suitable for larger networks
No routing-protocol update traffic	Routing information is exchanged
Administrator must update routes	Routes can adapt to network changes
Simple to understand	More complex
Less protocol overhead	More protocol overhead

Examples of Dynamic Routing Protocols

RIP – Routing Information Protocol
OSPF – Open Shortest Path First
EIGRP – Enhanced Interior Gateway Routing Protocol
BGP – Border Gateway Protocol

7. Complete Routing Process

The complete routing process can be represented as:

SOURCE

│

▼

Create Data Packet

│

▼

Destination IP Address

│

▼

ROUTER

│

▼

Check Routing Table

│

▼

Select Suitable Route

│

▼

Next Hop

│

▼

Forward the Packet

│

▼

Another Router

│

▼

DESTINATION

8. Important Points for BTEUP Exam