Layer-3 Routing

Natawut Nupairoj, Ph.D.

Department of Computer Engineering

Chulalongkorn University

Outline

Overview. Interconnection Devices. Routing Concepts. Routing Algorithms.

Overview

End-to-end delivery Across multiple links (or

hops). Must concern

Find paths in different networks.

Choose appropriate paths.

Avoid overloading links. Data-Link is just for

machine-to-machine over single link.

Interconnection Devices

Device Overview

Repeater Focus at physical layer. But not an amplifier.

Repeater

Bridge

Connect two (or more) LANs together Forward packages between LANs. Smart hub.

Focus at Layer-2 Use MAC addresses to decide if it should forward

packages.

Bridge Functions

Transparent Bridge

No need to configure the addresses Self-updating.

How does a bridge learn addresses? Initially, know nothing. If found unknown

address, send to all ports.

Also, note the port of the source address.

Bridge in OSI Model

Router

Similar to bridge, but focus on layer-3. Forward to neighbor network or next router

toward the destination.

Router in OSI Model

Gateway

Operate in all seven layers. Protocol converter.

Gateway in OSI Model

Switch

Smart multiport bridge Multiple ports. Transparent bridge

functions (Layer-2). Packet buffers.

Next generations L3 Switch.

Routing Concepts

Key design elements Performance criteria. Decision time. Decision place. Network information source. Network information update timing.

Performance Criteria (PC)

What route should I take? Hop count – simplest. Links’ bandwidths – better. Current delay in the queue – more realistic.

Example of least-cost algorithms Distance vector routing. Link-state routing.

Decision Time (DT) When finding the route, what level should I decide for ?

Per-packet. Per-session.

Decision Place (DP) Who will decide the route ?

Switching node (e.g. router). Central node. Source node.

Decision Time and Place

What should I obtain the information regarding to current network information ? Topology. Traffic load. Link cost.

Scope of the information Cost from the router to all other routers. Cost from the router to its neighbors.

Network Information (NI)

Where do I obtain the information regarding to current network information ? None. Local. Adjacent (neighbor) node. Node along the route (of packet). All nodes – centrally or distributed.

Network Information Source (NS)

How often should I collect network information ? Never. Continuous. Periodic. Major load change. Topology change.

The more often you collect The better routing decision you can make. The more overhead you generate.

Network Information Updating Time (NU)

Routing Strategies

Fixed Routing all routes are predetermined. simple but not flexible.

Source Routing Source node determines the route. Routing patterns can be pre-arranged. Good for special network.

Flooding send to everyone. require no network information. generate lots of traffic.

Routing Strategies

Random Routing simple and require no network information with less traffic. may not be the least-cost routing.

Adaptive Routing complex generate some traffic overheads react too quick / too slow ?

Distance Vector Routing

Keys PC: N/A. DP: router. DT: N/A. NI: to all routers. NS: exchange with neighbors. NT: periodic (e.g. every 30 seconds).

Example: Network

Example: NI-NS-NT

Distance Vector Routing Table

Routing Table Distribution

Network Information Updating

Final Routing Tables

Link-State Routing

Keys PC: N/A. DP: router. DT: N/A. NI: to neighbors. NS: exchange with all routers -- flooding. NT: major changes.

Example: NI-NS

Cost in Link-State Routing

Link-State Packet

Flooding of A’s Link-State Packets

Link-State Database

Cost in Dijkstra Algorithm

Shortest Path Calculation

Figure 21-31, Part III

Shortest Path Calculation, Part X

Routing Table for Router A