Layer-3 Routing Natawut Nupairoj, Ph.D. Department of Computer Engineering Chulalongkorn University
Dec 22, 2015
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.
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.
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.
Router
Similar to bridge, but focus on layer-3. Forward to neighbor network or next router
toward the destination.
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).
Link-State Routing
Keys PC: N/A. DP: router. DT: N/A. NI: to neighbors. NS: exchange with all routers -- flooding. NT: major changes.