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 and Place
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.
Network Information (NI)
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 Source (NS)
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 Updating
Time (NU)
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.
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