The big idea: Every router keeps a routing table: for each destination network, which neighbour to forward to.
The only question is how that table gets filled in — by hand, or by the routers telling each other.
Either way, one hop at a time: No router ever knows the whole path.
It looks up the destination, forwards to the next router, and that one does the same. The path is the result, not the plan.
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Written by hand: An administrator types each route in. The table never changes unless somebody changes it.
On a network with three routers that is a few minutes' work — and it then behaves identically every day.
What it gives you
- No resource usage — no messages between routers, no calculation
- Completely predictable — traffic always takes the route you chose
- Nothing to go wrong in the protocol, because there is no protocol
- More secure: a false routing message cannot mislead it
What it costs
- Every change is manual — a new network means editing every router
- No recovery: if a link fails, traffic keeps being sent down it
- Maintenance grows with the size of the network
- Unworkable beyond a small number of routers
The failure case is the deciding one: A static route does not know its link is broken. It keeps forwarding into the fault until someone notices and edits the table.
That is acceptable in a small office, and unacceptable anywhere that must stay up.
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The routers work it out between them: Routers exchange messages about what they can reach and build their own tables.
When a link fails they notice, tell each other, and settle on new routes. That settling is called convergence.
What it gives you
- Adapts automatically — a failed link is routed around
- Scales: adding a network is configured once, not everywhere
- Maintenance stays roughly constant as the network grows
- Essential for anything large or that must stay available
What it costs
- Uses resources — bandwidth for the messages, CPU and memory for the tables
- More complex to set up and to diagnose
- During convergence traffic may loop or be dropped
- A false routing message can be used to misdirect traffic
Convergence is the word to know: The time between something changing and every router agreeing on the new routes.
During it the network is briefly inconsistent, which is the one real weakness of dynamic routing — and the price of it recovering at all.
How this is tested — you must justify from network size and what happens when a link fails. It comes up two ways:
Paper 1 Section A
- Explain static or dynamic routing, 3-5 marks
- Give advantages and disadvantages of one
- Recommend one for a described network
Paper 1 Section B — case study
- Recommend routing for the case study's network
- Justify it from their size and uptime needs
The classic trap: Saying dynamic routing is better. On a three-router office it wastes bandwidth and CPU on a problem that does not exist. Network size is the factor that decides.
IB-style questionRecommend[5 marks]
A retail chain is growing from four shops to forty, each with its own router, and cannot afford to lose card payments if a link fails. Recommend a routing approach.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.