Aggregation and the Size of the Table
Four networks announced as one, and what that costs the receiver
Step 1 of 2 — understand the idea. Read this first and the packet trace will confirm what you already expect, instead of teaching you two things at once.
What you'll be able to do
- ▸Explain what aggregation removes and what it removes it from
- ▸State the trade an aggregate makes on the receiver's behalf
- ▸Say why the global routing table's size is an engineering constraint
Before this lesson — it builds directly on:
Nobody owns the global routing table, and everybody pays for it. A router carrying a full table has to store every prefix and look any of them up fast enough to forward at line rate, which usually means specialised memory that is expensive and comes in fixed sizes.
So when a network announces four prefixes where one would do, it has added four entries to every full-table router on the internet instead of one. Aggregation is the mechanism for not doing that — and, because it costs the announcer some control, it is a decision rather than an automatic behaviour.
AS 65003 holds four contiguous /26 networks and announces the /24 that covers them.
Held: 203.0.113.0/26 203.0.113.64/26
203.0.113.128/26 203.0.113.192/26
Announced: 203.0.113.0/24 (one prefix)
Neighbour's table before: 4 entries
Neighbour's table after: 1 entry
Reachability to 203.0.113.30: unchanged
Ability to route .64/26 differently: goneThe data plane cannot tell the difference. The control plane has lost three quarters of its options, deliberately.
✓ Concept check — before you open the packets
These test the idea, not the trace. You should be able to answer them from the explanation above.
Now that you understand the concept, observe how it appears in the packet exchange.
The lab runs a real simulated capture — pause, step, click any packet, and inspect every byte.