What are the benefits and the costs of route summarization?
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Summarizing advertises one shorter prefix instead of many specific ones. It shrinks routing tables, hides flaps inside the block from the rest of the network, and bounds EIGRP queries and OSPF inter-area flooding. The costs: routers outside lose detail and may pick a longer path, and when a subnet dies behind a summarizing router, that router keeps attracting its traffic and drops it. Summaries also only work when the addressing plan was built for them.
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The math
10.20.0.0/24, 10.20.1.0/24, 10.20.2.0/24 and 10.20.3.0/24 differ only in the last two bits of the third
octet (000000 00 to 000000 11). The first 22 bits are common, so the summary is 10.20.0.0/22.
Benefits
- Smaller RIB and FIB everywhere outside the block.
- Stability: a flapping /24 inside the summary causes no updates outside it.
- EIGRP queries stop at a router that only knows the summary.
- OSPF: one type 3 LSA instead of many at the ABR.
Costs
- Suboptimal routing. Two hubs advertise the same summary with similar metrics. A remote router sends traffic to whichever hub looks closer for the summary, even when the specific subnet sits behind the other hub.
- Black holes. DSW1 and DSW2 both advertise 10.20.0.0/16. DSW1 loses its link to 10.20.30.0/24. The core still sends part of that traffic to DSW1, which only has its summary discard route and drops it.
- Planning. Subnets scattered across the network cannot be summarized.
Where you can summarize
| Protocol | Where |
|---|---|
| OSPF | only at ABRs (area range) and ASBRs (summary-address) |
| EIGRP | on any interface |
| IS-IS | at the level 1 / level 2 boundary and on redistribution |
| BGP | anywhere with aggregate-address |
They will ask next
How do you avoid the black hole between two summarizing routers?
Give them a routed link between each other that carries the specific routes, with no summarization on that link. The router that lost the subnet then has a more specific route via its partner, which beats its own discard route.
Traps
- Selling summarization as free. Interviewers want to hear about suboptimal paths and black holes.
- Getting the summary length wrong by counting the subnets instead of the common bits.