Route summarization/aggregation combines several more specific routes into one summary route:

10.1.1.0/24
10.1.2.0/24   →   10.1.0.0/16

Why use it?

• Smaller routing tables · fewer routing updates · faster convergence
• Hides internal network details · better scalability

How it works

• The router advertises one aggregated prefix
• Longest Prefix Match always prefers the more specific route
• Traffic to unknown addresses inside the summary is handled locally

Null0 and summarization (IMPORTANT)

When summarizing, the router needs a local route to the summary prefix – the Null0 route:

ip route <SUMMARY-NET> <MASK> Null0

• Drops traffic to unknown subnets inside the summary
• Prevents routing loops
• Makes the summary route valid in the routing table

10.1.1.0/24, 10.1.2.0/24  →  10.1.0.0/16 Null0
Traffic to 10.1.1.10 → forwarded correctly
Traffic to 10.1.99.5 → matches summary → DROPPED (Null0)

Summarization vs aggregation

Point	Summarization	Aggregation
Used in	OSPF, EIGRP, RIP	BGP
Scope	Within an AS	Between ASes
Null0	Often	Almost always
Exam term	“summarize routes”	“aggregate routes”

Protocols: BGP (aggregate/network), OSPF (ABR/ASBR), EIGRP (manual), RIP (auto/manual, legacy). Null0 is protocol-independent (a static discard route).