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Routing & Switching · Intermediate

Route Summarization

Advertising a single aggregate prefix that covers multiple more specific routes to reduce table size and contain instability.

Definition

Summarization (aggregation) reduces routing table size, update volume and SPF or query scope. It is configured manually at boundaries: ABRs and ASBRs in OSPF, any interface in EIGRP, L1/L2 routers in IS-IS and aggregate-address in BGP. Summaries should cover only owned space to avoid blackholing, and a discard route is usually installed locally.

Example

ip summary-address eigrp 100 10.20.0.0 255.255.252.0 on the WAN interface

Exam tip

To calculate a summary, find the common leading bits; 10.20.0.0/24 through 10.20.3.0/24 summarize to 10.20.0.0/22.

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