BGP Route Summarizer & CIDR Aggregator - Cisco, Juniper, RouterOS - cidrcalculate.com
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BGP Route Summarizer & CIDR Aggregator

Combine multiple IP subnets into optimal summary supernet blocks and generate Cisco IOS, Juniper JunOS, MikroTik RouterOS, FRRouting, and Arista EOS BGP aggregate commands.

Input Subnets

Enter one subnet per line. Accepts CIDR notation, subnet masks, or mixed delimiters. Invalid lines are ignored.

Aggregation Mode

Original Subnets
0
Aggregated Summaries
0
Table Reduction
0%

What is BGP Route Aggregation and Why is it Essential for Routing Tables?

BGP route aggregation (also called route summarization or supernetting) is the process of combining multiple contiguous IP prefixes into a single shorter-prefix advertisement. Instead of announcing eight individual /24 subnets from your data center, you advertise one /21 that covers all eight. This reduces the global routing table size, lowers TCAM and memory usage on routers, speeds up BGP convergence after failures, and shields upstream providers from routing instability (route flapping) within your network.

The global internet BGP table has grown to over 950,000 routes as of 2025. Without aggregation at every level — from enterprise edge routers to Tier 1 ISP core routers — the routing table would be unmanageably large. CIDR (Classless Inter-Domain Routing), introduced in 1993 (RFC 1518/1519), made aggregation possible by replacing classful addressing (A, B, C networks) with variable-length prefixes.

Difference Between aggregate-address summary-only vs. Suppressing Specific Routes

On Cisco IOS and FRRouting, the aggregate-address command creates a summary prefix in BGP. The summary-only keyword suppresses all more-specific routes from being advertised — only the aggregate is sent to neighbors. Without it, both the aggregate and the more-specifics are advertised unless a suppress-map selectively blocks specific prefixes.

Use summary-only when you own the entire aggregate block and do not want any specific prefixes leaking out. Use a suppress-map when you need to advertise the aggregate to some neighbors while still allowing selected more-specific routes to others (common in multi-homed or traffic-engineering scenarios). Juniper achieves similar control via policy-statement with term reject-morespecifics.

How to Prevent Blackhole Routing When Advertising BGP Summary Routes

Blackhole routing occurs when a BGP summary route is advertised but some of the more-specific subnets within the summary do not actually exist in the router's forwarding table. Traffic destined for those missing subnets arrives at the advertising router (attracted by the summary) but has nowhere to go — it is dropped, creating a connectivity black hole.

Best practices to prevent blackholes: (1) Always ensure that all constituent subnets within the aggregate are reachable and present in the routing table before generating the summary. (2) Configure a null0 static route for the aggregate as a safety net — traffic to missing subnets hits the null0 route instead of being dropped by the CPU. (3) On Cisco IOS, use ip route x.x.x.x y.y.y.y Null0 200 with a high administrative distance so it only activates when no more-specific routes exist. (4) Use BGP conditional advertisement — only advertise the aggregate when the constituent routes are present in the BGP table. (5) Regularly audit your aggregate coverage using this route summarization tool.

Reference Table: Subnet Aggregation by Prefix Length

Supernet Mask/24 SubnetsTotal IPsReduction Ratio
/2182,0488:1
/20164,09616:1
/19328,19232:1
/186416,38464:1
/1712832,768128:1
/1625665,536256:1
/15512131,072512:1
/141,024262,1441,024:1

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Frequently Asked Questions

How do I calculate a BGP aggregate summary manually?

To manually calculate a BGP aggregate: (1) Write all subnet addresses in binary. (2) Find the longest common prefix across all addresses. (3) Count the matching bits — this is the aggregate prefix length. (4) The aggregate network address is the common prefix followed by zeros. For example, to aggregate 192.168.0.0/24 through 192.168.3.0/24: 192.168.0.0 = 11000000.10101000.00000000.00000000 and 192.168.3.0 = 11000000.10101000.00000011.00000000. The first 22 bits match (11000000.10101000.000000), so the aggregate is 192.168.0.0/22. Use this tool to perform the calculation instantly for any set of subnets.

What is the maximum number of /24 subnets I can aggregate into a single BGP prefix?

A single /20 aggregate covers 16 A— /24 subnets (4,096 IPs). A /16 covers 256 A— /24s (65,536 IPs). A /8 covers 65,536 A— /24s (16,777,216 IPs). In practice, the maximum depends on your IP allocation and contiguous range. For example, if you own 10.0.0.0/14 and have deployed all subnets contiguously, you can advertise a single /14 aggregate. ISPs typically receive /19 to /12 allocations from RIRs (ARIN, RIPE, APNIC, LACNIC, AFRINIC) and aggregate each allocation into one prefix to upstream providers.

How does BGP route aggregation prevent route flapping propagation?

Route flapping occurs when a specific subnet repeatedly goes up and down, causing BGP update messages to propagate across the internet. Without aggregation, every flap of the /24 generates BGP UPDATE withdrawals and announcements that must be processed by every router in the path. With aggregation, the /24 is hidden behind a /20 summary — the upstream router only sees the /20 aggregate, which remains stable as long as at least one of the /24s within it is still reachable. This containment of routing instability is one of the primary reasons ISPs require customers to advertise only aggregates (RFC 7454). Route dampening (RFC 2439) at the aggregate level provides additional protection.

What is the difference between BGP aggregation and CIDR route summarization?

These terms are closely related and often used interchangeably. CIDR route summarization is the mathematical process of finding the longest common prefix across a set of IP prefixes — it is a general networking concept. BGP aggregation specifically refers to the BGP configuration that creates and advertises summary routes using the aggregate-address or aggregate route commands. In other words, CIDR summarization is the calculation; BGP aggregation is the implementation. Both rely on the same binary prefix-matching principle. This tool performs both the CIDR calculation and generates the BGP aggregation CLI configuration.

Can I aggregate non-contiguous IP subnets into a single BGP prefix?

Technically yes, using supernetting (fill-in mode), but it is dangerous. If you aggregate 10.1.0.0/24, 10.1.2.0/24, and 10.1.3.0/24 (skipping 10.1.1.0/24), the tightest bounding prefix is 10.1.0.0/22. This includes 10.1.1.0/24 — addresses you do not own or operate. Anyone sending traffic to 10.1.1.x will be routed to your router and dropped (blackholed) unless you have null0 routes. In production, you should only use supernetting when you control all addresses within the aggregate. This calculator's strict mode prevents this and warns you when fill-in addresses are detected.