> ## Content Index > Fetch the complete content index at: https://www.pinglabz.com/llms.txt > Use this file to discover other available public pages before exploring further. # BGP Route Aggregation and Summarization - URL: https://www.pinglabz.com/bgp-aggregation-summarization/ - Published: 2026-03-28T06:51:59.000Z - Updated: 2026-07-04T22:56:32.000Z - Description: The IPv4 BGP table has over 950,000 prefixes and growing. Route aggregation lets you advertise a summary instead of many specifics, reducing the routing table burden on your peers and the internet. Configuration and gotchas. - Author: Jaime - Tags: BGP, #Import 2026-08-01 19:54 The global IPv4 routing table has over 950,000 prefixes - and it keeps growing. Every network operator has a responsibility to advertise as few prefixes as reasonably possible. BGP route aggregation lets you advertise a single summary prefix instead of multiple more-specific ones, reducing the routing table burden on your peers and the internet at large. (This article is part of the PingLabz BGP series - the [full BGP guide](https://www.pinglabz.com/bgp/) maps the whole cluster in reading order.) ## aggregate-address Command The `aggregate-address` command creates a summary from more-specific BGP routes already in the BGP table: ``` R1-HQ(config)# router bgp 65001 R1-HQ(config-router)# aggregate-address 10.1.0.0 255.255.0.0 ``` Requirements: - At least one more-specific prefix within the aggregate range must exist in the BGP table (not just the IP routing table) - The aggregate is generated locally - it has a next-hop of 0.0.0.0 and AS-path is empty (just your AS) - Without `summary-only`, both the aggregate AND the component routes are advertised ## summary-only The most common option - suppresses the component routes and only advertises the summary: ``` R1-HQ(config-router)# aggregate-address 10.1.0.0 255.255.0.0 summary-only ``` ``` R1-HQ# show ip bgp Network Next Hop Metric LocPrf Weight Path *> 10.1.0.0/16 0.0.0.0 32768 i s> 10.1.1.0/24 0.0.0.0 0 32768 i s> 10.1.2.0/24 0.0.0.0 0 32768 i s> 10.1.3.0/24 0.0.0.0 0 32768 i ``` The `s` flag means "suppressed" - these /24s exist in the local BGP table but are NOT advertised to any peer. Only the /16 aggregate is sent. ## as-set When you aggregate routes from multiple ASes, the AS-path information is lost - the aggregate shows only your AS. This can break loop detection and hide the origin of the component routes. The `as-set` option preserves AS-path information by creating an AS\_SET (unordered set of all ASes in the component routes): ``` R1-HQ(config-router)# aggregate-address 10.0.0.0 255.0.0.0 summary-only as-set ``` ``` R1-HQ# show ip bgp 10.0.0.0/8 BGP routing table entry for 10.0.0.0/8, version 30 Paths: (1 available, best #1, table default) {65002,65010}, (aggregated by 65001 1.1.1.1) 0.0.0.0 from 0.0.0.0 (1.1.1.1) Origin IGP, weight 32768, valid, aggregated, local, atomic-aggregate, best ``` The `{65002,65010}` is the AS\_SET - it tells downstream routers that ASes 65002 and 65010 are part of this aggregate. This preserves loop detection: if AS 65002 receives this route and sees itself in the AS\_SET, it rejects it. Warning: `as-set` means the aggregate's path attributes change whenever a component route is added or withdrawn. This can cause unnecessary churn - every component change triggers an UPDATE for the aggregate. Use judiciously. ## suppress-map Sometimes you want to suppress most component routes but keep advertising a few specific ones alongside the aggregate: ``` route-map SUPPRESS-MOST deny 10 match ip address prefix-list KEEP-ADVERTISING route-map SUPPRESS-MOST permit 20 ! ip prefix-list KEEP-ADVERTISING seq 10 permit 10.1.1.0/24 ! R1-HQ(config-router)# aggregate-address 10.1.0.0 255.255.0.0 suppress-map SUPPRESS-MOST ``` This suppresses all component routes EXCEPT 10.1.1.0/24 (denied by the suppress-map means "don't suppress this one"). Both the /16 aggregate and the /24 specific are advertised. Other /24s within 10.1.0.0/16 are suppressed. ## attribute-map and advertise-map `attribute-map` lets you set specific attributes on the aggregate (e.g., community, MED): ``` route-map AGG-ATTRIBUTES permit 10 set community 65001:500 ! R1-HQ(config-router)# aggregate-address 10.1.0.0 255.255.0.0 summary-only attribute-map AGG-ATTRIBUTES ``` `advertise-map` controls which component routes are used to generate the aggregate - only routes matching the advertise-map are considered as components: ``` route-map ONLY-INTERNAL permit 10 match ip address prefix-list INTERNAL-NETS ! R1-HQ(config-router)# aggregate-address 10.1.0.0 255.255.0.0 summary-only advertise-map ONLY-INTERNAL ``` ## Aggregation vs network Command with Null Route There's another common approach to advertising a summary: create a static null route and use the `network` command: ``` ip route 10.1.0.0 255.255.0.0 Null0 ! router bgp 65001 network 10.1.0.0 mask 255.255.0.0 ``` Differences: Component routes required aggregate-address Yes - at least one must be in BGP table network + null route No - just needs the static route in IP RIB Suppress components aggregate-addressYes (summary-only) network + null route No - must filter separately AS-SET support aggregate-addressYes network + null routeNo Origin aggregate-addressIGP (aggregated) network + null routeIGP Stability aggregate-address Withdrawn if all components disappear network + null route Always present (null route is always up) For advertising your own address space to the internet, the null route approach is often preferred because it's unconditionally stable - the route exists as long as the static route is configured, regardless of what's happening with more-specific routes in the BGP table. ## Verification ``` R1-HQ# show ip bgp 10.1.0.0/16 BGP routing table entry for 10.1.0.0/16, version 28 Paths: (1 available, best #1, table default) Advertised to update-groups: 1 2 Local, (aggregated by 65001 1.1.1.1) 0.0.0.0 from 0.0.0.0 (1.1.1.1) Origin IGP, weight 32768, valid, aggregated, local, best Aggregator: 65001 1.1.1.1 R1-HQ# show ip bgp neighbors 172.16.0.2 advertised-routes | include 10.1 *> 10.1.0.0/16 0.0.0.0 32768 i ``` Only the /16 aggregate appears in advertised-routes - component /24s are suppressed. ## Troubleshooting Aggregate not appearing in BGP table Cause No component routes in the BGP table within the aggregate range Fix Verify component routes exist with `show ip bgp longer-prefixes [aggregate]`. Ensure they're in the BGP table, not just the IP routing table. Component routes still being advertised despite summary-only Cause `summary-only` keyword missing, or suppress-map permitting them Fix Verify config: `show run | section aggregate`. Check for suppress-map exceptions. Aggregate flapping (appearing/disappearing) Cause Component routes are unstable - when the last one withdraws, the aggregate disappears Fix Use the null route + `network` approach for unconditional stability, or ensure component routes are stable. ## Key Takeaways - `aggregate-address` creates a BGP summary from existing more-specific routes in the BGP table. Use `summary-only` to suppress components. - `as-set` preserves AS-path information in the aggregate - important for loop prevention when aggregating multi-AS routes. - Use `suppress-map` to selectively keep advertising specific components alongside the aggregate. - For unconditional route stability, consider a null route + `network` command instead of aggregate-address. - Aggregating reduces global routing table size - advertise the shortest prefix your allocation supports, and suppress more-specifics unless you have a traffic engineering reason to advertise them.