To configure a summary address for BGP, use the aggregate-address command in Router Configuration mode. No aggregate addresses are configured by default. Unless the options are specified, the aggregate is advertised with the ATOMIC_AGGREGATE attribute and an empty AS path, and the more specific routes are advertised along with the aggregate.
To be considered a match for an aggregate address, a prefix must be more specific (i.e. have a longer prefix length) than the aggregate address. A prefix whose prefix length equals the length of the aggregate address is not considered a match.
When BGP originates a summary address, it installs a reject route in the common routing table for the summary prefix. Any received packets that match the summary prefix, but not a more specific route, match the reject route and are dropped.
BGP accepts up to 128 summary addresses for each address family.
| Default | No aggregate addresses are configured by default. Unless the options are specified, the aggregate is advertised with the ATOMIC_AGGREGATE attribute and an empty AS path, and the more specific routes are advertised along with the aggregate. |
| Format | aggregate-address {address mask|ipv6-prefix/pfx-len} [as-set] [summary-only] |
| Mode | BGP Router Config |
| Parameter | Description |
|---|---|
| address mask | Summary IPv4 prefix and mask. The default route (0.0.0.0 0.0.0.0) cannot be configured as an aggregate-address. The mask cannot be a 32-bit mask (255.255.255.255). The combination of prefix and mask must be a valid unicast destination prefix. |
| ipv6-prefix/pfx-len | Summary IPv6 prefix and prefix length. The range for prefix length is 1 to 127. |
| as-set | (Optional) Normally, the aggregate is advertised with an empty AS path and the ATOMIC_AGGREGATE attribute. If the as-set option is configured, then the aggregate is advertised with a nonempty AS_PATH. If the AS_PATH of all contained routes is the same, then the AS_PATH of the aggregate is the AS_PATH of the contained routes. Otherwise, if the contained routes have different AS_PATHs, the AS_PATH attribute includes an AS_SET with each of the AS numbers listed in the AS PATHs of the aggregated routes. If the as-set option is not configured, the aggregate is advertised with an empty AS_PATH. |
| summary-only | (Optional) When the summary-only option is given, the more-specific routes within the aggregate address are not advertised to neighbors. |
