This attribute specifies an EGP autonomous system number.
When this value is non-zero, the protocol attribute must be set to
either egp or to all. And this more specific route will be advertised
together with the aggregated route only if it is learned through egp
and this attribute matches the EGP autonomous system number
where the network is learned from.
The default value 0 indicates that the EGP autonomous system
number is not specified.