Compatible Systems 5.4 - Manual (Page 281)

Chapter 16 - BGP 275
Subnet Mask
The optional mask parameter tells the router how many bits of the IP routing
table entry to match against the IP address listed in the BGP Network. If a
mask is not provided, an all 255’s mask will be assumed.
v Note: This is not necessarily the actual mask of the network you wish to
advertise because subnet masks more specific than Class C are automatically
truncated. This truncation is not the same as aggregation, and only applies to
internal networks, and only to masks more specific than Class C.