From the BGP config bellow, what is this aggregate-address do?
Also, this is suppose to advertise 16 class C's, yet I only see 3 being advertised... ?!? Confused...
router bgp 17xxx
aggregate-address xxx.xxx.224.0 255.255.255.0
aggregate-address xxx.xxx.239.0 255.255.255.0
neighbor xxx.xxx.212.241 remote-as 701
neighbor xxx.xxx.212.241 description MCI_link
neighbor xxx.xxx.212.241 version 4
neighbor xxx.xxx.212.241 soft-reconfiguration inbound
neighbor xxx.xxx.212.241 distribute-list 199 in
neighbor xxx.xxx.212.241 distribute-list 101 out
In determining what has precedence there are 2 factors to consider: prefix length, and administrative distance.
Most people are pretty familiar with administrative distance as a way to determine precedence in which locally connected routes with AD of 0 are the best, and static routes (with default AD of 1) are next best. BGP (with default AD of 20 for EBGP) is next and has precedence over EIGRP (AD of 90) and RIP (AD 0f 120).
But prefix length is more important than AD. So if the ISP has a static route for 220.127.116.11/20 with AD of 1 pointing to router 3 and if it receives an advertisement for 18.104.22.168/24 with AD of 20 pointing to router 1, then the longer prefix (of /24) takes precedence over the AD (of 1 or 20) and packets would be routed to router 1.