1. A peering session to another BGP speaking router is initiated in the config router bgp section:

router bgp 10143

neighbor x.x.92.57 remote-as nnn

2. The filter-list command is used to control which ASN's the router will accept and send:

router bgp 10143

neighbor x.x.92.57 remote-as nnn

neighbor x.x.92.57 filter-list 40 in

neighbor x.x.92.57 filter-list 3 out

2.1 The filter-list command reads the corresponding ip as-path access-list:

ip as-path access-list 3 permit ^$

ip as-path access-list 40 permit ^nnn_

Ip as-path access lists use unix-like regular expressions to match ASN's. ^$ matches the start and end of a line with nothing in it. ^nnn_ matches a line starting with nnn and anything following it.

3. The route-map command is used to apply one or more route-map rules to network addresses the router will accept or send. In most cases, it is used to control specific IP address blocks:

router bgp 10143

neighbor x.x.92.57 remote-as nnn

neighbor x.x.92.57 remove-private-as

neighbor x.x.92.57 filter-list 40 in

neighbor x.x.92.57 filter-list 3 out

neighbor x.x.92.57 route-map bgp-full-default-in in

neighbor x.x.92.57 route-map bgp-peer1-out out

3.1 The route-map command reads the corresponding route-map rules in numerical order:

route-map bgp-peer1-out permit 10

match ip address prefix-list bgp-peer1-out

route-map bgp-peer1-out permit 20

match ip address prefix-list bgp-peer1-out-prepend1

set as-path prepend 10143

route-map bgp-peer1-out permit 30

match ip address prefix-list bgp-peer1-out-prepend2

set as-path prepend 10143 10143

if the route-map instruction is 'permit', it will allow any network permitted in the prefix-list to be announced by bgp. If the instruction is 'deny' it will block any matching network permitted in the prefix-list.

3.2 The ip address prefix-list command reads the corresponding ip prefix-list of network addresses:

ip prefix-list bgp-peer1-out seq 1 permit x.x.128.0/17

ip prefix-list bgp-peer1-out seq 2 permit x.x.0.0/17

ip prefix-list bgp-peer1-out seq 3 permit x.x.0.0/17

ip prefix-list bgp-peer1-out seq 9 permit x.x.64.0/18

ip prefix-list bgp-peer1-out seq 19 permit x.x.0.0/24

ip prefix-list bgp-peer1-out seq 20 permit x.x.1.0/24

ip prefix-list bgp-peer1-out seq 21 permit x.x.5.0/24

ip prefix-list bgp-peer1-out seq 22 permit x.x.6.0/24

ip prefix-list bgp-peer1-out seq 23 permit x.x.7.0/24

ip prefix-list bgp-peer1-out seq 24 permit x.x.8.0/24

ip prefix-list bgp-peer1-out seq 100 permit x.x.236.0/24

....

ip prefix-list bgp-peer1-out seq 1000 deny 0.0.0.0/0 le 32

The last line of this prefix-list will match any network of any netmask length. In other words, any network not explicitly permitted, will be denied.

4. To summarize, in BGP:

* The filter-list command uses an ip as-path access-list to permit or deny AS numbers

* The route-map command uses a route-map apply route-map rules

* The route-map itself uses one or more ip address prefix-list to permit or deny network address blocks