1、配ip
2、底层用rip和ospf
3、起bgp
4、选路
1）AS1去AS2走R1-R3，可以用Local_pref,即在R1上抓取AS2的流量，增大Local值，在R3的入向调用
[r1]ip ip-prefix 2 permit 100.2.3.3 32
[r1]ip ip-prefix 2 permit 100.2.4.4 32
[r1]route-policy b permit node 10
[r1-route-policy]if-match ip-prefix 2
[r1-route-policy]apply local-preference 120
[r1]route-policy b permit node 20
[r1-bgp]peer 10.1.13.2 route-policy b import
2）AS2去AS1走R4-R2，可以使用Med，在R3上抓取AS1的流量，增大MED值，
[r3]ip ip-prefix 1 permit 100.1.1.1 32
[r3]ip ip-prefix 1 permit 100.1.2.2 32
[r3]route-policy a permit node 10
[r3-route-policy]if-match ip-prefix 1
[r3-route-policy]apply cost 100
[r3]route-policy a permit node 20
[r3-bgp]peer 10.1.13.1 route-policy a import
3) AS1去AS3走R2-R6，使用as-path，在R2上，抓取AS3的流量，增加AS-path，
ip ip-prefix 3 index 10 permit 100.3.0.0 16 greater-equal 17 less-equal 32
route-policy c permit node 10
if-match ip-prefix 3
apply as-path 5 additive
[r2]route-policy c permit node 20
peer 10.1.25.2 route-policy c import
4）AS2去AS3走R4-R6，使用prefval，在R4上，抓取AS3的流量，增加PrefVal
[r4]ip ip-prefix 4 permit 100.3.0.0 16 greater-equal 17
[r4]route-policy d permit node 10
[r4-route-policy]if-match ip-prefix 4
[r4-route-policy]apply preferred-value 130
[r4-bgp]peer 10.2.46.2 route-policy d import
[r4]route-policy d permit node 20