В статье "Traffic Engineering: Specific Routes" описано как прописывается нестандартная маршрутизация на Cisco при использовании BGP. Ниже небольшой пример из статьи:

!
router bgp 60055
 network 220.37.0.0 mask 255.255.240.0
 network 220.37.0.0 mask 255.255.248.0
 network 220.37.8.0 mask 255.255.248.0
 neighbor 192.0.254.17 remote-as 40077
 neighbor 192.0.254.17 description BGP session to ISP A
 neighbor 192.0.254.17 prefix-list ispa-ms out
 neighbor 219.2.19.1 remote-as 50066
 neighbor 219.2.19.1 description BGP session to ISP B
 neighbor 219.2.19.1 prefix-list ispb-ms out
!
ip route 220.37.0.0 255.255.240.0 Null0
ip route 220.37.0.0 255.255.248.0 Null0
ip route 220.37.8.0 255.255.248.0 Null0
!
ip prefix-list ispa-ms description outbound filter for ISP A
ip prefix-list ispa-ms seq 5 permit 220.37.0.0/20
ip prefix-list ispa-ms seq 10 permit 220.37.0.0/21
ip prefix-list ispa-ms seq 15 deny   220.37.8.0/21
ip prefix-list ispb-ms description outbound filter for ISP B
ip prefix-list ispb-ms seq 5 permit 220.37.0.0/20
ip prefix-list ispb-ms seq 10 deny   220.37.0.0/21
ip prefix-list ispb-ms seq 15 permit 220.37.8.0/21
!