How to configure BGP AS_Path with regular Expressions on Cisco routers?
BGP AS_PATH Attribute
AS_PATH filtration basically used to filter the BGP route to control the network traffic. For example:
_99$ = Routes having as-path value ends with 99
^10_ = Routes having as-path value starts with 10
_88_ = Routes having as-path value 88, either at start or at end or in middle
.* = Routes having any value of as-path
RUNNING RIP ON ROUTER-A
A(config)# router rip
A(config-router)# network 200.100.50.0
A(config-router)# network 1.0.0.0
RUNNING EBGP ON ROUTER-A
A(config)# router bgp 10
A(config-router)# no synchronization
A(config-router)# neighbor 1.1.1.2 remote-as 20
A(config-router)# neighbor 4.1.1.1 remote-as 20
A(config-router)# network200.100.50.0
RUNNING EBGP ON ROUTER-B
B(config)# router bgp 20
B(config-router)# no synchronization
B(config-router)# neighbor 1.1.1.1remote-as 10
RUNNING EBGP ON ROUTER-D
D(config)# router bgp 20
D(config-router)# no synchronization
D(config-router)# neighbor 4.1.1.2 remote-as 10
RUNNING OSPF ON ROUTER-B
B(config)# router ospf 1
B(config-router)# network 2.0.0.0 0.255.255.255 area 0
B(config-router)# network 1.0.0.0 0.255.255.255 area 0
RUNNING OSPF ON ROUTER-C
C(config)# router ospf 1
C(config-router)# network 2.0.0.0 0.255.255.255 area 0
C(config-router)# network 3.0.0.0 0.255.255.255 area 0
RUNNING OSPF ON ROUTER-D
D(config)# router ospf 1
D(config-router)# network 3.0.0.0 0.255.255.255 area 0
D(config-router)# network 4.0.0.0 0.255.255.255 area 0
RUNNING IBGP ON ROUTER-B
B(config)# router bgp 20
B(config-router)# no synchronization
B(config-router)# neighbor 4.1.1.1 remote-as 20
B(config-router)# neighbor 2.1.1.2 remote-as 20
RUNNING IBGP ON ROUTER-C
C(config)# router bgp 20
C(config-router)# no synchronization
C(config-router)# neighbor 2.1.1.1 remote-as 20
C(config-router)# neighbor 3.1.1.2 remote-as 20
RUNNING IBGP ON ROUTER-D
D(config)# router bgp 20
D(config-router)# no synchronization
D(config-router)# neighbor 3.1.1.1 remote-as 20
D(config-router)# neighbor 2.1.1.1 remote-as 20
DEFINING 4 STATIC ROUTES ON ROUTER-A
A(config)# ip route 192.1.18.0 255.255.255.0 null 0
A(config)# ip route 192.1.19.0 255.255.255.0 null 0
A(config)# ip route 192.1.20.0 255.255.255.0 null 0
REDISTRIBUTION OF STATIC ROUTES INTO BGP 10 on ROUTER-A
A(config)# router bgp 10
A(config-router)# redistribute static
CREATING ACCESS-LIST ON ROUTER-A
A(config)# access-list 51 permit 200.100.50.0 0.0.0.255
A(config)# access-list 51 permit 192.1.17.0 0.0.0.255
A(config)# access-list 51 permit 192.1.18.0 0.0.0.255
A(config)# access-list 52 permit 192.1.19.0 0.0.0.255
A(config)# access-list 52 permit 192.1.20.0 0.0.0.255
CREATING ROUTE-MAP (AD) ON ROUTER-A
A(config)# route-map AD permit 10
A(config-route-map)# match ip address 51
A(config-route-map)# set as-path prepend 33
A(config-route-map)# exit
A(config)# route-map AD permit 20
A(config-route-map)# match ip address 52
A(config-route-map)# set as-path prepend 88 99
CREATING ROUTE-MAP (AB) ON ROUTER-A
A(config)# route-map AB permit 10
A(config-route-map)# match ip address 51
A(config-route-map)# et as-path prepend 44 55
A(config-route-map)# exit
A(config)# route-map AB permit 20
A(config-route-map)# match ip address 52
A(config-route-map)# et as-path prepend 66
APPLYING ROUTE-MAP (AD & AB) ON ROUTER-A
A(config)# router bgp 10
A(config-router)# neighbor 4.1.1.1 route-map AD out
A(config-router)# neighbor 1.1.1.2 route-map AB out
D(config)# ip as-path access-list 5 deny _99$
D(config)# ip as-path access-list 5 permit .*
D(config)# router bgp 20
D(config)# neighbor 4.1.1.2 filter-list 5 in
D# clear ip bgp *
D# show ip bgp
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