Followings are the example of the local-as and its option in BGP.
R1 = AS 100
R2 = AS 200
R3 = AS 300
Configure the network to have R1 thinks that R2 is in AS 65001 (Use local-AS)
R1
router bgp 100
no synchronization
bgp log-neighbor-changes
network 1.1.1.0 mask 255.255.255.0
network 2.2.2.0 mask 255.255.255.0
network 3.3.3.0 mask 255.255.255.0
neighbor 192.168.1.2 remote-as 65001
no auto-summary
Local AS
R2
router bgp 200
no synchronization
bgp log-neighbor-changes
neighbor 192.168.1.1 remote-as 100
neighbor 192.168.1.1 local-as 65001
neighbor 192.168.2.2 remote-as 300
no auto-summary
R3#sh ip bgp
BGP table version is 46, local router ID is 192.168.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.0/24 192.168.2.1 0 200 65001 100 i
*> 2.2.2.0/24 192.168.2.1 0 200 65001 100 i
*> 3.3.3.0/24 192.168.2.1 0 200 65001 100 i
Local AS with no prepend
R2
router bgp 200
no synchronization
bgp log-neighbor-changes
neighbor 192.168.1.1 remote-as 100
neighbor 192.168.1.1 local-as 65001 no-prepend
neighbor 192.168.2.2 remote-as 300
no auto-summary
R3#sh ip bgp
BGP table version is 40, local router ID is 192.168.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.0/24 192.168.2.1 0 200 100 i
*> 2.2.2.0/24 192.168.2.1 0 200 100 i
*> 3.3.3.0/24 192.168.2.1 0 200 100 i
With the no prepend and replace-as, (neighbor to R3) R2 will not prepend the local as but instead replace the real AS with the local as value and send an update to R3. Therefore, R3 will see 65001 (local-as value) and 100 (AS at R1)
Local AS with no prepend & replace as
R2
router bgp 200
no synchronization
bgp log-neighbor-changes
neighbor 192.168.1.1 remote-as 100
neighbor 192.168.1.1 local-as 65001 no-prepend replace-as
neighbor 192.168.2.2 remote-as 300
neighbor 192.168.2.2 local-as 65001 no-prepend replace-as
no auto-summary
R3#sh ip bgp
BGP table version is 28, local router ID is 192.168.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.0/24 192.168.2.1 0 65001 100 i
*> 2.2.2.0/24 192.168.2.1 0 65001 100 i
*> 3.3.3.0/24 192.168.2.1 0 65001 100 i
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