To share what I've learned !

according to my experience we can't but similar functionalities given by Local AS option . It enables to act one AS for some of the neighbors and another AS for other neighbors R2 connects to R1 using remote as 100 , but the R1 using remote-as as 2 rather than 300 R2# router bgp 300 no synchronization bgp log-neighbor-changes network 200.200.200.0 neighbor 192.168.100.1 remote-as 100 neighbor 192.168.100.1 local-as 2 R1#router bgp 100 no synchronization bgp log-neighbor-changes neighbor 192.168.100.2 remote-as 2 no auto-summary but when the route injected it shows originated AS as 300 R1#show ip bgp BGP table version is 2, local router ID is 192.168.100.1 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 *> 200.200.200.0 192.168.100.2 0 0 2 300 i with no-p...

this lab is directly taken from BGP Design & Implementation Chap 4. GNS3 configurations attached below. Summary - In the boarder router if the summary route injected as follows aggregate-address 172.16.0.0 255.255.0.0 as-set summary-only downwards the originality of the prefix may be lost therefore to specify the best exist path we can regenerate the path we use inject path . bgp inject-map Map1 exist-map Map2 Map1 injects the path Map2 checks whether path is available , it at least two match statements one is route-source & aggregate prefix. Whether u can inject weird prefix other than aggregate .. (eg aggregate is 172.16.0.0/16 but if you try to inject 10.0.0.0/24 ???) As usual you can't :) following is the attached diagram R5 Relevant configuration. router bgp 100 no synchronization bgp log-neighbor-changes bgp inject-map AS200-Specific exist-map AS200-aggregate neighbor 192.168.12.2 remote-as 100 neighbor 192.168.12.2 send-community neighbor 192.168.23....