In order to allow same AS prefix to be received from eBGP neighbor and installed in BGP table, R1 can use the "allowas-in" command will as below:
R1(config)#router bgp 123
R1(config-router)#address-family ipv4
R1(config-router-af)#neighbor 10.0.0.2 allowas-in
Same configuration should be applied to R3:
R3(config)#router bgp 123
R3(config-router)#address-family ipv4
R3(config-router-af)#neighbor 192.168.1.2 allowas-in
Note: In the second version of this sim, only R2 can be accessible so the above commands cannot be applied to R1 & R3. In that case we have to use "as-override" on R2 as follows:
R2(config)#router bgp 456
R2(config-router)#address-family ipv4
R2(config-router-af)#neighbor 10.0.0.1 as-override
R2(config-router-af)#neighbor 192.168.1.3 as-override
With "as-override" configured, R3 would accept routes advertised from R1 (and vice versa) as the AS 123 was replaced by AS 456 in the AS_PATH. R3 will see the AS_PATH "456 456 i" (instead of "123 456 i"). Since the AS_PATH does not contain its own AS number so it accepts the advertised routes.
You can practice this version (AS-Override) with our own simulator at BGP Configuration online simulator v2. This version does not work with EVE-NG as AS-Override feature requires a VRF so we did not create a lab file in EVE-NG.
Verification
R1,R3#show ip bgp -> see Loopback0 of remote routers.
R1 must ping Loopback0 of R3 successfully.
Save the configuration
Don't forget to save the configuration on R1 & R3
R1, R3#copy running-config startup-config
In the second version, only R2 was configured so save its configuration: R2#copy running-config startup-config