BGP "update-source" command odd behavior

[bassomarco1998]

Hello everyone,
I was setting up a simple lab and came across what, in my opinion, is an anomalous behavior. Below is the photo of the topology.

So, I have 2 routers, R1 and R3, with their configuration provided below. I tried to create a BGP session between the routers' loopback interfaces. However, on R1, I omitted the

update-source

command. Therefore, I would expect the session not to be established. Instead, it comes UP correctly.

R1:
router bgp 65000
 bgp log-neighbor-changes
 neighbor 3.3.3.3 remote-as 65100
 neighbor 3.3.3.3 ebgp-multihop 255

R3:
router bgp 65100
 bgp log-neighbor-changes
 neighbor 1.1.1.1 remote-as 65000
 neighbor 1.1.1.1 ebgp-multihop 255
 neighbor 1.1.1.1 update-source Loopback1

I tried capturing packets and performed a clear first on R1 and then on R3. I'm attaching the respective screens below.

When I perform the clear on R1, after terminating the session, R1 attempts to re-establish the session using the source IP associated with the physical interface. Obviously, R3 responds with a reset. The strange thing is that then R3 actively tries to open the session, and R1 responds using the source IP associated with its loopback. How do you explain this behavior if I haven't used the

update-source

command on R1?

If I instead clear on R3, as in the previous case, R3 actively attempts to open a BGP session, and R1 responds using the source IP associated with its loopback.

What do you think?"

[MHM Cisco World · Accepted solution]

Hi, it normal, if R3 initiate the traffic to R1 using it LO as update then the BGP will form 
if the R1 initiate to R3 using is interface IP then the BGP will never form 
try add access list make deny traffic from R3 toward R1 using tcp port 179 and see the different 
NOTE:- the ACL must config in F1/0 with direction IN 

deny tcp host 3.3.3.3 host 1.1.1.1 eq 179
permit ip any any 

MHM