How eNodeB knows to which MME it should send the request when eNodeB is connected to pool of MME?

Question

What is the eNB behavior in-case MME is not responding to S1 setup request message (ie. MME is not sending S1 setup failure or success).

Answer 1

I think this is implementation specific. Usually, eNB selects the MME either in round-robin manner or uses some load-balancing algorithm, e.g., weight-factor algorithm.

Answer 2

If I got your question then your query is about what things are considered while sending "Initial UE Message" to a particular MME when an eNodeB is already connected to multiple MMEs of a pool.

If you go through the content of RRC Connection Setup Complete message (3GPP TS 36.331), it may contain "RegisteredMME" information element. When eNodeB receives this optional IE (Registered MME) within the RRC Connection Setup Complete message, it sends Initial UE message (3GPP TS 36.413) to MME based on this registered MME information. If Registered MME information is not received then eNodeB does load balancing , selects a MME and forward Initial UE message to the selected MME.