Core Issue
These issues may prevent the Border Gateway Protocol (BGP) neighbors from being established:
- The neighbor IP address or Autonomous System (AS) number is incorrect.
- In a non-directly connected neighbor, no routes to the neighbor IP address exist, or the default route (0.0.0.0/0) is used to reach the peer.
- The neighbor update-source command is missing under the BGP.
- The typing error results in the wrong IP address or as-number in the neighbor statement. Check your configurations.
- The unicast is broken due to one of these reasons:
- The wrong Virtual Circuit (VC) maps in an ATM or Frame Relay environment and in a highly redundant network.
- The Access Control List (ACL) blocks the unicast or Transmission Control Protocol (TCP) packet at port 179.
- The Network Address Translation (NAT) runs on the router and translates the unicast packet.
Resolution
To resolve this issue, refer to Why Do BGP Neighbors Toggle Between Idle, Connect, and Active States?.
show ip bgp neighbors {neighbor address}
State is active