Two routers directly connected using network 10.0.0.0/8. R1 S1/0 is 10.0.0.1/8 and R2 s1/0 is 10.0.0.2/8. Both are using the same AS 1.

Symptom:
- BGP state keeps on ACTIVE.
- Unable to ping each other interface.
- Interface is UP/UP

Verification:
- Check reachability.

R1#ping 10.0.0.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds:
U.U.U
Success rate is 0 percent (0/5)

- Check TCP transactions packets.

R1# debug ip tcp transactions
*Mar  1 00:30:31.303: TCB64D6711C created
*Mar  1 00:30:31.303: TCB64D6711C setting property TCP_WINDOW_SIZE (0) 65CA1830
*Mar  1 00:30:31.303: TCB64D6711C setting property TCP_MD5KEY (5) 0
*Mar  1 00:30:31.303: TCB64D6711C setting property TCP_TOS (11) 65CA1810
*Mar  1 00:30:31.303: TCP: Random local port generated 27054, network 0
*Mar  1 00:30:31.303: TCB64D6711C bound to 10.0.0.1.27054
*Mar  1 00:30:31.307: Reserved port 27054 in Transport Port Agent for TCP IP type 1
*Mar  1 00:30:31.307: TCP: sending SYN, seq 3525721113, ack 0
*Mar  1 00:30:31.307: TCP0: Connection to 10.0.0.2:179, advertising MSS 1460
*Mar  1 00:30:31.307: TCP0: state was CLOSED -> SYNSENT [27054 -> 10.0.0.2(179)]
*Mar  1 00:30:31.323: TCP0: ICMP destination unreachable received
*Mar  1 00:30:31.323: TCP0: state was SYNSENT -> CLOSED [27054 -> 10.0.0.2(179)]
*Mar  1 00:30:31.323: Released port 27054 in Transport Port Agent for TCP IP type 1 delay 240000
*Mar  1 00:30:31.327: TCB 0x64D6711C destroyed

- Check interface status.

R1#sh ip int brief s1/0
Interface                  IP-Address      OK? Method Status                Protocol
Serial1/0                  10.0.0.1        YES manual up                    up

Diagnose:
- Neighbor is unreachable
- From debug ip tcp transactions. It seems that the TCP SYN was sent but the destroyed. Need to check ACL on Neighbor router.