Clearing hung TCP session on a Cisco router

R1#debug ip tcp trans
TCP special event debugging is on
R1#term mon
R1#
messages
connection queue limit

2) Take a look at: 

R1#sh tcp brief
TCB       Local Address   Foreign Address        (state)
6353F5E8 10.10.3.5.1720   10.11.3.24.12871      SYNRCVD
63555A14 10.10.3.5.1720   10.11.3.23.12814      SYNRCVD
6353AEFC 10.10.3.5.1720   10.11.3.24.12872      SYNRCVD
6350B2DC 10.10.3.5.1720   10.11.3.24.12875      SYNRCVD
63488D44  10.10.3.5.23    10.11.3.23.11265      ESTAB
63571718 10.10.3.5.1720   10.11.3.24.12914      SYNRCVD

All of the TCB's associated with TCP port 1720 are 'hung' (SYNRCVD).
The associated TCP Control Block (TCB) are highlighted in a PINKISH colour.

3) To clear them, clear the associated TCB:
R1#clear tcp tcb 6353F5E8
[confirm]y
[OK]
R1#
R1#clear tcp tcb 63555A14
[confirm]y
[OK]
R1#
R1#clear tcp tcb 6353AEFC
[confirm]y
[OK]
R1#
R1#clear tcp tcb 6350B2DC
[confirm]y
[OK]
R1#
R1#clear tcp tcb 63571718
[confirm]y
[OK]
R1#