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I have this iBGP session established between a Cisco ASR9K and a Cisco 7600 router.

On the ASR9K device, 'show tcp detail pcb all' displays 'MSS 1240, peer MSS 9152, min MSS 1240, max MSS 1240' for this specific iBGP connection.

Which is the actual TCP MSS value the ASR9K is using for that connection? Is it 1240 or 9152?

See more outputs below.

ASR9K#show tcp detail pcb all
(...snip...)
Datagrams (in bytes): MSS 1240, peer MSS 9152, min MSS 1240, max MSS 1240

7600#show ip bgp neighbors X.X.X.X | i Dat
Datagrams (max data segment is 1240 bytes):

7600#show tcp
(...snip...)
Datagrams (max data segment is 1240 bytes):
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    I'm not sure I understand the question. It tells you what the maximum is, and that would be the maximum used. Between two devices, the lowest MSS must be used. It looks like the 7600 has jumbo frames enabled, but the ASR MSS is much smaller, so the ASR MSS is used. – Ron Maupin Aug 19 '16 at 12:32
  • @RonMaupin I specifically want to confirm my reading that the ASR9K is actually using MSS=1240, and NOT 9152, for this connection. – Everton Aug 19 '16 at 12:34
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    It tells you that right there in what you posted: Datagrams (in bytes): MSS 1240, peer MSS 9152, min MSS 1240, max MSS 1240. – Ron Maupin Aug 19 '16 at 12:37
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    The MSS is negotiated when the TCP connection is negotiated. See RFC 793, Transmission Control Protocol, Section 3. FUNCTIONAL SPECIFICATION. – Ron Maupin Aug 19 '16 at 16:49
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    That would be basically how it happens.Most things actually use this, but it it is not required, and the RFC specifically allows using an MSS of any size, which wouldn't work in practice because the ASR couldn't deal with a frame the size that the 7600 could send, and the frames would be dropped. – Ron Maupin Aug 19 '16 at 17:13
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I have found an answer for this question:

> I would like to know how I must interpret the "show tcp detail pcb" and specifically the following line (IOS XR device): 
> 
> Datagrams (in bytes): MSS 1460, peer MSS 1460, min MSS 1946, max MSS 1946 
> 
> Is the IOS XR device using MSS 1460 or MSS 1946. 

Its using 1460. 

http://www.gossamer-threads.com/lists/cisco/nsp/186322#186322

Therefore, according my output (MSS 1240, peer MSS 9152, min MSS 1240, max MSS 1240), the ASR9K is limiting its outgoing TCP segments to 'MSS 1240'.

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