[c-nsp] MTU / BGP

[CiscoNSP List]

Thanks James!


Well...I was on the right track with MTU, and disabling path discovery..but, Id only ever soft reset the sessions...hard reset of the bgp session was needed(As James suggested below), and I have one of the sessions up for 7 minutes....so ill do the other now.


Thanks to all that responded....docco probably states somewhere that a hard reset is needed, but certainly wasnt obvious to my tired eyes lol!


Side note - It would be nice to be able to set mtu per neighbour purely for bgp (i.e. not have to change the physical Interface, or disable path mtu discovery)...


Cheers.


________________________________________
From: cisco-nsp <cisco-nsp-bounces at puck.nether.net> on behalf of James Bensley <jwbensley at gmail.com>
Sent: Monday, 20 July 2015 8:25 PM
To: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] MTU / BGP

On 20 July 2015 at 07:47, CiscoNSP List <CiscoNSP_list at hotmail.com> wrote:
> Hi Everyone,
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> Have a new POP, 2 x ASR1000's (Acting as RR's), and 2 x ME3600's - We have another POP with 2 x ASR1000's also acting as RR's.
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> The 2 "new" ME's are peered to the "new" ASR's RR's, but if I try to establish BGP  session with the existing RR's, session establishes, but then drops on both ME's after ~4minutes...I *think* it's due to MTU, and have tried disabling path mtu discovery, but it doesnt appear to do anything? (also tried it with it enabled, and the results are the same)
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> i.e. no neighbour xxx.xxx.xxx.xxx transport path-mtu-discovery on the ME's, I see
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> Datagrams (max data segment is 9036 bytes):
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> On the ASR RR's I see
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> Cheers.
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This should be easy to test for.

With the BGP sessions configured normally check the negotiated MSS
between the BGP peers, on the ME run:

device1#show bgp ipv4 uni neig 1.1.1.1 | i Datagram
Datagrams (max data segment is 1536 bytes):

Now on that ME configure:

router bgp 12345
 neighbor 1.1.1.1 transport path-mtu-discovery disable

Hard clear the TCP session then check again:

device1#show bgp ipv4 uni neig 1.1.1.1 | i Datagram
Datagrams (max data segment is 536 bytes)

If the BGP sessions is timing out with the lower MTU then its not the problem.

BGP normally uses the size of the egress interface towards the peer to
set the maximum segment size for the BGP sessions (less 40 bytes of
overhead for IP and TCP).

Cheers,
James.
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