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Community Member

BGP Datagram size

Dear all,

on one of my router(MPLS Service) the BGP datagram size shows as

Datagrams (max data segment is 516 bytes)

and on another router(ISP Service) it shows

Datagrams (max data segment is 536 bytes):

Can anybody tell me the reason of such difference

Regards

Mahesh

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: BGP Datagram size

The mtu of 516 is most likely due to the fact that you use MD5 authentication on this session. MD5 authentication adds 20 bytes of overhead in the TCP header, hence the MSS being reduced by 20 bytes.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
7 REPLIES

Re: BGP Datagram size

536 is the default MSS (max segment size) for a TCP connection which is Very small considering the amount of data we transfer.

This can cause delays in BGP convergence.

Enable path MTU discovery and clear the BGP session

ip tcp path-mtu-discovery

clear ip BGP *

check the size now by

show ip bgp neighbor | include max data segment

http://www.cisco.com/en/US/products/ps6922/products_feature_guide09186a0080666002.html

HTH

Narayan

Community Member

Re: BGP Datagram size

I accept ,

But my basic question is on one of my router it shows 516 and another router it shows 536. why it is so?

i will enable mtu discovery once i take approval from customers..

so please provide me details of why it is 516 if document tells that default is 536

regards!!

Re: BGP Datagram size

Mahesh,

Have you specified any MTU vlaue on any interface?

Narayan

Community Member

Re: BGP Datagram size

Dear Narayan,

I have not specified any MTU on interface

Regards!!

Cisco Employee

Re: BGP Datagram size

The mtu of 516 is most likely due to the fact that you use MD5 authentication on this session. MD5 authentication adds 20 bytes of overhead in the TCP header, hence the MSS being reduced by 20 bytes.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Community Member

Re: BGP Datagram size

Thanks hritter and narayan for your extended support.

Regards!!

Mahesh

Re: BGP Datagram size

Harold

Thanks for the information. Never thought this could be the reason

Narayan

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