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Jumbo frame support with BGP MSS size

Eric Guo
Level 1
Level 1

‎12-10-2013 06:18 AM

Hi All,

I am working at small SP. we are going to enable Jumbo Frame support from end to end. Our core segment have MPLS cloud and packet size could be able support  up over 9000 already. Since our Core segment router  are running pure BGP with our edge/access segment router , when I enable jumbo frame support on their  interface level, I still can see BGP MSS size is 1260 right now. so my question, do I need  increase BGP MSS size between our core router  and edge router for transiting our SP cloud  traffic packet ?

many thanks!

Eric

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5 Replies 5

Harold Ritter
Cisco Employee
Cisco Employee

‎12-10-2013 07:33 AM

Hi Eric,

You need to configure "tcp path-mtu-discovery", which is off by default.

Regards

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
0 Helpful

Eric Guo
Level 1
Level 1
In response to Harold Ritter

‎12-10-2013 08:19 AM

Hi Harold,

It makes sense, and is matching what I am thinking.

Many thanks for your help !

Eric

0 Helpful

Eric Guo
Level 1
Level 1
In response to Eric Guo

‎12-10-2013 08:59 AM

hi Harold,

I have other question about MSS size for IPv4 and IPv6 BGP session, if the physical link MTU size is 1500 ( same as 1514 on ASR 9K platform), why IPv6 BGP MSS is 1240 and IPv6 BGP MSS is 1220? As I only understand IPv4 headr 20, tcp header 20, but didn't match these MSS size number, I am sure I mis-understand some value in between, could you please let me know how we get 1240 and 1220?

many thanks!

Eric

0 Helpful

Harold Ritter
Cisco Employee
Cisco Employee
In response to Eric Guo

‎12-10-2013 01:16 PM

Hi Eric,

If tcp PMTUD is disabled and the session is not to a directly connected interface, the path mtu is set to the minimum MTU defined for ipv6 (1280). From that value, you substract 40 bytes for the ipv6 header and 20 bytes for the tcp header, hence the 1220 value that you saw for your ipv6 tcp session.

Regards

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
0 Helpful

Eric Guo
Level 1
Level 1
In response to Harold Ritter

‎12-11-2013 08:01 AM

I got you.

Thanks Harlod again!

Eric

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