[c-nsp] BGP convergence with jumbo frames

[Pete Kruckenberg]

Thanks for all the responses. Some clarifications:

The interface MTU (on both ends of the point-to-point fiber)
was set to 9000 bytes for all tests.

I adjusted "ip tcp mss" from 536 to 8636 (which IIRC was the
"max data segment" shown in the "show ip bgp neighbor"  
output prior to fiddling with anything). I double-checked
the "max data segment" on the BGP neighbor after each MSS
change to make sure it matched up.

MTU path discovery was on, and I double-checked the max data
segment under the show ip bgp neighbor. Though in this case 

I didn't play with "ip mtu" because I couldn't determine
from Cisco documentation if that affects just traffic
originating on the router or if it also affects traffic
passing through the router. These links have to support
9000-byte jumbo frames. The documentation for "ip tcp mss"
was more clear that it only affects router-originated
traffic.

Pete.

On Mon, 2 Aug 2004, Tantsura, Jeff wrote:

> Date: Mon, 2 Aug 2004 08:52:11 +0200
> From: "Tantsura, Jeff" <jeff.tantsura at capgemini.com>
> To: Pete Kruckenberg <pete at kruckenberg.com>, cisco-nsp at puck.nether.net
> Subject: RE: [c-nsp] BGP convergence with jumbo frames
> 
> 
> IP MTU ?
> 
> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net
> [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Pete Kruckenberg
> Sent: Monday, August 02, 2004 7:42 AM
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] BGP convergence with jumbo frames
> 
> Spent some time recently trying to tune BGP to get convergence down as
> far as possible. Noticed some peculiar behavior.
> 
> I'm running 12.0.28S on GSR12404 PRP-2.
> 
> Measuring from when the BGP session first opens, the time to transmit
> the full (~128K routes) table from one router to another, across a
> jumbo-frame (9000-bytes) GigE link, using 4-port ISE line cards (the
> routers are about 20 miles apart over dark fiber).
> 
> I noticed that the xmit time decreases from ~ 35 seconds with a 536-byte
> MSS to ~ 22 seconds with a 2500-byte MSS. 
> >From there, stays about the same, until I get to 4000, when
> it beings increasing dramatically until at 8636 bytes it takes over 2
> minutes.
> 
> I had expected that larger frames would decrease the BGP converence
> time. Why would the convergence time increase (and so significantly) as
> the MSS increases?
> 
> Is there some tuning tweak I'm missing here?
> 
> Pete.
> 
> 
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