[j-nsp] MTU changes affecting BGP sessions

Jonathan Looney jonlooney at gmail.com
Wed Jul 26 10:53:36 EDT 2006

If your router is trying to send frames larger than the actual MTU
supported by the infrastructure or remote side, the frames will simply
be dropped.  It seems likely your initial update packets are being
dropped and never received by the remote side.  This would cause your
session to timeout.  The fact that snd_nxt=snd_una in the log message
adds credence to this theory, since I believe that usually indicates
you are trying to retransmit unacknowledged frames.


On 7/26/06, Raniery Pontes <raniery at rnp.br> wrote:
> Hi everybody,
>   I´d like to know if someone has seen the following issue.
> I´ve got a M320 (Junos 7.5R2.8) with a few BGP peerings. These peerings
> run on vlans over a L2 infrastructure built on Ethernet switches (Cisco
> and Foundry). Some of these switches are configured to support jumbo
> frames, around 9000 bytes, some are not yet.
> Then I changed physical MTU in the M320 GigE interface to 9000 bytes.
> Most  peerings just flapped and came up fine, but one started to flap
> continuously. This one stays up for about 90seg (bgp holdtime?) and goes
> down again.
> I´ve tried "mtu-discovery" bgp command on my side, but didn´t help.  No
> clue about the other side, so far.
> Did anyone see this kind of thing? Hints about a solution?
> Another interesting fact : there is another BGP peer (multihop 2) in
> this same problematic vlan, and it works fine after mtu change. But I´m
> almost sure it´s a Juniper, so no interop issues ;)
> Raniery Pontes
> -----------------------------------------
> Log example:
> Jul 26 10:57:30  jm320_sp rpd[3001]: bgp_event: peer xxx (External AS
> xxx) old state OpenConfirm event RecvKeepAlive new state Established
> Jul 26 10:59:00  jm320_sp rpd[3001]: bgp_traffic_timeout: NOTIFICATION
> sent to xxx (External AS xxx): code 4 (Hold Timer Expired Error),
> Reason: holdtime expired for xxx (External AS xxx), socket buffer sndcc:
> 0 rcvcc: 0 TCP state: 4, snd_una: 3621665251 snd_nxt: 3621665251
> snd_wnd: 16200 rcv_nxt: 3247700880 rcv_adv: 3247759208, keepalive timer 0
> Jul 26 10:59:00  jm320_sp rpd[3001]: bgp_event: peer xxx (External AS
> xxx) old state Established event HoldTime new state Idle
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