[j-nsp] MTU changes affecting BGP sessions

Raniery Pontes raniery at rnp.br
Wed Jul 26 10:14:43 EDT 2006

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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