Flapping POS Interface on Frame-relay between a Juniper and Cisco

Jeff Saxe jsaxe at briworks.com
Mon Dec 5 19:20:56 CST 2011


Ah, memories are flooding back from my Voice over Frame Relay days on a Cisco MC3810. We would crush compressed G.729a voice and barely-enough data for 24 remote call center employees into, I believe, two quarter-T1 frame DLCI's to keep costs down. Anyway, I believe this is the explanation for your flapping: a PPP link is intended to go between two routers over, for instance, a private leased line, so both of the devices are peers, neither one particularly special. Frame-relay, by contrast, was originally designed so that your router was an "end user" device and its directly-connected partner device was not your other router, which you control, but the frame carrier's frame-relay switch. Your router was a DTE device, and their switch, which was in a more "important" position in control of the frame-relay NBMA cloud, was the DCE device. Your router then slaved to the frame switch via LMI signaling, so that the upstream switch instructed you which DLCIs existed and were up at the moment.

So if you connect up two routers with frame-relay encap and each thinks it is the DTE, and neither one is taking the role of the frame switch, then when you bring them up, they will initially optimistically think their DLCIs are up and working, and the routing protocol and traffic will come up... but both of them will be waiting for the frame switch to send them LMI indicating that their idea of the DLCI up/down status is correct. When a couple minutes go by and they don't hear the responses to their LMI enquiries, they will bring all the DLCI's down. I thought they would then stay down forever, i.e., not flap, but maybe you are shutting / no shutting the POS circuit to try again. Anyway, I believe the very simple fix is

interface POS0/0/0
frame-relay intf-type dce

So this will turn your Cisco side of the circuit into "DCE" mode, and if the Juniper side stays in "DTE" mode (the default, so probably not listed in the config), then the LMI should start behaving between the two. And yes, as Jay Hennigan suggested, you might need to use "encap frame-relay ietf" to be compatible with non-Cisco gear, or you might need to adjust the frame-relay lmi-type -- one type sends the LMI on DLCI number 0, one of them on DLCI 1023, whatever. I think you'll need to adjust the two ends until you see LMI enquiries both sent and received; right now the "show interface" from the Cisco side shows it has not received any LMI enq yet.


Good luck, and I hope it's that simple.   :-)


Jeff Saxe
blue ridge internetworks

321 east main st • suite 200
charlottesville va  22902
434.817.0707 x 2024
www.briworks.com

Central Virginia’s technology authority since 2000.

________________________________________
From: Righa Shake [righa.shake at gmail.com]
Sent: Saturday, November 19, 2011 11:11 AM
To: afnog at afnog.org
Subject: Flapping POS Interface on Frame-relay between a Juniper and Cisco

Hi,

Am having a problem that is buffling.

I recently changed a POS link encapsulation from PPP to Frame-relay.
Since that time the POS interface keeps resetting from time to time.

On my BGP session am receiving cease notifications from my upstream
provider.

The setup is such that we have a cisco on one end and a Juniper on the
other.

interface POS0/0/0
 mtu 4474
 no ip address
 no ip unreachables
 encapsulation frame-relay
 logging event link-status
 crc 32
 pos scramble-atm
 frame-relay lmi-type ansi
end

ROUTERshow run int pos0/0/0.101
Building configuration...


!
interface POS0/0/0.101 point-to-point
ip address X.X.X.X 255.255.255.252
frame-relay interface-dlci 101
end

ROUTER#show int pos0/0/0
POS0/0/0 is up, line protocol is up
  Hardware is SPA-2XOC12-POS
  MTU 4474 bytes, BW 622000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 6/255, rxload 38/255
  Encapsulation FRAME-RELAY, crc 32, loopback not set
  Keepalive set (10 sec)
  Scramble enabled
  LMI enq sent  81981, LMI stat recvd 77480, LMI upd recvd 0, DTE LMI up
  LMI enq recvd 0, LMI stat sent  0, LMI upd sent  0
  LMI DLCI 0  LMI type is ANSI Annex D  frame relay DTE  segmentation
inactive
  FR SVC disabled, LAPF state down
  Broadcast queue 0/256, broadcasts sent/dropped 26/0, interface broadcasts
0
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters 1w2d
  Input queue: 0/375/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 94336000 bits/sec, 13151 packets/sec
  5 minute output rate 16470000 bits/sec, 7049 packets/sec
     12211574207 packets input, 10967607038364 bytes, 0 no buffer
     Received 0 broadcasts (0 IP multicasts)
     6970870 runts, 2179 giants, 0 throttles
              0 parity
     892493293 input errors, 882184781 CRC, 0 frame, 0 overrun, 0 ignored,
3335463 abort
     6379191154 packets output, 1614018181446 bytes, 0 underruns
     0 output errors, 0 applique, 4 interface resets
     0 unknown protocol drops
     0 output buffer failures, 0 output buffers swapped out
     0 carrier transitions


Any assistance on this will be greatly appreciated.


Regards,
Righa Shake



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