Frame-relay FECN BECN...

agustinchernitsky

Hello Everyone!

Just a quick one: From my understanding, the Frame switch sets FECN bit to 1 if a congestion is detected. Also, it sets the BECN bit to frames returning to the src router to indicate that a congestion lays ahead.

If I do a sh frame-relay pvc, the stats show 3 cols: input pkts, output
pkts and in bytes.

Input pkts has: in BECN and out BECN.
Output pkts has: out FECN
In Bytes has: in FECN

Why:

Why a DTE (router) will send a BECN? or Why a router will send out a FECN? Shouldn't a router only receive a BECN and a FECN? not send them out?

Maybe if someone could tell me how the congestion control flow is... I could figure this out...

Saludos amigos!

jbkm

i thought the frame relay switch set the FECN and the recieving router would set the BECN bit when sending back the info. i could be wrong tho!

jbkm

disregard, this is wrong. the frame relay switch sets both fecn and becn bits!

Thanks for the question so now i know the answer!

agustinchernitsky

Yep... but does the router re-send those bits?

JNeko

agustinchernitsky wrote: »

Yep... but does the router re-send those bits?

Bump, I asked the same question to myself when I saw the sh frame-relay pvc output:

RouterA#sh frame-relay pvc
PVC Statistics for interface Serial0/0 (Frame Relay DTE)
Active Inactive Deleted Static
Local 1 0 0 0
Switched 0 0 0 0
Unused 0 0 0 0
DLCI = 100, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
input pkts 1300 output pkts 1270 in bytes 21212000
out bytes 21802000 dropped pkts 4 in pkts dropped 147
out pkts dropped 0 out bytes dropped 0 in FECN pkts 147
in BECN pkts 192 out FECN pkts 147
out BECN pkts 259 in DE pkts 0 out DE pkts 214
out bcast pkts 0 out bcast bytes 0
pvc create time 00:00:06, last time pvc status changed 00:00:06


If the FR switch sets BECN/FECN bits, then how can there be outbound FECN/BECN packets being sent from the DTE router?? Is it setting bits also? Should I interpret this as the DTE router itself is experiencing congestion?

Thanks for any help.
J

billscott92787

When the frame switch detects network congestion it sets the FECN bit in a frame to the receiving router. Once the receiving router receives that frame, and sends a frame back to the other router, the Frame switch will remember that the FECN bit was set, which will set the FECN bit in this frame, which tells the original sender "SLOW DOWN."


For example:


R1 sends a frame to FR Switch destinated for R2

Frame switch detects congestion, sets FECN bit.

Frame is forwarded to R2

R2 sends a frame to the Frame switch destined for R1

Frame switch sets BECN bit

Frame is forwarded to R1

R1 knows to slow down.


A router can also detect that a particular frame has experience congestion and set the FECN bit. It's more common that you see the switch set the FECN bit though.

JNeko

billscott92787 wrote: »

When the frame switch detects network congestion it sets the FECN bit in a frame to the receiving router. Once the receiving router receives that frame, and sends a frame back to the other router, the Frame switch will remember that the FECN bit was set, which will set the FECN bit in this frame, which tells the original sender "SLOW DOWN."


For example:


R1 sends a frame to FR Switch destinated for R2

Frame switch detects congestion, sets FECN bit.

Frame is forwarded to R2

R2 sends a frame to the Frame switch destined for R1

Frame switch sets BECN bit

Frame is forwarded to R1

R1 knows to slow down.


A router can also detect that a particular frame has experience congestion and set the FECN bit. It's more common that you see the switch set the FECN bit though.

hmmmm, not sure if I follow you 100%. At one point exactly in your example does the "out BECN pkts" and/or "out FECN pkts " fields on R1 and/or R2 change from 0 to 1?

Thanks for the reply!
J

Edit: cleared up: http://ieoc.com/forums/t/5692.aspx

Firemarshalbill.com

JUST AN FYI - most service providers are eliminating Frame Relay services and moving to all IP and MPLS