two sides connected by frame relay

Answered Question
Mar 30th, 2009
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Hi every body!


Two sides r1 and r2 are connected by frame relay

r1 router at side 1

r2 router a side 2


r1---- frsw1----------------------------frsw2--r2

frsw1 frame relay switch connected to r1

frsw2 frame relay switch connected to r2


Should the LMI type between r1 and frsw1 match the LMI type between frsw2 and r2?



My book also defines " Access rate" as the speed at which the access link is clocked. Thischoice affects the connection's price.


When we order framerelay service, does telco give us this " access rate" or value for the clock on access link need to be configured on the access link?

Am i correct in assuming access rate and clockrate are same thing?



Thanks a lot

Correct Answer by Richard Burts about 8 years 3 months ago

to add a bit to the good response from Vishwamurti, there are several potential weaknesses inherent in FECN and BECN. One of these is that, as you identified, if there is not traffic going back to the source in which the Frame Relay switch can set the BECN then the source may not know that there was congestion and that it should slow down.


Even if the routers receive a BECN or a FECN there is not guarantee that they will do anything about it. FECN and BECN are a "request" to slow down but there is no requirement that the source and destination router will honor the request.


And even if the source wants to slow down there are limited mechanisms to accomplish this. Probably the best one if to run Frame Relay Traffic Shaping. FRTS can recognize BECN and will slow down the transmission rate.


As far as DE is concerned: a Frame Relay switch will start by trying to discard frames with DE set to reduce congestion. But if that is not enough then the Frame Relay switch will go on to discard other frames that are not DE. So the stingy customer who sends lots more than their CIR will lose frames (and other customers will also probably be impacted).


HTH


Rick

Correct Answer by viyuan700 about 8 years 3 months ago

Since frs2 sets the FECN bit therefore it is only entitled to set BECN bit in any frame traveling on same dlc towards r1? Is it correct to assume?


>>Yes frs2 is setting BECN as frs2 knows link frs2to frs3 is congested.


"FR switches use FECN and BECN to inform a router that a particular VC has experienced congestion.

To do so, when a switch notices congestion caused by a VC, the switch sets the FECN bit in a

frame that is part of that VC. The switch also tracks the VC that was congested so that it can

look for the next frame sent over that VC, but going the opposite direction.

"


Since no return traffic is expected back to r1 from r2, r1 will not receive any indication that it is causing congestion in frame relay network.


>>

"The DTE device can relay this information to a higher-layer protocol for processing. Depending on the implementation, flow control may be initiated, or the indication may be ignored.

"


In such situation, what will frame relay provider do who only depends upon " DE" bit to discard traffic causing congestion


>>If customer set DE bit then his critical data is not dropped during congestion as customer knows which traffic is important and which is not.


But if customeris not setting then switch will start dropping during congestion everything whether it is important or not.


Correct Answer by Richard Burts about 8 years 3 months ago

Yes you are correct to assume that access rate and clock rate are the same thing. You would not normally configure anything about the access rate since it is set by the clocking that is typically provided by the telco or Frame Relay provider.


LMI is between the router and its connected Frame Relay switch. While it is frequently the same on both ends that is not a requirement.


HTH


Rick

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Correct Answer
Richard Burts Mon, 03/30/2009 - 17:36
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Yes you are correct to assume that access rate and clock rate are the same thing. You would not normally configure anything about the access rate since it is set by the clocking that is typically provided by the telco or Frame Relay provider.


LMI is between the router and its connected Frame Relay switch. While it is frequently the same on both ends that is not a requirement.


HTH


Rick

sarahr202 Mon, 03/30/2009 - 18:51
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Thanks Rick! As usual(:-) if you don't mind i have few more questions about frame relay.


FECN and BECN bits:


Consider a frame relay cloud with three frame relay switches, frs1,frs2, and frsw3


r1--- frs1--frs2----frs3---r2


Frs1 receives a frame from r1 and forwards it to frs2.frs2 receives the frame and find this frame experiences congestion so set the FECN bit.

Since frs2 sets the FECN bit therefore it is only entitled to set BECN bit in any frame traveling on same dlc towards r1? Is it correct to assume?


r1 relies on BECN bit to slow down. How about if there is no return traffic to r1 and r1 is sending a lot of frames which carry udp traffic. Since no return traffic is expected back to r1 from r2, r1 will not receive any indication that it is causing congestion in frame relay network.


your expert insight might help me understand the above.

==========================


DE bit:


Frame relay switch when experiences congestion, it will drop the frames with DE bit set. How about if a stingy customer does not want to pay for more bandwidth and keep sending huge data way higher than CIR, without any DE bit set. In such situation, what will frame relay provider do who only depends upon " DE" bit to discard traffic causing congestion ?


Thanks a lot !

Correct Answer
viyuan700 Mon, 03/30/2009 - 23:42
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Since frs2 sets the FECN bit therefore it is only entitled to set BECN bit in any frame traveling on same dlc towards r1? Is it correct to assume?


>>Yes frs2 is setting BECN as frs2 knows link frs2to frs3 is congested.


"FR switches use FECN and BECN to inform a router that a particular VC has experienced congestion.

To do so, when a switch notices congestion caused by a VC, the switch sets the FECN bit in a

frame that is part of that VC. The switch also tracks the VC that was congested so that it can

look for the next frame sent over that VC, but going the opposite direction.

"


Since no return traffic is expected back to r1 from r2, r1 will not receive any indication that it is causing congestion in frame relay network.


>>

"The DTE device can relay this information to a higher-layer protocol for processing. Depending on the implementation, flow control may be initiated, or the indication may be ignored.

"


In such situation, what will frame relay provider do who only depends upon " DE" bit to discard traffic causing congestion


>>If customer set DE bit then his critical data is not dropped during congestion as customer knows which traffic is important and which is not.


But if customeris not setting then switch will start dropping during congestion everything whether it is important or not.


Correct Answer
Richard Burts Tue, 03/31/2009 - 11:30
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to add a bit to the good response from Vishwamurti, there are several potential weaknesses inherent in FECN and BECN. One of these is that, as you identified, if there is not traffic going back to the source in which the Frame Relay switch can set the BECN then the source may not know that there was congestion and that it should slow down.


Even if the routers receive a BECN or a FECN there is not guarantee that they will do anything about it. FECN and BECN are a "request" to slow down but there is no requirement that the source and destination router will honor the request.


And even if the source wants to slow down there are limited mechanisms to accomplish this. Probably the best one if to run Frame Relay Traffic Shaping. FRTS can recognize BECN and will slow down the transmission rate.


As far as DE is concerned: a Frame Relay switch will start by trying to discard frames with DE set to reduce congestion. But if that is not enough then the Frame Relay switch will go on to discard other frames that are not DE. So the stingy customer who sends lots more than their CIR will lose frames (and other customers will also probably be impacted).


HTH


Rick

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