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How to calculate BANDWIDTH statement on Frame Relay Subinterfaces??

Where do you put your bandwidth statement on your Frame Relay interfaces? I have OSPF on Frame Relay and want to put the bandwidth statement in, but am not sure how to apply it. My confusion is with these two questions:

Do you put it on the main serial interface, or do you put it on your FR subinterfaces? Seems the answer is almost surely to put it on the subinterfaces, so I'll assume that you DON'T put it on the interface itself and put the statement on the subinterface.

Unless I'm making it more complex than it needs to be, it seems difficult to come up with a number! You have to represent that the PVCs can swell to beyond their share when it is overloaded and others are quiet, and you have to represent that PVCs compete for bandwidth if more than one are busy.

What do YOU personally do to calculate the bandwidth? Would you split up the total access rate equally, or maybe split them up relatively to reflect the CIR, or put in the max throughput that the PVC could have if the other links were empty?

Let's say I have this full mesh Frame Relay scenario:

-Site A has full T1 access, and have four 16k CIR PVCs to sites B, C, D and E and a 284k CIR PVC to site F.

-Further, on the other end of the PVCs, site B and C has 56k access, site D has 512k access, site E has 768k access and site F has full T1 access.

-Site F looks identical to site A, with a 284k PVC to Site A and 16k PVCs to the other 4 sites.

-Sites B, C, D and E all have nothing but 16k PVCs

I'm sure hoping someone out there can say that there's a rule for how to calculate the bandwidth to put there, or give an idea on how they do it and whether you are happy with your logic. It seems you've got to arrive at some method to determine what amount of the total access speed is available for the individual PVC. Then again, if all you do is put the bandwidth statement on the serial port and not on the subinterfaces, then it would be a much easier answer. Seems that would defeat the purpose, though.

2 REPLIES
New Member

Re: How to calculate BANDWIDTH statement on Frame Relay Subinter

I believe the standard has been to apply your Access port speed to the main interface. On each sub-interface use the PVC CIR.

Many network reporting tools while then take these values to correctly graph out your utilization reports.

Your OSPF will only use the bandwidth statement for the individual PVC that you have OSPF running on. Remember, the bandwidth statement is not used for any rate-limiting, just for route costing.

New Member

Re: How to calculate BANDWIDTH statement on Frame Relay Subinter

Your statement at the end is the part that seems to be catching me: Our CIR is low, and doesn't really depict the cost of the link. Heck, if the contract with the telco allowed us to order a 0k CIR, we would've done it.

And route costing is what I'm looking to do. So, OSPF wouldn't even look at the access port bandwidth statement. In my case, I have dedicated T1's running parallel to the frame connection to critical sites (effectively means that the T1 is the primary link and the frame is the secondary/backup link). If I use the CIR in the subinterface, then I'm depicting the frame connection as a 256k PVC versus the 1.5M T1 line. This depicts the frame as being 1/6 the size of the T1. In reality, I could see it being, say 2/3 to 3/4 of it after you subtract out the other link's portion of it.

Well, what you said sounds like a reasonable rule of thumb, but it doesn't really seem to accurately depict the situation.

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