Transit Gateway Metrics compared to bill

Question

Hi. I am trying to establish monitoring for our high transit gateway cost and I can't seem to get them to agree. It looks like either the CloudWatch metrics for Transit Gateway are incorrect, or I am being highly overcharged for Transit Gateway usage. After doing some analysis on an hour of flow logs, I am thinking that the metrics are incorrect, or I am not understanding how to interpret them. I have tried to go over everything publicly available.

Please correct me if I am mistaken. It is my understanding that the transit gateway billing is based on the number of bytes that the gateway processes. This should be fully accountable by adding up all the TransitGateway/Per-TransitGateway Metrics/BytesIn metrics for all the transit gateways in my account. CloudWatch does not give units for this metric, so I am assuming that it reports the number of bytes in each minute sent to the TGW, since docs say that this metric is reported every minute. However, if I look at this number, taking an average value over weeks and extrapolate it out to a month, it is off by like a factor of 2000x from the number reported in my bill.

Can I get an explanation of how I can accurately break down and monitor my Transit Gateway traffic bill?

Accepted Answer

Just to answer my own question here. I had two key misunderstandings that led to the confusion.
(1) I misapplied how to use the TGW CloudWatch metrics. I had to use the SUM statistic to see all the BytesIn because there were so many samples per minute.
(2) The TGW FlowLog reports double the number of bytes flowing through the TGW, because it reports one line for bytes into the TGW (ingress) and another line for bytes out of the TGW (egress). So if you just add up all the bytes blindly, you end up with double the bytes.

With these two concepts resolved, I was able to get my metrics and flowlogs to agree with my bill.

Answer

Hi Randall,

For details on how Transit Gateway charges & an example of pricing, please take a look at the AWS Transit Gateway pricing page here:

https://aws.amazon.com/transit-gateway/pricing/

For further assistance with your billing and usage, I suggest opening a support case with our Accounts & Billing team. They are best tooled to assist with these matters & may be able to provide you with further guidance:

http://go.aws/support-center

Kind regards,

— Thomas E.

Transit Gateway Metrics compared to bill

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