フリー問題

問題文

A router classifies traffic into the expedited-forwarding class at ingress. An input firewall filter then raises the loss priority of part of that traffic to high. The egress interface carries a DSCP rewrite rule that lists one code point for expedited-forwarding with low loss priority and a different one for expedited-forwarding with high loss priority. What decides the code point that is written?

選択肢

1. The rewrite rule reads the forwarding class and loss priority that the packet carries as it leaves, so the loss priority the filter raised selects the second code point.
2. The loss priority is consumed by the drop profile of the scheduler before the rewrite table is read on the way out, so both halves of the flow leave the port with the entry listed first against that class, af11.
3. The rewrite rule reads the code point that arrived in the header and maps it to a new one, so a loss priority that was changed inside the device has no bearing on the value written.
4. The rewrite rule takes effect at ingress together with the classifier, so it runs before the filter and writes the code point for the low loss priority.