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Juniper MX Series by Harry Reynolds, Douglas Richard Hanks Jr.

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Chapter Review Answers

  1. Answer: D. Because DDoS is on by default, the policers are set to the same high values as when the feature is disabled, effectively meaning the host-bound traffic from a single PFE is limited by the processing path capabilities and not DDoS protection. You must reduce the defaults to suit the needs of your network to gain additional DDoS protection outside of alerting and policing at aggregation points for attacks on multiple PFEs.

  2. Answer: C. When an lo0 policer is present, it is executed first, as traffic arrives at the line card, before any DDoS (even Trio PFE-level) are executed. In addition, a copy of the RE policer is also stored in the kernel where its acts on the aggregate load going to the RE, after the DDoS policer stage.

  3. Answer: E. A strong security filter always uses a discard-all as a final term. Using rejects can lead to resource usage in the form of error messages, a bad idea when under an attack. Adding the log action to the final term is a good idea, as it allows you to quickly confirm what traffic is hitting the final discard term. Unless you are being attacked, very little traffic should be hitting the final term, so the log action does not represent much burden. The firewall cache is kept in kernel, and only displayed when the operator requests the information, unlike a syslog filter action, which involves PFE-to-RE traffic on an ongoing basis for traffic matching the final discard term.

  4. Answer: A. When a routing instance has filter applied to an lo0 unit in that instance, that filter is used; otherwise, control plane traffic from the instance to the RE is filtered by the main instance lo0.0 filter.

  5. Answer: B. Use prefix-lists and the apply-path feature to build a dynamic list of prefixes that are defined somewhere else on the router (e.g., those assigned to interfaces or used in BGP peer definitions), and then use the dynamic list as a match condition in a filter to simplify filter management in the face of new interface or peer definitions.

  6. Answer: D. Output filters are most often used to alter the default CoS/ToS marking for locally generated traffic.

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