From the [edit routing-options] hierarchy, which command correctly configures an aggregate route for 64.18.128/17
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A. B. C. D.A
The set aggregate route 64.18.128/17 command correctly defines an aggregate route for 64.18.128/17. The default next hop for aggregate routes is reject, which sends an ICMP unreachable message when incoming traffic does not match one of the contributing routes.
The correct command to configure an aggregate route for the prefix 64.18.128/17 under the [edit routing-options] hierarchy in Junos OS is:
C. set route aggregate 64.18.128/17
Explanation:
The "routing-options" hierarchy in Junos OS is used to configure routing options, such as static routes, aggregate routes, and routing policies.
An aggregate route is a summarized route that represents a set of more specific routes. It helps reduce the size of the routing table by replacing multiple smaller routes with a single larger route.
The "set route aggregate" command is used to configure an aggregate route in Junos OS. The syntax of this command is as follows:
set routing-options static route aggregate <aggregate-prefix> [active-tag | discard | next-hop <next-hop>] [preference <preference>] [qualified-next-hop <qualified-next-hop>] [reject | tag <tag>] [to <to-address>] [via <next-hop>] [weight <weight>]
In the given question, the aggregate prefix is "64.18.128/17". Therefore, the correct command to configure an aggregate route for this prefix would be "set route aggregate 64.18.128/17".
Let's take a closer look at the other answer options and why they are incorrect:
A. set aggregate route 64.18.128/17 - This command is not a valid Junos OS command. The correct syntax for configuring an aggregate route is "set route aggregate <aggregate-prefix>".
B. set summary route 64.18.128/17 - The "set summary route" command is used to configure a summary route, which is similar to an aggregate route, but is created using a routing policy instead of a static configuration. This command is not used to configure an aggregate route in the [edit routing-options] hierarchy.
D. set route summary 64.18.128/17 - This command is not a valid Junos OS command. The correct syntax for configuring a summary route is "set policy-options policy-statement <name> then summary <prefix>" and this command is not used to configure an aggregate route in the [edit routing-options] hierarchy.