NSE7 Advanced Networking and SD-WAN Practice Question
What is the purpose of using a prefix list in route redistribution?
⚠ Common exam trap
Test-takers frequently confuse prefix lists with route maps or ACLs, thinking prefix lists can modify route attributes or specify next-hops, when in reality prefix lists only perform matching based on prefix and length, while route maps handle attribute manipulation.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
To match routes based on IP prefix and prefix length
A prefix list is used in route redistribution to match routes based on their IP prefix and prefix length (e.g., 192.168.0.0/16). This allows granular control over which routes are redistributed from one routing protocol to another, such as from OSPF to BGP, by filtering based on the network address and subnet mask.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
To match routes based on IP prefix and prefix length
Why this is correct
A prefix list filters routes by network address and subnet mask length, giving granular control over exactly which prefixes enter or leave the routing domain during redistribution. This satisfies the stem's requirement by matching on both IP prefix and prefix length, unlike access lists that match only the network portion.
- ✗
To define a list of allowed source IPs for management access
Why it's wrong here
Prefix lists filter routing prefixes during redistribution, not source IPs of management traffic; that is the role of administrative access lists or trusted-host settings. They are tempting because both are named 'lists' and both filter, but the match field differs: prefixes versus packet source addresses.
- ✗
To specify the next-hop for a set of routes
Why it's wrong here
Prefix lists match network prefixes and mask lengths for filtering redistributed routes; they cannot set next-hop attributes, which is done with route maps. They are tempting here because route maps do contain next-hop statements, so administrators associate the two, but a prefix list alone only permits or denies prefixes.
- ✗
To set BGP community values on matched prefixes
Why it's wrong here
Prefix lists only match prefixes for permit or deny decisions; BGP community values are attached using route maps with set community statements. They are tempting because communities are often applied to selected prefixes, but the prefix list supplies the match, while the route map performs the attribute modification.
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