Route Redistribution Question

[davidcriollo]

So I have the following question from an Enarsi practice exam "You wish to redistribute all routes within AS 1 into AS 2 with the exception of 192.168.1.0/24. Which of the following approaches can you use to selectively filter that route?

A. Create an ACL to match the 192.168.1.0/24 network, and reference that ACL under a
“route map deny” statement.
B. Use the “filter-list 192.168.1.0/24” option as part of the “redistribute” command.
C. Create an ACL to match the 192.168.1.0/24 network, and reference that network in a
“distribute list” command.
D. Use the “distribute-list 192.168.1.0/24” option as part of the “redistribute” command.

I know that the answer is number A, But I want to know why the other ones wouldn't work. 

"

[MHM Cisco World · Accepted solution]

From respective of AS2 

You receive this prefix and 

Filter list with redistrubte

and

distribute list use for advertise prefix not receive it.

From respective of AS1 only answer A specify deny  other answer not mentioned that.

MHM

[M02@rt37 · Accepted solution]

Hello @davidcriollo,

A. Create an ACL to match the 192.168.1.0/24 network, and reference that ACL under a "route map deny" statement
- This approach uses a route map with an ACL to deny the specific route from being redistributed. It's a valid method for selectively filtering routes during redistribution.

B. Use the "filter-list 192.168.1.0/24" option as part of the "redistribute" command.
- The "filter-list" option is typically used with standard or extended ACLs, but it doesn't directly match the format mentioned in option B. The correct syntax would be more like "filter-list 10 out" where 10 is the ACL number. This option seems to have a syntax issue and may not work as intended.

C. Create an ACL to match the 192.168.1.0/24 network, and reference that network in a "distribute list" command.
- This option involves using a distribute list with an ACL to control routing updates. However, the distribute list is typically applied to control updates coming into or leaving the routing process, not specifically during redistribution. It may not work as expected for the given scenario.

D. Use the "distribute-list 192.168.1.0/24" option as part of the "redistribute" command.
- Similar to option B, this syntax doesn't align with the typical usage of distribute-list in the context of the "redistribute" command. The correct syntax would involve specifying an ACL number. This option also seems to have a syntax issue and might not achieve the desired result.

To sum up, options B, C, and D appear to have syntax issues or involve concepts not directly applicable to the scenario described. Option A, with a route map and ACL, is a suitable and common method for selectively filtering routes during redistribution.