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350-401 Practice Question: An enterprise has two BGP routers, R1 and R2,…
An enterprise has two BGP routers, R1 and R2, both in AS 65000. R1 peers with ISP1 (AS 100) and R2 peers with ISP2 (AS 200). The enterprise advertises a prefix 192.168.0.0/24 to both ISPs. The engineer wants to ensure that traffic from the Internet to this prefix enters the network primarily via R1, and only uses R2 if the link to ISP1 fails. Which BGP attribute should be manipulated on the updates sent to the ISPs?
⚠ Common exam trap
Cisco often tests the distinction between attributes that influence inbound vs. outbound traffic; the trap here is confusing local preference (outbound) with AS path prepending (inbound), leading candidates to incorrectly choose local preference 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
✓
Prepend AS 65000 multiple times on R2's updates to ISP2.
To influence inbound traffic from the Internet, you must manipulate attributes sent to the ISPs. AS path prepending makes a route appear less preferred by artificially lengthening the AS path. By prepending AS 65000 multiple times on R2's updates to ISP2, ISP2 will see a longer AS path for the prefix and prefer the shorter path via ISP1, causing traffic to enter primarily via R1 unless the ISP1 link fails.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Prepend AS 65000 multiple times on R2's updates to ISP2.
Why this is correct
AS_PATH prepending artificially lengthens the AS path by adding multiple copies of the enterprise's ASN (65000) to the prefix advertised to ISP2. Because ISP2's BGP best-path algorithm compares AS_PATH length among equal-preference routes, the path through R2 appears longer than the same prefix advertised via ISP1's shorter AS path. This induces ISP2 to select the ISP1 route for inbound traffic, thereby directing traffic to R1 as intended. It is a standard, lightweight technique that only changes how ISPs view the return path, without affecting outbound routing.
- ✗
Set a higher MED on R1's updates to ISP1.
Why it's wrong here
MED is a non-transitive metric that a BGP speaker only compares among routes learned from the same neighboring AS. Since ISP1 and ISP2 are distinct autonomous systems, ISP2 will never compare a MED value that R1 attaches to routes sent to ISP1 against its own routes from R2. Even if R1 assigns a higher MED to ISP1, that value is consumed within ISP1's policy and does not propagate, so ISP2's path selection for the prefix is completely unaffected. Thus MED is not a valid tool for influencing inbound traffic across two different providers.
- ✗
Set a higher local preference on R1 for routes learned from ISP1.
Why it's wrong here
Local preference is a well-known mandatory BGP attribute that is significant only within the local autonomous system and influences the selection of the best exit path for outbound traffic. A higher local-preference value on R1 for routes learned from ISP1 would make the enterprise prefer ISP1 as the exit for its own traffic, but it is never communicated to any eBGP peer. ISP2 will independently evaluate the routes it receives from R2 using its own BGP decision process, completely unaware of any local-preference setting on R1. Therefore, local preference cannot influence ISP2's choice of path for inbound traffic back into the enterprise.
- ✗
Use the no-export community on R1's updates to ISP1.
Why it's wrong here
The no-export community signals any receiving BGP speaker to advertise the route only within the local AS and never to external (eBGP) peers. If R1 sends this community with the prefix to ISP1, ISP1 will suppress the announcement of that prefix to its own upstream and peer networks, which removes the route from those parts of the Internet. This does not influence ISP2's best-path selection at all — ISP2 still receives the route from R2 and will forward inbound traffic to R2, but if ISP1's side is suppressed, traffic from other parts of the Internet may prefer ISP2 inadvertently, causing an asymmetric or black-holed path. It is a routing-policy restriction, not a path-selection manipulation.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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