Refer to the exhibit. With BGP always-compare-med enabled, which BGP entry is installed in the RIB?
A. Entry 1 because it was installed first (oldest) in the BGP table.
B. Entry 1 because it has the best MED of the external routes.
C. Entry 2 because it has the lowest router ID.
D. Entry 3 because it has the lowest MED.
Answer: D
Example 2: bgp deterministic-med Disabled, bgp always-compare-med Enabled
Entry1 is compared to entry2. These entries are from different neighbor autonomous systems, but since the bgp always-compare-med command is enabled, MED is used in the comparison. Of these two entries, entry1 is better because it has a lower MED. Next, entry1 is compared to entry3. The MED is checked again because the entries are now from the same autonomous system. Entry3 is chosen as the best path.
https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/16046-bgp-med.html
answer is B because first the entry1 is compare against entry2 using lowest MED because they are from different AS, winning entry 1, then its continue to compare winner entry1 against entry3 but they are from same AS 400 so no MED will be use this time and the winner will be based on traditional lower rouerID winning entry1 again.
I used this link as reference for guidance:
https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/16046-bgp-med.html
Funny enough, exactly based on the referenced link, the answer to my understanding should be D.
Having always-compare-med and no deterministic-med, we are following example 2.
A few notes to remember:
1) MED will be compared between entries of the same AS (default)
2) MED will be compared between entries of different ASes (always-compare-med)
Comparisons = result – based on:
1 with 2 = 1 – MED (always-compare-med)
1 with 3 = 3 – MED (default)
Of course MED will be used while comparing advertisements from the same AS. In fact, that is the whole point of MED.
By default, Cisco routers won’t compare MED on advertisements coming from DIFFERENT AS’s, but since ‘always-compare-med’ is in use, this is not the case here. However, using ‘always-compare-med’ does not change the fact that MED will be compared on advertisements coming from the same AS (hint: ALWAYS)
Nah. I think “D” is correct here – the lowest med is the best med. Entry 1 is clearly not the lowest MED (150). Entry 3 MED is 100.
Answer D
Lewest MED the best is the route