Exam 350-501 topic 1 question 300 discussion

Example 3: bgp deterministic-med Enabled, bgp always-compare-med Disabled When the bgp deterministic-med command is enabled, routes from the same autonomous system are grouped together, and the best entries of each group are compared. The BGP table looks like this: entry1: AS(PATH) 100, med 200, external, rid 1.1.1.1 entry2: AS(PATH) 500, med 100, internal, rid 172.16.8.4 entry3: AS(PATH) 500, med 150, external, rid 172.16.13.1 There is a group for AS 100 and a group for AS 500. The best entries for each group are compared. Entry1 is the best of its group because it is the only route from AS 100. Entry2 is the best for AS 500 because it has the lowest MED. Next, entry1 is compared to entry2. Since the two entries are not from the same neighbor autonomous system, the MED is not considered in the comparison. The external BGP route wins over the internal BGP route, making entry1 the best route. Clearly A, the example is copy/paste from this

§ Consider the tie breaker with bgp best path selection. In this question, relevant attributes are External vs Internal learned routes, router-id, and MED. See https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html as it is a crucial point in bgp. (MED > external vs Internal > router-Id). With bgp deterministic-med command, routes are grouped per AS. Group 1 = AS51 route, Group2 = AS321. Out of Group2, since they are in the same AS, MED is compared by default. Lowest MED wins, so route with MED300 is chosen. Then, route from AS51 (med500) vs route with med300 are compared. Since these routes are coming from different AS, the MED is NOT compared. Therefore, we move on to next attribute which is external vs internal. Since we prefer external, AS51 with med500 is chosen.

Answer: A https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/16046-bgp-med.html

Agree with mohdema, really good explanation and reference.

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/16046-bgp-med.html Clearly A, the example is copy/paste from this

I think the answer should be D, as the question shows that we are trying to influence routing choices for two different AS peers. The Cisco website says: bgp deterministic-med command ensures the comparison of the MED variable when choosing routes advertised by different peers in the same autonomous system. BGP always-compare-med is useful when wanting to evaluate MED values from two different AS_PATH’s of equal length towards a prefix. The answer is D