Home » Cisco » 400-101 » Which path is selected as best path?
Refer to the exhibit. Which path is selected as best path?
A. path 1, because it is learned from IGP
B. path 1, because the metric is the lowest
C. path 2, because it is external
D. path 2, because it has the higher router ID
Answer: B
Explanation:
Metrics is a property of a route in computer networking, consisting of any value used by a routing protocol to determine whether one particular route should be chosen over another. The routing table stores only the best possible routes, while link-state or topological databases may store all other information as well. For example, Routing Information Protocol uses hopcount (number of hops) to determine the best possible route. The route will go in the direction of the gateway with the lowest metric (default gateway).
C is Correct (tested)
R1#sh ip bgp 3.3.3.3 255.255.255.255
BGP routing table entry for 3.3.3.3/32, version 3
Paths: (2 available, best #2, table default)
Advertised to update-groups:
31
Refresh Epoch 1
200
10.10.10.2 from 10.10.10.2 (2.2.2.2)
Origin IGP, metric 180, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
Refresh Epoch 1
Local
10.10.20.2 from 10.10.20.2 (3.3.3.3)
Origin IGP, metric 190, localpref 100, valid, internal, best
rx pathid: 0, tx pathid: 0x0
I finally figured it out. B is indeed correct!!
Here, metric refers to MED. Since 2 paths’ neighboring AS are same (both are 65000), so we have to consider MED. Because path 1’s MED 100 is lower than path 2’s MED, path 1 is the best path.
I feel the answer should be C.
The BGP best-path selection algorithm ‘N WLLA OMNI’.
N: eBGP over iBGP
I: IGP metric
metric is the last step after neighbor type