350-401 · Question #689
Refer to the exhibit. After configuring the BGP network an engineer verifies that the path between Server1 and Server2 is functional. Why did RouterSF choose the route from RouterDAL instead of the ro
The correct answer is A. The Router-ID for Router DAL is lower than the Roter-ID for RouterCHI.. When BGP has multiple paths to a destination and all other path attributes are equal, BGP uses the numerically lowest BGP Router ID of the advertising router as a tie-breaker.
Question
Refer to the exhibit. After configuring the BGP network an engineer verifies that the path between Server1 and Server2 is functional. Why did RouterSF choose the route from RouterDAL instead of the route from RouterCHI?
Exhibits
Options
- AThe Router-ID for Router DAL is lower than the Roter-ID for RouterCHI.
- BThe route from RouterDAL has a lower MED.
- CBGP is not running on RouterCHI.
- DThere is a static route in RouterSF for 10.0.0.0/24.
How the community answered
(58 responses)- A57% (33)
- B14% (8)
- C22% (13)
- D7% (4)
Why each option
When BGP has multiple paths to a destination and all other path attributes are equal, BGP uses the numerically lowest BGP Router ID of the advertising router as a tie-breaker.
After comparing all other BGP path attributes (such as Weight, Local Preference, AIGP, AS-Path length, Origin, and MED), if a tie still exists, BGP will prefer the path advertised by the BGP router with the numerically lowest BGP Router ID.
While a lower MED is preferred, MED is typically compared earlier in the BGP path selection process, usually for routes originating from the same AS, and if it were the sole decider, it would have been chosen before resorting to the Router ID tie-breaker.
If BGP was not running on RouterCHI, RouterSF would not have received any routes from it to compare against RouterDAL's routes.
A static route with a default administrative distance (1) is usually preferred over a BGP route (AD 20 for eBGP, 200 for iBGP); if a static route for 10.0.0.0/24 existed on RouterSF, it would be used without comparing BGP paths.
Concept tested: BGP Path Selection Algorithm
Source: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html#bgp_path_selection
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