You need to ensure that an iBGP-learned route is properly advertised to other eBGP peers without triggering routing loops. Which mechanism does BGP use automatically to prevent routing loops within an Autonomous System?
Trap 1: MED attribute comparison
MED is used for ingress traffic steering between ASes.
Trap 2: Local Preference check
Local preference is strictly internal and never sent to eBGP peers.
Trap 3: Cluster list checking
Cluster list is specific to route reflectors to prevent loops in iBGP.
- A
MED attribute comparison
Why it fails: MED is used for ingress traffic steering between ASes.
- B
Local Preference check
Why it fails: Local preference is strictly internal and never sent to eBGP peers.
- C
AS-path loop prevention on eBGP export
iBGP-learned routes carry no AS-path change when re-advertised within the AS, so BGP relies on the AS-path attribute to detect its own AS number on eBGP export, dropping routes that would loop back. This automatic check satisfies the loop-prevention constraint without extra configuration.
- D
Cluster list checking
Why it fails: Cluster list is specific to route reflectors to prevent loops in iBGP.