Flashcards on BGP Path Selection and Attributes

BGP Path Selection and Attributes: The Ultimate Guide

SummaryKnowledge testFlashcardsPodcastMindmap
1 / 14

When BGP has multiple paths to a destination, where are all paths stored and which one is installed in the routing table?

All paths are stored in the BGP table, but only the single best path (selected by the BGP path selection process) is installed in the routing table.

Tap to flip · Swipe to navigate

BGP path selection

14 cards

Card 1

Question: When BGP has multiple paths to a destination, where are all paths stored and which one is installed in the routing table?

Answer: All paths are stored in the BGP table, but only the single best path (selected by the BGP path selection process) is installed in the routing table.

Card 2

Question: What is the very first attribute BGP checks when selecting the best path?

Answer: Weight — prefer the path with the highest weight (Cisco-proprietary, local to the router).

Card 3

Question: If weights are equal, what attribute does BGP evaluate next?

Answer: Local preference — prefer the path with the highest local preference (used within an AS, default 100).

Card 4

Question: What does 'originate' mean in BGP path selection and how does it affect selection?

Answer: Prefer a path that the local router originated (next hop 0.0.0.0). Locally originated routes are preferred over routes learned from other routers.

Card 5

Question: After local preference and originate checks, which attribute does BGP consider next?

Answer: AS path length — prefer the path with the shortest AS path length.

Card 6

Question: How does the origin code influence BGP path selection and what is the precedence order?

Answer: Prefer the lowest origin code. Order: IGP (best) < EGP < INCOMPLETE (worst).

Card 7

Question: Which attribute is used to prefer one inter-AS path over another, and how is it chosen?

Answer: MED — prefer the path with the lowest MED (multi-exit discriminator) exchanged between autonomous systems.

Card 8

Question: How does BGP prefer between eBGP and iBGP paths?

Answer: Prefer eBGP (external) paths over iBGP (internal) paths.

Card 9

Question: When multiple paths remain after previous checks, what IGP-related criterion does BGP use?

Answer: Shortest IGP path to the BGP next hop — prefer the path with the lowest IGP metric within the AS to reach the BGP next hop.

Card 10

Question: If paths are still tied, which path attribute does BGP prefer regarding timing?

Answer: Oldest path — prefer the path that was received first.