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Author(s) Olivier Bonaventure
Deadline No deadline
Submission limit No limitation
Category tags BGP

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The BGP decision process


Question 1: The BGP decision process

The BGP decision process is the process that is used by a BGP router to select the best path among all the paths learned towards a given destination prefix. The table below represents all the BGP routes learned by a BGP router and their corresponding attributes. For simplicity, the nexthops are indicated as router names instead of IP addresses.

prefix AS Path local-pref nexthop
2001:db8:1::/48 AS1:AS4 100 R1
2001:db8:1::/48 AS1:AS3:AS4 200 R2
2001:db8:1::/48 AS1:AS4 100 R3
2001:db8:1::/48 AS4 150 R4
2001:db8:2::/48 AS1:AS4 100 R1
2001:db8:2::/48 AS1:AS3:AS4 100 R2
2001:db8:2::/48 AS1:AS4 100 R3
2001:db8:2::/48 AS4 150 R4
2001:db8:0::/47 AS1:AS4 200 R1
2001:db8:0::/47 AS1:AS3:AS4 200 R2
2001:db8:0::/47 AS1:AS4 100 R3
2001:db8:0::/47 AS4 150 R4
2001:db8:0::/48 AS1:AS4 100 R1
2001:db8:0::/48 AS1:AS3:AS4 100 R2
2001:db8:0::/48 AS1:AS4 150 R3
2001:db8:0::/48 AS4 150 R4

What is the next hop that this router will prefer for prefix 2001:db8:1::/48 ?

Question 2: Best route

What is the next hop that this router will prefer for prefix 2001:db8:2::/48 ?

Question 3: Best route

What is the next hop that this router will prefer for prefix 2001:db8:0::/48 ?

Question 4: Best route

What is the next hop that this router will prefer for prefix 2001:db8:0::/47 ?

Question 5: Route for a specific packet

What is the next hop that this router will use to forward a packet whose destination address is 2001:db8:0::1234 ?