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AS Network Diagram.

Hi,

I am stuck on some networking BGP Valley Free Routing homework, so more explanation focused answers would be greatly appreciated!

In this assignment I am trying to construct the FIB table from the given Network Diagram above.

For clarification on the network diagram, the red numbers are the Local Preferences. For example, moving from router 1 to 2 has a local preference of 100, while moving form router 2 to 1 has a local preference of 300. Arrows show a customer/provider relationship where the arrow points to the provider and a normal line shows a peer relationship.

Where I am stuck at is picking the ideal paths for the table because I don't know when Local Preference gets out weighed by Shortest path.

For example: Finding the shortest path between routers 3 and 6. Since Local_preference is highest on the order or operations for selecting the best choice next hop, I initially go 3 -> 1. Which leaves me to select 2 as the next router to hop to, so my path becomes 3 -> 1 -> 2. (It is specified in the assignment that in cases of tie, use the lowest router number). Continuing the local pref train gets my final path to be 3 -> 1 -> 2 -> 5 -> 6. But the shortest path would be to take the local preference hit and route the path as 3 -> 6.

3 -> 6 seems much more correct but Local_Pref has a higher operation order which leaves me unsure.

Any insight on this question is greatly appreciated, I'm sure I'm just over thinking this.

Thanks!

user4493284
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1 Answers1

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This is a matter of BGP path manipulation and inbound vs. outbound influence.

BGP doesn't route like OSPF or EIGRP (router to router). BGP routes prefixes between autonomous systems (lots of routers). Your diagram would need to change in order to make all those "routers" reside in separate AS's. BGP doesn't care about individual interfaces, but the router itself as a whole.

In the real world, you would not have any control over another AS but you do control what you can apply to prefixes entering or exiting your AS.

Local preference is in-fact a more preferred BGP attribute but it's applied to prefixes which we RECEIVE. This will then influence OUTBOUND traffic. It's for autonomous systems with multiple exit points when you want to prefer one path over the other.

AS-Path Prepending is something we send to a neighbor for influencing INBOUND traffic. You can set a longer AS-Path on your less preferred ingress point.

I guess what I am trying to say is, this isn't a situation you'll ever be in, and it's not going to make sense like if (for example) OSPF were being used. The AS will always choose the path with the highest local preference outbound. This is why your "routers" are taking that path.

Hope this helps.

fencepencil
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