BGP Notes – Path Decision

This is required blogging…and reading for that matter.  A good chunk of this is taken from my CCNP posts from last year.  Corrections, please.

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How does a BGP router decide which BGP route is the best?

Next-hop : Does the router have a route to the next-hop?

Weight : This is a numeric value where bigger is better.  Weight is not passed onto other peers and is a Cisco proprietary feature.

LOCAL_PREF : This is a numeric value where bigger is better.  All iBGP peers pass this value around amongst themselves.

Local : Is the next hop me (0.0.0.0)?

AS_PATH length : This is the number of AS hops to the destination.  If you don’t know this one by now, then you missed something big.

: This is the number of AS hops to the destination.  If you don’t know this one by now, then you missed something big.

ORIGIN : Did this route come from a netowork statement in an IGP (I), from  EGP (E, which shouldn’t exist any more), or somewhere else (?) like a redistributed route?  I is better than E is better than ?.

MED : The Multi Exit Discriminator can be used by one AS to influence routes to that AS.  The smaller the better.

Neighbor type : eBGP are better than iBGP routes.

IGP metric : Prefer the next-hop address that’s closest via an IGP like OSPF or EIGRP (or RIP, Ivan).

Route age : Prefer the oldest (and, thusly, the most stable) route.

Lowest BGP neighbor router ID : Do I have to explain that one?

Lowest BGP neighbor IP : You know what this is, right?

Aaron Conaway

I shake my head around sometimes and see what falls out. That's what lands on these pages. @aconaway@mstdn.social

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