The official Study guide for 350-401 inserts AIGP at #4 in the BGP Preference list and removes the multipath at #9. Can anyone offer clarification on the BGP Preference list?
AIGP in BGP Path Selection? 350-401 ENCOR
When it comes to path selection. There is a acryonym that has been helpful: "We Love Oranges AS Oranges Mean Pure Refreshment"
- highest Weight
- Local Pref
- "locally" Originated route
- shortest AS Path
- lowest Origin code (e.g. IGP<EGP< incomplete)
- lowest MED (Metric)
- Preference of EGP over iBGP
- lowest Router ID
Now having said that, AIGP changes the BGP Best Path Selection once it gets to the decision to the
shortest AS Path. If you've configured AIGP it will occur following the locally advertised routes (e.g. routes advertised using the "network" statement). So that's why it occurs at the 4th step in the selection process.
Also, the list mnemonic doesn't helps when it covers step #9 as you've said. The cisco documentation puts it this way:
`7. Prefer eBGP over iBGP paths.
If bestpath is selected, go to Step 9 (multipath).
8. Prefer the path with the lowest IGP metric to the BGP next hop.
Continue, even if bestpath is already selected.
9. Determine if multiple paths require installation in the routing table for BGP Multipath.
Continue, if bestpath is not yet selected.
The BGP Multipath is only required if there are multiple paths have met all the previous conditions (1-6) have been met. So after the MED, If eBGP is the best path, then you can determine if there are multiple paths require installation in the routing table for BGP Multipath. In the OSG, it may mean that if the best path doesn't require BGP Multipath, then it removes that.
Not always, but in many cases you will not get to this step before the best path has been selected.
got it, thank you for the quick reply.