Meant to copy this to the list. Hopefully this explanation helps others as well.
Hi Fulvio, This matter took me a long time to really understand. Administrative distance is used only to select the best route among multiple protocols for installation into the routing table. Each protocol has it's own selection algorithm to identify the candidate route it is offering for selection by the IP routing table, at which point AD is used to select the best overall route. In other words, Admin Distance cannot be used to select a specific route *within* a protocol, only among all protocols that have the route. For example, EIGRP will always select Internal routes over External routes. Even if you set the AD for external routes to 1, if the best route to a destination prefix is learned as an EIGRP internal route, that one will be installed with the AD of 90. AD doesn't matter when selecting *which* EIGRP route will be used, only when selecting EIGRP vs. another protocol. Likewise, OSPF will always select O, the O IA, then E1/N1, then E2/N2. Can't be changed (as far as I know). But then whichever route is selected will be installed in the IP routing table with a (default) AD of 110. For BGP, consult this: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml See Step 7. BGP External routes will always be chosen over Internal, all other values in steps 1-6 being equal. Unlike most other protocols, BGP has knobs that can be used to tune route selection before getting to Step 7. That's where weight, local preference, etc. come in. It took me a long time to wrap my brain around this. AD has nothing to do with intra-protocol route selection. Hope this helps, Bob On Tue, May 1, 2012 at 5:11 PM, Fulvio allegretti <[email protected]>wrote: > > Hi all, > Doing lab 9 Volume 2 - Task 4.4, configure R6 such that if it learned the > same route via eBGP, it would still prefer the same route it has from R2 > (iBGP). For my little brain this was very clearly to do with distance, eBGP > 20 - iBGP 200, change the distance for the eBGP peers the task refers to > and job done. The solution suggests adjusting weight which has left me a > bit confused, what do you think? I though that admin distance would be > looked at before the path selection algorithm > Using the same logic, why does step 7 of the algorithm uses eBGP and iBGP? > Again, adming distance is different, so we shouldn't even start the path > selection alogrithm. Unless the distance was changed manually I suppose. > Fulvio > _______________________________________________ > For more information regarding industry leading CCIE Lab training, please > visit www.ipexpert.com > > Are you a CCNP or CCIE and looking for a job? Check out > www.PlatinumPlacement.com > > http://onlinestudylist.com/mailman/listinfo/ccie_rs > _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com http://onlinestudylist.com/mailman/listinfo/ccie_rs
