Unanswered Question
Nov 30th, 2007
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i have a question

Adminstrative distance determines the trust worthiness of routing protocol

For example if routerA receives update from rip and ospf, routerA will consider ospf update because ospf had ad Of 110 while rip has ad of 120.

Ebgp has ad of 20 while ibgp has ad of 200 .

Thus ebgp is more trust worthy than ibgp

however when i look at the selection process, i found the following:


2 ) local preference

3) as path


5) ibgp over ebgp

i may have misplaced the attributes in the list .

So if router receives update

from ebgp and ibgp, router should intall ebgp update because ebgp has ad of 20 but i read in cisco book this is not the case.

router will consider the attributes of the path to select the best path.

any idea why AD is not considered in bgp the way Ad is considered for rip and ospf to determines the best routing source then best path ?

thanks alot and have good day.

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bvsnarayana03 Fri, 11/30/2007 - 20:54
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you are probably confused with Origin attribute which says how a BGP route is learnt. It says routes learnt from IGP are preferred over routes learnt from EGP.

Then there is path selection, if same route is learnt from eBGP & iBGP then eBGP path is preferred over the other.

You seem to have confused between the two. Refer this link for clarity:

Harold Ritter Sun, 12/02/2007 - 11:44
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The AD only comes into play when the same route is learnt from different protocols, so it is irrelevant to the BGP best path selection.

This selection process will definitely prefer the path from ebgp over the one learnt via ibgp assuming other attributes compared previously are the same.


zeeshanr202 Sun, 12/02/2007 - 21:53
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thanks a lot for response. please correct me if iam wrong.

Router first selects the routing protocol by Ad if it is receiving same route from more than one

routing protocols. Then, it chooses the best path from others reported by routing protocol.

So router first chooses routing source then the best path from the paths reported by routing protocol.

I expect, the router receiving same route from

ibgp which has Ad of 200 and from Ebgp which has Ad of 20 to choose the routing protocol which should be ebgp.

After choosing the routing source based on Ad, router should now choose the best path from the paths reported by ebgp.

why is it not so in bgp?

thanks and have a good day !

bvsnarayana03 Sun, 12/02/2007 - 23:28
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Your understanding is correct. But is there any isntance you can bring to our notice which could support the statement in your posting about IBGP path being chosen.

Harold Ritter Mon, 12/03/2007 - 06:10
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It is actually the other way around. Each protocol first selects its best path and if the route is learnt via two or more routing protocols then the AD is used to pick the one that will be installed in the riuting table.



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