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RE: routing sequence [7:131928] posted 07/16/2008
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You are correct.  Unfortunately, that is right.

I had my vendors a bit mixed up, as the last time I demonstrated that
functionality was on Juniper routers not Cisco ones.

So the underlying rule is that longest-prefix match ALWAYS wins.

Scott 

-----Original Message-----
From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf Of
Ibrahim Abo Zaid
Sent: Wednesday, July 16, 2008 4:40 AM
To: cisco@xxxxxxxxxxxxxx
Subject: Re: routing sequence [7:131928]

Hi Scott


I labed the  below trick and it verifies that longest match always wins as
you state but i have a small comment


when configuring local interface with less spessific address 172.16.1./24and
have a more spesific route
172.16.1.0/25 , ping 172.16.1.1 goes to more spesific route not local
interface as you state . i just want to ensure this is right .


thank you
--Ibrahim


On Wed, Jul 16, 2008 at 3:58 AM, Paul Yeo  wrote:

> Thank you very much Iain, Scott and all,
>
> I really need to brush up this...I am confused at times.
>
> Thanks much!
>
> -----Original Message-----
> From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf 
> Of Scott Morris
> Sent: Tuesday, July 15, 2008 6:58 PM
> To: cisco@xxxxxxxxxxxxxx
> Subject: RE: routing sequence [7:131928]
>
> PBR is actually done first.  This technically AVOIDS the routing 
> process (manual routing).
>
> Otherwise, most-specific matching is done on your route.  (longest 
> prefix
> match)
>
> If you have more than one protocol (static or dynamic) that gives the 
> same prefix, administrative distance (AD) is used to select the best 
> path.  If different AD values, only one will appear in "show ip route"
>
> So you can play some interesting tricks with that.  Namely you can 
> have a connected interface (which logically trumps everything) 
> configured as  "ip address 172.1.1.1 255.255.255.0" and then you have 
> a route (static or dynamic doesn't matter) for 172.1.1.0/25 
> (255.255.255.128) pointing to another router.  This is a more specific
match.
>
> Now, if you ping 172.1.1.1 that's your local interface IP.  It will go 
> locally.
>
> If you ping 172.1.1.2, it will go to a remote router because 
> 172.1.1.0/25is more specific than 172.1.1.0/24.
>
> If you ping 172.1.1.150, it will go to the directly connected interface.
>
> Kinda messes with your head sometimes!  But the bottom line is most 
> specific match wins!
>
> HTH,
>
>
> Scott Morris, CCIE4 #4713, JNCIE-M #153, JNCIS-ER, CISSP, et al.
> CCSI/JNCI-M/JNCI-ER
> Senior CCIE Instructor
>
> smorris@xxxxxxxxxxxxxxxxxxxxxx
>
>
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com
> Toll Free: 877-224-8987
> Outside US: 775-826-4344
> Online Community: http://www.IEOC.com
> CCIE Blog: http://blog.internetworkexpert.com
>
>
> Knowledge is power.
> Power corrupts.
> Study hard and be Eeeeviiiil......
>
>
> -----Original Message-----
> From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf 
> Of Paul Yeo
> Sent: Tuesday, July 15, 2008 2:00 AM
> To: cisco@xxxxxxxxxxxxxx
> Subject: routing sequence [7:131928]
>
> Hi folks,
>
> Am I understood this correctly, the routing will process in this way:
>
> 1) very specific static route - e.g. 172.1.1.0 255.255.255.0
> 2) general static route --- e.g. 172.1.0.0 255.255.0.0
> 3) routing protocol route - e.g. eigrp
> 4) pbr
>
> I have a remote router configured with a very specific route than my 
> central router, it seems that the router actually choose the remote 
> router to route to the destination.
>
> Tx,




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