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BGP Best Path Selection Question

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#1 jimmyt

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Posted 13 August 2014 - 07:46 PM

Hi There,

 

I'm doing a practice CCIE lab and I'm still struggling a little with the BGP best path selection process. Can anyone tell me what path attribute causes the best route to be selected from the following output? Please note that the router with BGP router-id of 150.11.2.2 is a an IBGP peer. The other two are eBGP peers. Also, why does the route with a next hop of 54.11.7.254 have an empty metric attribute and how is this compared to the metrics of zero?

 

Rack11R6#show ip bgp
BGP table version is 11, local router ID is 150.11.6.6
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
 
   Network          Next Hop            Metric LocPrf Weight Path
* i28.119.16.0/24   204.12.11.254    0    100              0 54 i
*                   204.12.11.254              0                        0 54 i
*>                  54.11.7.254                                           0 54 i
 
 
Rack11R6#show ip bgp 28.119.16.0/24
BGP routing table entry for 28.119.16.0/24, version 2
Paths: (3 available, best #3, table Default-IP-Routing-Table)
  Advertised to update-groups:
        1    2
  54
    204.12.11.254 from 204.12.11.2 (150.11.2.2)
      Origin IGP, metric 0, localpref 100, valid, internal
  54
    204.12.11.254 from 204.12.11.254 (31.3.0.1)
      Origin IGP, metric 0, localpref 100, valid, external
  54
    54.11.7.254 from 54.11.7.254 (212.18.3.1)
      Origin IGP, localpref 100, valid, external, best
 

 


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#2 martinlo

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Posted 15 August 2014 - 12:14 AM

I am  thinking : 1st external wins over internal; then oldest route

 

MED -or metric- have some restrictions and extra commands like med deterministic, always-compare med, missing med as worst;

show run all or full to see hidden and/or default  bgp commands ; some routers or cisco routers default behavior is to assign a value of 0 to the missing MED. so, in your case, med is not being compared at all.


Edited by martinlo, 16 August 2014 - 10:16 PM.

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#3 Gökhan

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Posted 31 August 2016 - 11:19 PM

Hi,

 

For BGP Path Selection, you need to check BGP Path attributes...

Read these...

http://ipcisco.com/b...ributes-weight/

http://ipcisco.com/b...cal-preference/

http://ipcisco.com/b...ibutes-as-path/

http://ipcisco.com/b...ributes-origin/

http://ipcisco.com/b...attributes-med/

 

Kind Regards,


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