Solved: BGP Best route clarification

SOL10 · ‎06-11-2008

Hi all

with reference to how BGP select the best route (weight, local pref etc) can someone please clarify or give further explanation on the following:

1) Lowest IGP metric to Next hop (is this not the same as MED)

2) Most Recent Route

Regards

foxbatreco · ‎06-11-2008

Lowest IGP metric to NH is the one which actually sort of least cost path to reach the NH which wud have this peer reach the other BGP peer and evolve in exchange of BGP routes.

MED is one which is actually used to comeinto your bgp router from ur connected BGP peer in different AS.the peer uses MED to direct the router which exit point to take when goin for you peer.

Most recent route indicates d current best path is kept as it is unless there are no drastic changes in other parameters.

Rate if this helps!!!

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Harold Ritter · ‎06-11-2008

Suleiman,

1) IGP metric to BGP next hop is different than MED. MED is actually a BGP attribute that can be set automatically or manually set by the peer AS to influence the best exit point, whereas the IGP metric is the metric for getting from BGP peer receiving the route to the BGP next hop set in the BGP update.

2) This is actually an IOS optimization. All other parameters being equal, IOS doesn't replace the current path solely based on the router-id. The current path is kept has the best one.

Regards,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
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Cuauhtémoc, Juárez
Ciudad de México, 06600
México

foxbatreco · ‎06-11-2008

Lowest IGP metric to NH is the one which actually sort of least cost path to reach the NH which wud have this peer reach the other BGP peer and evolve in exchange of BGP routes.

MED is one which is actually used to comeinto your bgp router from ur connected BGP peer in different AS.the peer uses MED to direct the router which exit point to take when goin for you peer.

Most recent route indicates d current best path is kept as it is unless there are no drastic changes in other parameters.

Rate if this helps!!!

SOL10 · ‎06-12-2008

thanks guys for your replies. This clears it up for me.