BGP Link Redundancy

[dedra_live]

Hello,

1) In an MPLS cloud, if the primary and secondary links are configured without the metric, does the BGP AS number act as a metric value

2) What is the influence of AS number on AS-path prepend (in absence of metric)

Thanks.

[Kishore Chennupati]

Hi dedra

1) In an MPLS cloud, if the primary and secondary links are configured without the metric, does the BGP AS number act as a metric value
A.  When you say metric do you mean the BGP attribute? If yes, then if you look at the BGP path calculation then AS-PATH is one of the attribute and does act as a decider. It also helps in avoiding routing loops as well.

If you are talking about MED as a metric then , remember that AS_PATH comes before MED in the BGP path selction algorithm so it will take preference. Please see below for the BGP algorithm

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

2) What is the influence of AS number on AS-path prepend (in absence of metric)
  AS_PATH prepend is used to make links prefer one over other. This is generally used in conjunction with Local pref where you want a primary/ secondary link scenario for high availiblity. You use high local pref at the primary end and use AS_PATH prepend at the secondary site so that the routing is not assymetric/sub optimal.
Below is a very good link that talks about influencing traffic paths
https://supportforums.cisco.com/docs/DOC-15352#Q_What_is_asymmetric_routing_and_what_are_the_effects_of_it

HTH,

Regards

Please rate if helps

[Marwan ALshawi]

to add to the above post

AS path alomst accepted by all service providers to be used for BGP path selction

while MED is not really accepted by many SP

if you thinking about BGP path slection for primary and Backup just use AS path prepending simple and achieve your needs

hope this help

if helpful Rate