BGP PATH SELECTION

In this lab we explore how BGP does path selection based on NLRI and attributes.

The lab is a simple one whereby AS_Path and weight are used to influence path selection. Weight is local to each router and it influences the path that SITE-A uses while sending out traffic to SITE-B. Higher weight is preferred and in my case I assigned a weight of 1500 for prefixes learned via ISP1 and weight of 1000 for prefixes learnt via ISP2.

I also applied route-maps for advertisements to ISP1 and ISP2, with ISP1 having the shorter AS-Path. The route-map applied to ISP2 prepends AS 65000 and makes it a less favourable route for inbound traffic to SITE-A.

Keep in mind that in the real world, the customer does not have access to the ISP routers.

NOTE the repeated 65000 in the path associated with ISP2 (203.0.114.2)

NOTE the weight set for prefixes outside the local router.

I encountered a problem while trying to set up BGP AS 65000 on SITE_B_RTR whereby routes were not being advertised into the BGP instance. I decided to use AS 65005 instead of allowing re-advertisement back into the same AS.

The lab was configured on a GNS3 instance on my local machine, a HP 840 G3 with 12gb ram. RAM and CPU consumption 70% and 38% respectively.

I hope you enjoyed this piece and thank you for your time.