CCNP/CCIE Exam Strategy: Understanding BGP Weight

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BGP Best Path Selection Cisco Documentation
https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

In this tutorial video I cover traffic engineering in BGP using the Weight attribute. The weight attribute is a Cisco-defined attribute. This attribute uses weight to select a best path. The weight is assigned locally to the router. The value only makes sense to the specific router. The value is not propagated or carried through any of the route updates. A weight can be a number from 0 to 65,535. Paths that the router originates have a weight of 32,768 by default, and other paths have a weight of 0.

Routes with a higher weight value have preference when multiple routes to the same destination exist.

I demonstrate this in the video using my EVE-NG lab. Here is the config I used on R1 to change the weight.
The first option I did was setting the weight at the neighbor level under the BGP process using the following command:

router bgp 100
neighbor 20.0.0.2 weight 9000

That worked great but it applied the weight value of 9000 to all prefixes from that neighbor. Sometimes its better to be as specific as possible.

Option 2 I decided to use a route-map and a prefix-list to set the weight but only on a single prefix: 1.1.1.0/24

I applied that inbound on neighbor ISP-B. The effect was that out of all prefixes I received inbound from ISP-B, only the 1.1.1.0/24 prefixes would have its weight value changed.

Here is the config for option 2.

ip prefix-list WEIGHT_ISP_B seq 5 permit 1.1.1.0/24
route-map WEIGHT_ISP_B permit 10
match ip address prefix-list WEIGHT_ISP_B
set weight 40000
route-map WEIGHT_ISP_B permit 20
!
router bgp 100
neighbor 20.0.0.2 route-map WEIGHT_ISP_B in

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#ccna #ccnp #ccie
00:00 Intro
00:25 Theory
01:22 Lab Time
09:05 Outro

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