Continuing work on below image:
Autonomous Systems:
R1, R4 – ASN 200
R3, R2, R5 – ASN 100
R8, R10 – ASN 810.1
The next traffic engineering methods used will be MED and AS-Path. These are both methods to influence traffic coming inbound towards the Autonomous System. In the example below, R4 reaching R5 will have two paths via R2 and R3. The two paths can be seen to R5’s loopback with a ‘show ip bgp 5.5.5.5’. They’re both valid routes with equal attributes, but R2 is preferred probably because of a tie breaker that the neighbor has been there longest.
Currently both paths have the same AS length to reach R5, but to modify and prefer traffic to R5 through R3, we can set AS Prepend on R2.
Configuration:
On R2 a prefix list, route-map and set to out will need to be applied. The prefix-list will match the loopback subnet of 5.5.5.0/24
The Route-map will match ip address to this prefix list.
And the route-map will then be applied outbound towards the neighbor we want to have a longer AS Path, router R1.
So again, R1 has two paths to subnet 5.5.5.0/24, one through R2 and one through R3. Adding AS-Prepend to the neighbor statement, we’re essentially saying the AS Path is now 4 Autonomous systems away instead of 1. This can be seen within a ‘show ip bgp’.
If we remove the AS-Prepend route-map we’ll see things go back to normal. The BGP table for R1 will have two even attributable routes to 2.2.2.2, but will only prefer one.
To modify inbound traffic with MED, we’ll need to go through a similar process. We’ll create a prefix list, route-map and assign to neighbor statements on both R2 and R3 toward R1. On R2 we’ll set the MED (metric) to 100, and R3 will be set to 150. Configs below, R2:
Config below, on R3:
After this config is applied and the BGP clear ‘out’ has occurred, the route metrics will show differently in R1.
The route going through the next hop 10.40.1.1 is now the most preferred because Metric now shows 100, while 10.20.1.1 shows Metric of 150.
Note:
- Both sides advertising a MED value should have something set. By default BGP without any value will be best.
- bgp bestpath med missing-as-worst can override the default behavior.
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