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AS prepending is very important for path selection; you can manage the incoming traffic to your site. Many customers use this feature for active/passive DC design. Today I will show you this BGP feature with a basic CISCO lab.

We announce the same IP subnet for redundancy If the DC-1 fails or any problem occurs. I prepared the basic lab with PNET.

When I check the BGP route table on the PROVIDER router, the AS Path is the same length.

bgp route table

I will configure the prepend feature on the BGP-DC-2 router as below.

route-map PREPEND permit 10
 set as-path prepend 5252 5252
router bgp 5252
 bgp log-neighbor-changes
 neighbor remote-as 5353
 neighbor route-map PREPEND out

Also, I change the BGP timer on the Provider router to see the result quickly.

router bgp 5353
 timers bgp 3 30

After the configuration is completed, the result is below;

This is subnet base redundancy.

Let's test for redundancy.

Thanks for Reading.

The topology and configuration are below.

Download ZIP • 62KB

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