BGP – Transit AS

In the previous blog, we discussed, how to configure weight attribute. Now when I started to prepare the design for the next attribute, I felt that, let’s first clear about transit AS.

By default, eBGP peers share all routes/prefixes with each other. In the above example, considering AS-7090, ISP1 and ISP2 will share all routes with AS-7090. In this situation ISP1 and ISP2 can route the traffic via AS-7090, becoming AS-7090 as a Transit AS.BGP Attributes 2 - Local Preference.001

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BGP Path Attributes 1 – Weight

“WEIGHT” is Cisco Proprietary Attribute and it’s not included in the BGP updates. Weight can be set between 0-65535 (default value is 0) and higher value is more preferable. Weight is used within individual router, means its locally significant. When BGP choosing best path, Weight is considered first above all the other attributes.

BGP router set the weight to 0 on learned routes from peers and 32768 for locally generated routes.

In the below design and configuration output, all routes are configured with minimum configuration and we will focus on INBL and INTERNET routers to understand weight attribute in more detail.

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