BGP Local Autonomous System

CUSTOMER uses XYZ ISP. Assume that if in a case XYZ goes down or overtakes by another ISP like ABC above example is set for the understanding of the LOCAL AS feature. You see here CUSTOMER has 300 AS and XYZ has 200 AS.

In the case of overtaking XYZ via ABC then we cannot able to configuration on a customer router. To come out with this problem we use the LOCAL AS feature. No-PREPEND feature to use for that only customer AS will go to in ABC router. REPLACE AS is used to avoid the local AS and customer AS.


If you are studying so first try a little topology to understand those topics.

What is a LOCAL Autonomous System?

BGP Local Autonomous System

Review of BGP neighborship: remote autonomous system is 300 and also the local autonomous system is 200 and also we are using next hope self-command on XYZ router for traffic pass (from R2 to R3). The same configuration is for the ABC router. Now R2 sends both autonomous systems to the customer for all prefixes.

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BGP Configuration on ABC router: BGP configuration with R2 router as basic BGP config. Use remote Autonomous system 100 and Pass a network for testing. Same as on the R2 router for the R3 router.

BGP configuration on customer Router: BGP configuration with R3 router as basic BGP config. Use remote Autonomous system 200 and Pass a network  for testing.

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BGP configuration on XYZ ISP router: BGP configuration with R2 router as basic BGP config. Use remote Autonomous system 300 and Pass a network for testing. But on R2 own autonomous now use 100 due to migration with R1 and after that what will happen and what needs to do to implement we are showing you that all below.

Autonomous system number miss-match error coming because now ABC ISP takes over the XYZ ISP. Now XYZ ISP work on 100 autonomous systems and the R3 customer router show miss match errors for the Autonomous system. The above screenshot clears the configuration of how it happens. To comes out of these problems we are using LOCAL AS features on XYZ ISP.

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LOCAL AS command configuration on XYZ R2 router:

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Neighbourship with R3 has been established:

LOCAL AS has been properly worked on now. After the take of the XYZ ISP by ABC ISP we cannot suffer the customer and are not able to configure the customer end router easily or in a little time. You can see in the above picture that neighbor think that 200 autonomous connected with them as per customer end configuration but here right now XYZ changed with ABC ISP and ABC have own Autonomous system 100. But ABC wants its customer to work in the per same condition without suffering. For this, we are using the LOCAL AS function on providing an edge router.

One more thing if we want to totally ignore the XYZ autonomous system then we are using no prepend feature

What is No-Prepend Autonomous System?

R2# neighbour local-as 200 no-prepend

Via this command used to exit own autonomous to send another. R2 sends the only customer R3 autonomous system to the R1 router.

R2# neighbor local-as 200 no-prepend replace-as

This command is used to send real autonomous systems locally. (LOCAL Autonomous system and customer autonomous system avoid)

Note:- NO-PREPEND work on incoming EBGP neighbor & REPLACE AUTONOMOUS SYSTEM work, ongoing EBGP neighbor.

After all of this, we can ask the customer to change your configuration.

What is Dual Autonomous System?

Dual AS: R2# neighbor local-as 200 no-prepend replace-as dual-as

Accept either real autonomous or from the EBGP peer. Now if the R3 customer changes our Autonomous system with 100 autonomous system neighbors will be up due to the dual autonomous system already on the R2 router.

All this is the way of migrating an ISP with another ISP. An ISP has many customers on its own autonomous system, so during the migration time of an ISP, it is not possible to change the customer-end configuration. All the above commands are used on edge provider routers like R2.

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