What is BGP Allowas-in Feature? How to configure BGP Allowas-in? GNS3

Описание: What is BGP Allowas-in Feature? How to configure BGP Allowas-in? GNS3
BGP Allowas-in is a configuration option in Border Gateway Protocol (BGP) routing that allows a router to accept routes with its own AS (Autonomous System) number in the AS_PATH attribute.


There's a risk of creating routing loops if not managed properly. It should be used cautiously and with a clear understanding of network topology.

Purpose: Normally, BGP prevents loops by rejecting routes that contain the local AS number in the AS_PATH. However, in scenarios like multi-homed environments or when dealing with MPLS VPNs, you should allow routes to come back into your AS through different paths.


Functionality: When you configure allowas-in, you specify how many times your AS number is allowed to appear in the AS_PATH. This number can be set between 1 to 10, although this varies by router vendor and model. For example, setting allowas-in 2 would allow routes with up to two instances of your AS number in the path.

Usage Scenario:
Multi-homed Networks: If you have connections to the same ISP in different locations, allowas-in can prevent unnecessary route blackholing by allowing these routes back into your network.
MPLS VPNs: In scenarios where customer routes are sent through a service provider network and back, allowas-in ensures these routes are not dropped.


Configuration Example (Cisco IOS-like syntax):

plaintext
router bgp 65100
neighbor 12.1.1.2 remote-as 65200
neighbor 12.1.1.2 allowas-in 2
end

To allow the re-advertisement of all prefixes that contain duplicate ASNs, use the neighbor allowas-in command in router configuration mode in Router


Risks:
There's a risk of creating routing loops if not managed properly. It should be used cautiously and with a clear understanding of network topology.
It can also lead to suboptimal routing if not configured correctly.
Best Practices:
Use sparingly and only where necessary.
Monitor BGP updates closely when employing this feature.
Combine with route-maps or other BGP policies to control which routes are accepted.
This configuration can significantly affect network reachability and BGP convergence, so it should be deployed with a good understanding of BGP dynamics and network design.
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