Private ASNs for Internal BGP Routing

Introduction :

The internet widely utilizes BGP (Border Gateway Protocol) as an external gateway protocol for transmitting routing and reachability data between autonomous systems (ASes). Autonomous systems consist of collections of IP networks and routers controlled by a single organization, which presents a unified routing policy to the internet.


bgp private asns-BGP uses AS numbers to identify and differentiate between different autonomous systems. AS numbers are divided into two categories: public ASNs and private ASNs.

1. Public ASNs:

The Regional Internet Registries (RIRs) assign globally unique AS numbers, which serve to identify autonomous systems on the public internet. These public ASNs facilitate the routing of traffic between various organizations and ISPs.

2. Private ASNs:

Private AS numbers are employed within restricted networks like an organization’s internal network or a data center.

The ranges of private ASNs in BGP are as follows:

  1. 16-bit Private ASNs: These ASNs are used in the traditional 16-bit format and fall within the range from 64512 to 65534 (inclusive), as well as 65535.
  2. 32-bit Private ASNs: With the introduction of 32-bit ASNs to accommodate the growing demand for unique AS numbers, the range for 32-bit private ASNs is from 4200000000 to 4294967294 (inclusive).

Here’s a summary of the private ASN ranges:

  • 16-bit Private ASNs: 64512-65535, 65535
  • 32-bit Private ASNs: 4200000000-4294967294

Example of how private ASNs might be used in a scenario involving an organization with multiple data centers and a need for internal BGP routing.


Multi-Site Organization with Private ASNs

Organization XYZ operates three data centers located in different geographic regions. The organization wants to implement BGP routing within its internal network to optimize traffic flows between these data centers.

To maintain privacy and security, they decide to use private ASNs.

Data CenterLocationPrivate ASN
ANew York65500
BLos Angeles65501

1. Data Center A:
– Location: New York
– Private ASN: 65500

2. Data Center B:
– Location: Los Angeles
– Private ASN: 65501

3. Data Center C:
– Location: Chicago
– Private ASN: 65502


1. Configuring BGP:
– Each data center is configured to use BGP internally. Routers within each data center run BGP to exchange routing information.
– Each data center uses its respective private ASN (65500, 65501, 65502) for internal BGP sessions.

2. Traffic Engineering:
– Organization XYZ can use BGP policies to influence traffic flows between data centers. For example, they can prioritize certain links or paths based on network performance or cost.

3. Maintaining Privacy:
– External peers and ISPs on the public internet will never see the private ASNs (65500, 65501, 65502) as they are not advertised externally. Only internal routers use these private ASNs for BGP routing decisions.

4. Multi-Homed Connectivity:
– If Organization XYZ has multiple internet connections in each data center, they can balance traffic across these connections using BGP. Each data center can make independent routing decisions based on its private ASN.

5. Expansion and Flexibility:
– If the organization expands by adding more data centers, each new location can be assigned a unique private ASN, ensuring segregation of routing domains.

6. Migration or Changes:
– If Organization XYZ needs to restructure its network or migrate to different addressing schemes, the use of private ASNs allows them to do so with minimal disruption to external BGP peers.

Remember that this is a simplified example. In practice, BGP configurations can become quite complex, involving various policies, route filtering, and route redistribution. Proper planning, documentation, and adherence to BGP best practices are crucial for maintaining a stable and efficient network.

Always consult the latest documentation and resources specific to your networking equipment and software to ensure accurate implementation.

Lets summarize the example scenario of an organization with multiple data centers using private ASNs for internal BGP routing:

Data CenterLocationPrivate ASNPurpose/Function
ANew York65500Internal BGP routing within Data Center A
BLos Angeles65501Internal BGP routing within Data Center B
CChicago65502Internal BGP routing within Data Center C
Implementation Details:
– BGP Configuration– BGP used within each data center for internal routing
– Traffic Engineering– Optimize traffic flows between data centers
– Maintaining Privacy– Private ASNs not advertised externally
– Multi-Homed Connectivity– Balance traffic across multiple connections
– Expansion and Flexibility– Each new location gets a unique private ASN
– Migration/Changes– Restructure network with minimal external impact

More information about private ASNs and their significance in BGP:

1. Internal Network Routing:

Organizations primarily utilize private ASNs to implement BGP routing within their internal networks. BGP, a powerful and flexible protocol, serves as a means for finely tuning routing decisions within an autonomous system. By using private ASNs, organizations can take advantage of BGP’s capabilities to manage traffic flows and optimize routing within their own network infrastructure.

2. Traffic Engineering:

Private ASNs enable organizations to perform traffic engineering and control how traffic flows within their internal network. BGP’s policy-based routing capabilities allow administrators to influence the path that data takes within the network, improving performance and ensuring efficient utilization of network resources.

3. Security and Privacy:

By using private ASNs, organizations can maintain a level of security and privacy by hiding their internal network structure from external entities.

4. Multi-Homed Networks:

Private ASNs can be used in scenarios where an organization has multiple connections to different ISPs (Internet Service Providers).

5. Data Centers:

BGP can be used to manage routing between different parts of the data center, and private ASNs help in keeping the internal structure hidden.

BGP Private ASNs

Leave a Comment