Skip to main content

AS Path Prepending: Controlling Inbound Traffic in BGP

AS Path Prepending is a BGP feature used to make a specific path appear less preferred by artificially lengthening the AS path. This is done by adding your AS number multiple times to the AS path. It is a common method to influence inbound traffic from external networks.

  • Longer AS Path = Less preferred route.

Example Scenario:

You have two ISPs: ISP1 (through CE1) and ISP2 (through CE2). You want inbound traffic from the internet to prefer ISP1 over ISP2.

Network Topology:

  • CE1 (connected to ISP1): 10.0.1.1/30
  • CE2 (connected to ISP2): 10.0.2.1/30
  • iBGP Router (Internal) connected to both CE1 (10.0.1.2/30) and CE2 (10.0.2.2/30).

Configuration on CE2 (AS Path Prepending to Make ISP2 Less Preferred):

Create a route map to prepend your AS path multiple times for CE2:


route-map PREPEND_AS permit 10
 set as-path prepend 65001 65001 65001

Apply this route map to the neighbor in the BGP configuration for CE2:


router bgp 65001
 neighbor 10.0.2.1 remote-as 65002
 neighbor 10.0.2.1 route-map PREPEND_AS out

Configuration on CE1 (No AS Path Prepending):

No AS Path Prepending is needed for CE1:


router bgp 65001
 neighbor 10.0.1.1 remote-as 65000

Result:

Inbound traffic will prefer ISP1 (CE1) because CE2 has a longer AS path due to AS Path Prepending.

Labels:

Comments

Post a Comment

Popular posts from this blog

How to import Putty Saved Connections to mRemoteNG

Just started using mRemoteNG and its being very cool to connect to different remote connection with different protocols e.g Window Remote Desktop, VNC to Linux, SSH, HTTP connection etc. from a single application. As new user I configured some remote desktop connection which was quite easy to figure out. But when I wanted to add SSH connections, it came in my mind to import all of the saved connections in the putty. But I couldn't figure it out how can it be done, though it was quite easy and here are the steps. Open your mRemoteNG Create a folder if you want segregation of multiple networks Create a new connection Enter the IP address of remote server under connection in Config pane Under the config pane, select protocol " SSH version 2 ".  Once you select protocol to SSH version 2 you are given option to import putty sessions, as shown in the snap below. In the above snap, I have imported CSR-AWS session from my saved sessions in Putty.
1 comment
Read more

SSL VPN vs IPSec VPN: Understanding the Key Differences

When it comes to securing communication over the internet, Virtual Private Networks (VPNs) are a cornerstone of modern networking. Two popular types of VPNs are SSL VPN and IPSec VPN . While both serve to encrypt and secure data, they differ significantly in terms of technology, use cases, and implementation. This blog post will help you understand these differences and choose the right solution for your needs. What is SSL VPN? An SSL VPN uses the Secure Sockets Layer (SSL) or its successor, Transport Layer Security (TLS) , to create a secure connection. It operates at the Application Layer (Layer 7) of the OSI model and is designed to provide secure access to specific resources over the internet. Key Features of SSL VPN: Ease of Use: Accessible via a standard web browser without the need for dedicated client software. Granular Access Control: Allows users to access specific applications or resources rather than the entire network. Port Usage: Uses TCP port 443, which is rarely...
Post a Comment
Read more

BGP Soft Reconfiguration vs. Route Refresh: Key Differences and Best Practices

In BGP (Border Gateway Protocol), managing route updates and reapplying new policies can sometimes be challenging, especially if you want to avoid resetting the BGP session. Two methods allow you to update routing policies without tearing down the session: BGP Soft Reconfiguration and BGP Route Refresh . While both methods serve the same purpose, they work differently and have distinct impacts on your router's resources. This post explains the key differences between Soft Reconfiguration and Route Refresh , when to use each, and why Route Refresh is preferred in most modern networks. 1. What is BGP Soft Reconfiguration? BGP Soft Reconfiguration is an older method of applying new policies (like route maps, filters, or prefix lists) without resetting the BGP session. It works by storing a local copy of all the routes received from a BGP neighbor before applying inbound policies. This local route copy allows the router to reprocess the routes when a policy change occurs. How So...
Post a Comment
Read more