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Posts Tagged ‘ISP’

Using ISP Community Strings to Alter Local Preference

posted in Cisco Networking, Technical
by on September 10th, 2010 tags: , , ,

Introduction

In this post, I want to discuss ISP local preference and their use of inbound community strings to influence it. We’ve covered this a few times on the forum, but this post will document why ISPs implement this, why it matters to their customers, and how it works in a basic GNS3 simulated ISP environment.

Why Do ISPs Raise Their LP on Customer Routes?

Why do ISPs raise the local preference on the routes customers advertise to them? Revenue. Think about the different types of external connections that ISPs have and the costs associated with each one. First, there is the customer link which is pure revenue. Every bit traversing these links adds to the ISP’s bottom line so naturally ISPs are going to prefer to send traffic to them. Second, peer links. An ISP has a few if not many peering links that are considered mutually beneficial to both parties. Therefore no money exchanges hands besides the cost of the link itself. The ISP can send and receive as much data on these links as they want without having to consult the accounting team. Lastly, there are transit links. Tier 1 providers may not have transit links per se because they are top dogs with worldwide networks, but the tier 2 and lower providers do. They have to pay the higher tier providers to send traffic to them much like an end customer has to pay their provider to send traffic to them. And that’s the key, customer = revenue, peer = no revenue/small cost, and transit = cost.

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BGP Multihoming with a Single ISP

posted in Cisco Networking, Technical
by on July 21st, 2010 tags: , , , , ,

Company XYZ has decided to add a backup link to add redundancy to their Internet connectivity. The current link with their ISP is over the 12.45.78.0/31 network. Their ISP has given them another link with a network address of 12.23.56.0/31 and has already configured their routers. Company XYZ has decided to use one of three methods to enable redundant Internet connectivity and needs to lab each of the three methods to see which they prefer.

The three methods they will be testing are:
1) Keep the current link (12.45.78.0/31) as their main link. The new link (12.23.56.0/31) will be the backup link in case the main link goes down. They will use Cisco’s Weight attribute to differentiate between the main and backup link.
2) Keep the current link as their main link. The new link will be the backup link in case the main link goes down. They will use the Local Preference attribute to differentiate between the main and backup link.
3) Use both links at the same time to load balance traffic via BGP multipath.

As a final step, Company XYZ wishes to aggregate their local networks that BGP will advertise.

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