ISP router routing table

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Oct 10th, 2014
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Hi

 

I have very limited knowledge on the same , So I am looking for info

I connected with tier-1 ISP via EBGP, what kind of routing table I will get - default,Entire routing table, partial ?

What are they & what are the pros/cons of the same

 

Tx

subhojit

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Jody Lemoine Fri, 10/10/2014 - 08:57
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It depends largely on the policies of the provider and the nature of your connection with them.

If you have only a single connection, all you really need is a default route because there is no point in asking for anything else. This approach makes for minimal traffic and memory usage, but there's no advantage as far as outbound routing because you only have one destination.

If you're connecting with multiple providers, it makes sense to request a full feed so that you can take advantage of the optimum path to and from your network. The downside is that a full feed takes more memory and processing to maintain.

Ultimately, BGP is more about establishing routing policy than simply taking what you're given, so it depends on what you need and how you want to be reached. Providers that peer with you via BGP are usually open to those negotiations.

Richard Burts Fri, 10/10/2014 - 18:11
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The answer is pretty much the same whether you are connecting to a tier-1 or some other tier of ISP. The ISP is able to advertise various groupings of routes if you are running BGP. As Jody has correctly pointed out if you are connecting to a single ISP there is not so much need for BGP, and if you are connecting a single router to a single ISP there is no need for BGP. But the original poster asked about the options when you are running BGP to an ISP, so lets discuss the options. Two of the choices are very easy to understand:

- default route means that the ISP will advertise only the default route to you - no other routes are advertised to you.

- full routes means that the ISP will advertise to you the entire Internet routing table - and that is BIG.

 

The ISP will also have at least one choice for partial routes which means that the ISP sends more than just the default and less than the full table. There might be several options for partial routes depending on the ISP. Partial routes might mean just the set of routes that belong to that ISP. Or it might be the routes of the ISP plus the routes of all of the customers who connect directly to the ISP. Or it might be the roues of the ISP plus the routes of all the ISPs that directly connect to the ISP. When you are signing up for the service the ISP will explain to you what options they offer and what they mean.

 

You ask about the pros and cons of the choices and I like to explain it this way. I start with a question:

- how much information does it take to make a good decision?

Most of us would probably agree that the more information we have the better we are able to make a good decision (and conversely the less information we have the harder it is to make a good decision). But there is a cost to this. The more routes are advertised to us the more memory it takes to store them, and the more CPU cycles it takes to keep up with the updates, and the more complicated it becomes searching through all the alternatives and selecting the best one. So generally we make a compromise about how many routes we want to learn.

 

Some customers will decide on just the default since that allows them to use a less powerful router with less memory. Some customers will select partial routes because they want to optimize their choices some of the time but do not want to allocate the resources to process full routes. And some customers will select full routes because they want to optimize all of the routing decisions that they make.

 

HTH

 

Rick

Joseph W. Doherty Mon, 10/13/2014 - 05:15
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Rick makes a good point about information for making a good decision.

If you only have a single ISP connection, what a full Internet table supports is "knowing" whether a destination prefix is "known".  I.e., only forward a packet to the Internet if the destination prefix is in the Internet routing table.  Using just a default, you'll forward packets that are unknown.

But, as Rick also notes, you might trade-off the capability of making such a better routing decision for a less capable router.

As both Jody and Rick have described, ISPs can often offer different options for what they will provide you regarding Internet routing information, but it's up to you to decide what you need and the trade-offs.

For a single link to the Internet, probably just using a default is the most common approach.

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