Internet Routing Table Storage

[alistair777]

Hi,

If I am multihoming with two providers with two seperate routers and running iBGP between them - will both my routers have to store two seperate copies of the routing table thus consuming twice the TCAM space? So roughly over 1 million routes?

[Masoud Pourshabanian]

Hello,

You did not mention what kind of router you have. The number of routes can be stored in routers depens on the router and its memory. You can check your router data sheet to find out the numbers of route and also consider a safe margin.

As an example, Cisco ASR 1000 series can support

With 8-GB memory:

●  Up to 1,000,000 IPv4 routes or 1,000,000 IPv6 routes

●  BGP RR Scalability up to 8,000,000 IPv4 routes or 6,000,000 IPv6 routes

With 16-GB memory:

●  Up to 4,000,000 IPv4 routes or 4,000,000 IPv6 routes

●  BGP RR Scalability up to 24,000,000 IPv4 routes or 17,000,000 IPv6 routes

http://www.cisco.com/c/en/us/products/collateral/routers/asr-1000-series-aggregation-services-routers/data_sheet_c78-441072.html

Hope it helps,

Masoud

[paul driver]

Hello

is their a reason why you have to store a full internet routing table?

res

Paul

[alistair777]

Yes Paul I would like to prefer some routes over others to certain autonomous systems.

Would this meaning storing two copies of the routing table on each router in the TCAM?

I've read material saying that only one table is stored at once into the FIB (the best routes) still not sure on this and I could do with some clarification.

[Richard Burts · Accepted solution]

To arrive at an appropriate answer to your question you need to understand that the FIB is one place where routes learned from a neighbor are stored. But it is not the only place. You are correct that the FIB will contain what the router has determined to be the "best" routes and typically a single route for each prefix. But if you are running BGP then the router must also store in BGP tables (separate from the FIB) what it has learned from its neighbors. So if 2 ISP advertising full routes then the router BGP table will have two copies of full routes (one from external neighbor and one from IBGP).

HTH

Rick

[Richard Burts]

I am glad that my explanation was helpful. Thank you for using the rating system to mark this question as answered. This will help other readers in the forum to identify discussions that have helpful information.

HTH

Rick

[Joseph W. Doherty]

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Posting

Just to add what Rick has already noted, TCAM would not be used to hold the BGP tables, it would only be used for the resultant "merge" of the two.  So, your TCAM demand wouldn't normally grow unless each BGP table has a route unknown to the other.  (NB: TCAM might grow if you have multiple paths to the same prefix with equal cost.  Cisco's eBGP normally doesn't do ECMP by default, but Cisco's can be configured to do so.)

PS:

Many low-end switches do not have sufficient TCAM to deal with the number of routes known to the Internet.  Even switches like the 6500/7600 have "special" supervisor variants with much larger than "normal" TCAM resources (on those sups, the "XL" variants).

[AmandaBrown2015]

Hi, Paul

If I do not want to store the full internet routing table, what are the best practices?

can you please assist?

Thank you

[Richard Burts]

Can we begin by clarifying the environment in which you operate? Are you using a single router or a pair of routers (as was described in the original post in this discussion)? Are you running BGP with one ISP or multiple ISP?

If you are running BGP with an ISP the best practice is to negotiate with your ISP about what they will advertise. Every ISP that I can remember dealing with offers options about what they will advertise to you and the options usually include 1) advertising full routes 2) advertising partial routes (usually routes that are withing a couple of AS hops of the ISP 3) advertising only a default route.

And if you have a single router and are using a single ISP I would ask why you are using BGP? In this case it is easy to just configure a static default route and to not run any dynamic routing protocol with the ISP.

HTH

Rick

[AmandaBrown2015]

Hi Rick

We have a single router running BGP with one ISP.

[Richard Burts]

Thanks for clarifying about your environment. If you have a single router communicating with a single ISP then I would ask why you are using BGP? For a single router communicating with a single ISP it would seem that configuring a static default route on your router and having the ISP forward to you for your address block would be sufficient and removes the question about storing the Internet routing table, removes sources of overhead on your router, and simplifies the configuration.

If there are reasons why you want to run BGP with this ISP then I would stand by my advice in a previous post that the best practice would be to negotiate with the ISP about what they advertise to you and prefereably having them advertise to you only a default route.

HTH

Rick

[AmandaBrown2015]

Thank you very much Richard

I will look into the default route option.