Jon Lewis's Blog
09 02 2008

Sat, 09 Feb 2008

One Million Routes

So, you just upgraded your cisco 6500/7600 gear to Sup720-3BXL's because that's the lowest end supervisor module that has the tcam for full internet routes (>244k routes). You may have read in the data sheet that it's capable of "1,000,000 IPv4 routes; 500,000 IPv6 routes." That should be plenty of room for growth, right? Well, maybe not as much as you think.

Somewhere burried in the fine print (ok, I can't actually find it even in fine print or an * or anywhere on the data sheet), is the fact that it's an either or thing. i.e. The 3BXL can do 1,000,000 IPv4 routes (and no IPv6 at all), or it can do 500,000 IPv6 routes (and no IPv4 at all). In a real world installation, neither of those configs are terribly useful. The default settings allow for 524,288 IPv4 routes and 262,144 IPv6 routes...meaning in its default config, with full internet routes a Sup720-3BXL is already at nearly half it's capacity of IPv4 routes. You can examine this (using recent IOS versions), with the command:
show platform hardware capacity

Look for the output section labeled "L3 Forwarding Resources". i.e.
L3 Forwarding Resources
             FIB TCAM usage:                     Total        Used	%Used
                  72 bits (IPv4, MPLS, EoM)     524288      230589	  44%
                 144 bits (IP mcast, IPv6)      262144           5	   1%
This can be tuned with the config command mls cef maximum-routes ip <N> where N is a number in thousands of IPv4 routes you want to be able to handle. i.e. With "mls cef maximum-routes ip 750", the above output changes to:
L3 Forwarding Resources
             FIB TCAM usage:                     Total        Used       %Used
                  72 bits (IPv4, MPLS, EoM)     770048      230459         30%
                 144 bits (IP mcast, IPv6)      139264           5          1%

Such a split may make more sense, as it leaves more room for anticipated IPv4 routing table growth, and in a perfect world, we really shouldn't see much more than a single IPv6 prefix per ASN.

Note: The numbers above from a set of Sup720-3BXL's in a lab environment have slightly filtered BGP feeds. "Full routes" would be closer to 240,000 routes.

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