The Global Internet Routing Table Reaching the 768k Milestone

Back in 2014, I wrote an article that highlighted that global Internet routing table passed the 512,000 or 512k route mark. Today we know that another significant milestone has been reached, as we passed the 768,000 or 768k route mark!  Many have predicted Internet outages may be expected. In short, the “sky is not falling”. The possibility of TCAM resource exhaustion at 768k routes is a known issue that we all know has been coming for some time. There is no related security vulnerability, and it cannot be easily triggered by a remote, untrusted user.

As I mentioned in my previous article, we’ve known for some time that the Internet routing table growth could cause Ternary Content Addressable Memory (TCAM) resource exhaustion for some networking products. TCAM is a very important component of certain network switches and routers that stores routing tables. It is much faster than ordinary RAM (random access memory) and allows for rapid table lookups. No matter who provides your networking equipment, it needs to be able to manage the ongoing growth of the Internet routing table. We recommend confirming and addressing any possible impacts for all devices in your network, not just those provided by Cisco. The products that could be affected include those with a default configuration supporting 512k routes.

Modern routers such as the Cisco ASR Routers support millions of routes. For instance, the Cisco ASR 1000 Series Aggregation Services Routers support up to 4 million routes.

The best practices and workarounds in my previous article are still valid. If you are running legacy devices, consider action if any of those devices were previously hard set to 768K routes.

For example, this document describes how to identify and resolve a common problem caused by growth of the Internet routing table. The document describes how you can detect when a Cisco ASR 9000 Series Aggregation Services Routers with the Trident-based line cards reaches its prefix limit. The router logs messages such as the following when the limit for a Trident-based line card is reached:

LC/0/3/CPU0:Apr  22 01:15:18.103 : fib_mgr[169]: %ROUTING-FIB-4-RSRC_LOW :
 CEF running low on DATA_TYPE_TABLE_SET resource memory. CEF will now begin
 resource constrained forwarding. Only route deletes will be handled in this state, which may result in mismatch between RIB/CEF. Traffic loss on certain prefixes can be expected. CEF will automatically resume normal operation, once the resource utilization returns to normal level.
 

Once the Trident-based line cards begin to display the %ROUTING-FIB-4-RSRC_LOW message, an outage for some prefixes occurs.

Route filtering and the use of a default route can also be used to decrease the number of routes in an affected device. Prefix lists can be used as an alternative to access lists in many BGP route-filtering commands. The use of prefix lists provides significant performance improvements when loading and performing route lookup of large routing tables. Additional information about BGP best practices and configuring prefix lists is available at: click here.

Implementing the recommended workarounds ahead of time will help your network avoid any performance degradation, routing instability, or impact to availability.

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