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Routing table growth
One of the largest problems faced by BGP, and indeed the Internet infrastructure
as a whole, comes from the growth of the Internet routing table. If the global
routing table grows to the point where some older, less capable, routers cannot
cope with the memory requirements or the CPU load of maintaining the table,
these routers will cease to be effective gateways between the parts of the
Internet they connect. In addition, and perhaps even more importantly, larger
routing tables take longer to stabilize (see above) after a major connectivity
change, leaving network service unreliable, or even unavailable, in the interim.
Until late 2001, the global routing table was growing exponentially, threatening
an eventual widespread breakdown of connectivity. In an attempt to prevent this
from happening, there was a cooperative effort by ISPs to keep the global
routing table as small as possible, by using CIDR and route aggregation. While
this slowed the growth of the routing table to a linear process for several
years, with the expanded demand for multihoming by end user networks the growth
was once again exponential by the middle of 2004. The global routing table hit
200,000 entries on or about October 13, 2006.
A network black hole is often used to improve aggregation of the BGP global
routing table. Consider an AS that has been allocated the address space
172.16.0.0/16, from which it has assigned the prefixes 172.16.0.0/18,
172.16.64.0/18, and 172.16.192.0/18. The AS can advertise the whole block,
172.16.0.0/16. This AS will still receive traffic sent to the "hole",
172.16.128.0/18, but will silently discard it.
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