Routers at risk
Internet routing table growth puts routers at risk
As the Internet continues to expand, more pressure is being put
on the underlying infrastructure to outpace the growth and ensure
performance. As a result, the Internet routing table may have
reached a point where some routers may be unable to process the
information load and are in danger of breaking down.
The global Internet routing table is closing in on 300,000
routes; it currently is sitting at roughly 280,000 prefixes but
risingquickly. A “route” or “prefix” is a
summary of an IP address range that represents a network or group
of networks. With an average monthly growth rate of about 3,500
prefixes, the table should exceed 300,000 prefixes early in
2009.
Routes are advertised through the Border Gateway Protocol (BGP)
by enterprises to their upstream Internet Service Provider (ISP),
who in turn advertises them to other ISPs. The global Internet
routing table is comprised of thousands of prefixes that include
the path information that tells Internet routers how to route
packets to their destination.
As the size of the routing table grows, some networks may be at
risk if their routers are not up to the performance and memory
requirements. Some networks may have routers that, even if they
have performed effectively for years, may be at risk of melting
down once the routing table reaches a certain size.
“The problem is subtle because Internet routing table
growth is gradual, and so far most routers have kept up. But one
day it will reach a tipping point for some routers,” said
Oliver Ramirez, president of Affiniti Systems, a consulting company
that has addressed the issue for clients.
Some router processing engines -- most notably Cisco’s
Supervisor II module which is popular in the 7600/6500 router class
-- default to holding a maximum 244,000 routes in the
router’s hardware-based content addressable memory or ternary
content addressable memory. After the routing table exceeds that
value, additional routes are process-switched in software.
Software-based processing increases the router’s CPU
utilization to the point where performance can suffer and
eventually the router may crash.
“If all of your routers are roughly the same model and
memory, you may one day wake up to an outage that propagates across
your network,” said Ramirez.
Fears of routing table explosion causing an Internet meltdown
are nothing new. Internet scalability has been a concern since the
early 1990s, starting with the depletion of IPv4 address space,
routing table growth and, more recently, the depletion of
autonomous system numbers.
Ramirez cautioned that the issue would not be widespread, but
would only affect a small portion of enterprises. Major ISPs and
most enterprises have upgraded their networks during the natural
technology refresh to routers that have the appropriate processing
capabilities.
“Most enterprises are not at risk because, even if they
are multi-homed to the Internet, they are probably only propagating
a default route and not full routing tables,” Ramirez
continued.
“But there may be some networks with a few routers at
risk,” Ramirez said. “If you are processing full
Internet routing tables, it is time to take a good look at your
router hardware to make sure they can continue to process the
routes.”
The amount of memory required to support full Internet routing
table is not as easily defined as mapping the number of global
prefixes to MBytes of memory. Other parameters factor into the
equation. According to Cisco:
“The amount of memory required to store BGP routes depends
on many factors, such as the router, the number of alternate paths
available, route dampening, community, the number of maximum paths
configured, BGP attributes, and VPN configurations. Cisco typically
recommends a minimum of 512 MB of RAM in the router to store a
complete global BGP routing table from one BGP peer.”
“At the beginning of the decade when there were less than
80,000 routes, 128 MB of DRAM was more than enough,” stated
Ramirez. “Soon we needed 256, then 512 MB. With all of the
features that routers need to process in addition to its routing
table, we are at a point where you should have at least a 1GB to be
safe.”