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Requirements of a router for use of BGP for Internet and
backbone-of-backbones purposes
Routers, especially small ones intended for Small Office/Home Office (SOHO) use,
may not include BGP software. Some SOHO routers simply are not capable of
running BGP using BGP routing tables of any size. Other commercial routers may
need a specific software executable image that contains BGP, or a license that
enables it. Open source packages that run BGP include GateD, GNU Zebra, and
Quagga. Devices marketed as Layer 3 switches are less likely to support BGP than
devices marketed as routers, but high-end Layer 3 Switches usually can run BGP.
Products marketed as switches may or may not have a size limitation on BGP
tables, such as 20,000 routes, far smaller than a full Internet table plus
internal routes. These devices, however, may be perfectly reasonable and useful
when used for BGP routing of some smaller part of the network, such as a
confederation-AS representing one of several smaller enterprises that are
linked, by a BGP backbone of backbones, or a small enterprise that announces
routes to an ISP but only accepts a default route and perhaps a small number of
aggregated routes.
A BGP router used only for a network with a single point of entry to the
internet may have a much smaller routing table size (and hence RAM and CPU
requirement) than a multihomed network. Even simple multihoming can have modest
routing table size. See RFC 4098 for vendor-independent performance parameters
for single BGP router convergence in the control plane.
It is not a given that a router running BGP needs a large memory. The memory
requirement depends on the amount of BGP information exchanged with other BGP
speakers, and the way in which the particular router stores BGP information. Do
be aware that the router may have to keep more than one copy of a route, so it
can manage different policies for route advertising and acceptance to a specific
neighboring AS. The term view is often used for these different policy
relationships on a running router.
If one router implementation takes more memory per route than another
implementation, this may be a legitimate design choice, trading processing speed
against memory. A full BGP table from an external peer will have in excess of
222,000 routes as of June 2007. Large ISPs may add another 50% for internal and
customer routes. Again depending on implementation, separate tables may be kept
for each view of a different peer AS.
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