An AS is considered stub, or single-homed, when it reaches networks outside its domain via a single exit
point. Figure
illustrates a single-homed or stub AS. A single-homed AS does not need to learn all the routes in the internetwork backbone,
because there is only one way out -- all traffic destined to a network
not within the AS can be routed along the default route toward this
single exit point.
To connect to the Internet, an AS (or
domain) will be connected to an Internet Service Provider (ISP),
which then connects to the Internet backbone. Single-homed ASs on
the Internet then have a single connection to an ISP. The service
provider this single-homed AS is connected to must still provide
information to the rest of the Internet about destinations which are
reachable within its downstream network.
There are different methods for the
provider to advertise the customer's routes. For instance, the
provider could list the customer's subnets as static entries in its
router and advertise these static entries towards the core of the
network. This method scales well if the customer's routes can be
represented by a small set of aggregate routes. When the customer
has too many discontiguous subnets, listing all these subnets as
static routes in the provider's routers becomes inefficient. Another
alternative is to use an IGP to advertise the customer's networks
towards the service provider. This has all the benefits of dynamic
routing, where network information and changes are dynamically sent
to the provider.
The third method by which the ISP can
learn and advertise the customer's routes is to use BGP. It is
difficult to get a registered AS number from IANA because the
customer's routing policies are an extension of the policies of the
provider. Instead, the provider can give the customer an AS number
from the private pool of ASs (65412-65535), and strip these ASs out
of the routing path information when advertising the customer's
routes towards the core of the internetwork.
Quite a few combinations of protocols
can be used between the ISP and the customer; Figure
illustrates some of the possible configurations for stub ASs
(external BGP [EBGP] and internal BGP [IBGP] are discussed in later
sections). The service providers might place a customer router in
their point of presence (POPs), or they might place their router in
the customer's network. Note that not every situation requires a
customer to run BGP with its provider, as mentioned earlier.
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