Information Technology Reference
In-Depth Information
The enterprise should then originate a default route from each border router. To prevent
traffic from following a default route to a border router with a failed upstream circuit, the
default origination should be conditional on the circuit's being up and active. This condi-
tional advertisement can be based on a static default pointed at the interface, or it can be
received from BGP and redistributed into the IGP. In this context, conditional advertise-
ment does not refer to the conditional advertisement feature. If additional prefix informa-
tion is received from the upstream provider, do not redistribute this into any IGP process
running on the border routers.
Standard Multihomed Network
The standard multihomed network design scenario described in this section is what is most
commonly referred to when multihoming is discussed. This design scenario involves the
enterprise connecting to multiple upstream providers.
This design scenario requires the enterprise to obtain its own ASN from its regional registry.
The enterprise also needs a block of address space that is large enough to pass standard
peering filters. An example of standard peering filters is discussed in detail in the later
section “Peering Filters.” The enterprise can usually obtain the required address space from
one or more of its upstream providers.
It is also possible to obtain an address space assignment from a regional registry; however,
significant justification is usually required. It is much easier to obtain an ASN than an IP
address space. The regional registries assign both ASNs and IP address space. There are
currently three regional registries:
•
American Registry for Internet Numbers (ARIN)
—www.arin.net
•
Asia Pacific Network Information Centre (APNIC)
—www.apnic.net
•
Réseaux IP Européens (RIPE)
—www.ripe.net
The following sections describe the single border router and multiple border router options
for enterprise edge Internet connectivity.
Single Border Router
The standard multihomed network with a single border router design involves the use
of multiple upstream providers, all terminated on a single enterprise border router (see
Figure 6-4). This provides redundancy for protection against circuit failure and upstream
provider failures. This design also provides the additional potential for optimizing
outbound traffic flow.
