User`s guide

84 XSR User’s Guide
IP Routing Protocols Chapter 5
Configuring IP
CIDR addressing also enables route aggregation in which a single high level
route entry can represent many lower-level routes in the global routing
tables. This reduces the routing table size. The XSR supports CIDR which is
always enabled. The
ip address <0-32> command implements CIDR.
Network Address Translation
Network Address Translation (NAT) maps IP address from one address realm
to another, providing transparent routing to end hosts. Using Port and
Address Translation (NAPT), the protocol provides a way for many users to
share one global IP address. NAT also enhances access security by only
allowing certain global addresses to access the private network.
NAT is limited in some respects: it requires additional processing in the fast
path which can impact packet delivery speed. Also, applications which
bundle the host IP address inside the payload do not interoperate with NAT
because the host IP address does not match the address on the IP header. A
special translation agent known as an Application Level Gateway (ALG) is
employed to allow such programs on a host in one address realm to
transparently connect to its counterpart running on a host in a different realm.
The XSR implements traditional NAT (RFC-3022). It has two forms:
 Basic NAT - Hosts on the private network are mapped statically to
global addresses. There are two kinds of basic NAT:
– One-to-one mapping - Each host is supplied a one-to-one mapping,
on the private network, to a global address. Hosts without
mappings are not NATted.
– Pool mapping - A pool of global addresses is defined. Hosts on the
private network are mapped to global addresses on a first-come,
first-serve basis. Once a global address is selected, static mapping
is performed.
 NAPT - Both the source address and source port of hosts on the
private network are translated. The global address is that of the
egress interface. Hosts on the private network all share the same
global address (based on the egress interface).
Features
The following NAT features are supported on the XSR:
 Basic NAT - One-to-one mapping based on global (independent of
interface) static mapping table. Mapping is permanent and is deleted
only if the configuration is removed.