Enterasys Networks Security Router X-PeditionTM - Manual (Page 126)

IP Routing Protocols
5-24 Configuring IP
Default Network
The default network is used to specify candidates for the default route when a default route is not
specified or learned. If the network specified by the
ip default-network command appears in
the routing table from any source (dynamic or static), it is flagged as a candidate default route and
is subject to being chosen as the default route for the XSR.
You may enter ip default-network multiple times. All candidate default routes appear in the
routing table preceded by an asterisk. If the network specified is a subnet, default routing applies
only to the classful network. If a directly connected interface is specified, RIP will generate a
default route.
If the XSR has no interface on the default network but has a route to it, it will consider this
network a candidate default route for itself. The best route candidate is chosen based on
administrative distance and metric.
The gateway to the best default path will be named the gateway of last resort for the router. The
gateway of last resort is the gateway for the route used by packets as the last possible alternative,
when there is no route to the destination, including a default route.
Refer to the XSR CLI Reference Guide for more information and a sample default route
configuration.
Classless Inter-Domain Routing (CIDR)
CIDR is an advanced address scheme for the Internet allowing more efficient allocation of IP
addresses than the earlier A, B, and C address scheme. CIDR currently uses prefixes anywhere
from 13 to 27 bits. This allows for address assignments that much more closely fit an
organization's specific needs. CIDR addressing also enables route aggregation in which a single
high level route entry can represent many lower-level routes in the global routing table, thus
reducing the table size.
The XSR supports CIDR in all IP address/subnet mask CLI commands and the feature is always
enabled. The
ip address {address mask | address&mask | negotiated} command is just
one CIDR-applicable command you can enter.
Router ID
A router identifier is used by routing protocols such as OSPF to uniquely identify a routing
instance. When the Router ID is not assigned, it is selected in the following manner by the XSR:
• If loopback interfaces are configured, the Router ID is selected based on the highest IP address
among active loopback interfaces.
• If no loopback are interfaces configured, the Router ID is selected based on the highest IP
address among active physical interfaces.
Note: Use this command only when the default network is more than one hop away and the XSR
has either a static or dynamic route to it. Do not confuse creating a valid static route of all zeroes (ip
route 0.0.0.0 0.0.0.0 <IPaddress/next-hop>) with an invalid all-zero default network.
Note: CIDR is not supported where a reverse mask is used.