subscribers only), Routing Principles and IOS Implementation
Considerations:
B. Default Whatevers
You will run across several terms that are often, and incorrectly,
considered synonymous: default routes, default gateways (default
routers), default networks, and gateways of last resort. These terms
refer to slightly different mechanisms, all of which are useful. This
section explains what each mechanism does.
1. Default Route
By convention, the address 0.0.0.0/0 is the default route, the least
specific possible route. Cisco sometimes uses the term pseudonetwork
to refer to 0.0.0.0/0. It is the route that you go to when you don't
have anyplace else to go. When it came time to pick softball teams in
my high school physical education classes, I was the default route.
As opposed to being something to put in right field and forget,
default routes are quite useful in networking. They can be declared
with static routes, or they can be learned from dynamic routing
protocols. To create a static route defining the local default, code:
ip route 0.0.0.0 0.0.0.0 {next hop IP | outgoing interface}
Created as a static route with an administrative distance less than
dynamic routing, a default route in the next-hop-ip format will be
used for the local router box, but not advertised unless it is
explicitly redistributed (or you use the outgoing interface form of
the static route command).
Statically declared default routes of the interface-name format will
be advertised as if they were directly connected.
Local configuration is not the only way your router can learn the
0.0.0.0/0 default route. It can be learned from dynamic routing
protocols such as OSPF and RIP. In the more recent IOS releases, you
can originate default from any of these routing processes with the
default-information-originate command. When you do this, the process
will advertise default to other routers, although it might use,
itself, the static route.
default-information-originate has an optional parameter, the always
keyword. If you don't use always, the router will advertise default
only if it itself has an active default route.
With always, the router will always advertise default, and will have
the behavior that it will blackhole routes to unknown destinations if
there is no default. A typical application for always would be where
you have a single ISP link to which you default, so you might as well
blackhole if you can't get to it.
2. Default Gateway
The default gateway is specifically intended for the situation when
no IP routing is enabled. It has the specific next hop address of the
gateway router.
You would use this on a switch, or a router box that is only doing
bridging, so the box can reach network management servers not on the
same subnet. Another application for the default gateway comes during
booting from ROM, to find the TFTP server.
In the IOS, you configure an IP default gateway with the command:
ip default-gateway gateway-address
where gateway-address is the address of a router interface on a
subnet to which your router is physically connected.
3. Default Network
The default network, used by IGRP and EIGRP, has only a prefix -- a
network or subnet -- so unless internal assumptions are made, there's
no way to know the specific next hop address.
Always remember the KISS (Keep it simple, stupid) rule. Once you
understand what command is intended to do something, it isn't always
useful to keep looking for commands that might do the same thing. The
major reason to look for obscure command interpretations is that they
may be the cause of problems you are troubleshooting.
In the real world of network design, the KISS rule is critical. On
the CCIE test, however, be prepared to be faced with scenarios that
violate this rule. One of the drivers of CCIE scenario writers seems
to be that you are very familiar with obscure parts of the IOS
command language. In addition, some CCIE lab scenarios may seem quite
contrived, due to the relatively small number of available routers.
For those of you who have taken ACRC, you have had an experience that
will give you perspective on odd scenarios you may see. ACRC's
redistribution and BGP scenarios are very unrealistic with respect to
plausible real-world configurations. They are the best that can be
done with the standard classroom lab and its topology.
See Scenario 2 for examples of the various default mechanisms.
To specify a default network for IGRP or EIGRP, or that will be known
locally on your router, code:
ip default-network ip-prefix
The ip-prefix is not a host address as used in the next hop field of
an ip route statement, or as the argument of a ip default-gateway. It
is a network or subnet address (i.e., with all zeroes in the host bit
positions).
4. Gateway of Last Resort
The gateway of last resort (GOLR) is selected by the process that
actually installs routes in the routing table. The GOLR represents
the default destination that comes from the source of default that
has the lowest administrative distance (AD).
So if you had a default static route, it would become the GOLR
regardless of anything you received from any routing protocol. If you
received a default network from EIGRP or IGRP, that network would
become GOLR in preference to anything from RIP or OSPF, unless you
changed the administrative distance for RIP or OSPF. An OSPF default
would be preferred to anything from RIP. An OSPF Type 1 default would
be preferred over an OSPF type 2 default.
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