I thought the Gateway of Last Resort was where you bought your computer...as
a last resort.
----- Original Message -----
From: "Howard C. Berkowitz" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Sunday, January 28, 2001 4:48 PM
Subject: Re: Gateway of last resort ?
> 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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