a.. Subject: Re: gateway of last resort....
b.. From: "Howard C. Berkowitz" <hcb@xxxxxxxxx>
c.. Date: Thu, 7 Jan 1999 11:09:44 -0500
d.. In-Reply-To: <[EMAIL PROTECTED]>
e.. References: <[EMAIL PROTECTED]>
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Default gateway, default route, default gateway, and may sound alike, but
each is a distinct function. Three of the four potentially relate to one
another, and have to do with the behavior of a Cisco box when it has
routing enabled.
You use default gateway, however, on a box that is acting as a host, not as
a router. For example, if you had a switch or access server, or a router
"box" with "no ip routing" that you were using as a bridge or X.25 switch,
that box has an IP address for management. If the box needs to get to a
TFTP, DNS, SNMP, or other server that is not on its own subnet, the box has
to follow the same rules as any other end host: send traffic to the
default router. The Cisco default gateway statement is one way of telling
a non-routing box how to send traffic not on its own subnet.
The default route is a concept in RIP, OSPF, and BGP. IGRP definitely does
not understand it. According to some documentation, EIGRP doesn't
understand the convention I'm about to describe, but I have heard that some
versions of EIGRP do. Needs to be checked in a lab.
The default route is the route to the address 0.0.0.0/0, which can be set
with a static route:
ip route 0.0.0.0 0.0.0.0 10.1.1.1
where 10.1.1.1 is a reachable router interface.
You can also create the default route with a default-information-originate
router subcommand.
The default network has a similar function to the default route -- it's
where you send things you don't otherwise know where to send -- but it's
specified differently. Note that the default router address in the static
route statement above is just that -- it's a specific host address. The
default network statement, however, specifies not a host address but a
prefix to which you can route. (E)IGRP automatically elects a candidate
default network if you do not specify it.
GOLR is subtle. Remember that the individual routing protocols do not
themselves decide what the default route is, or, for that matter, things
such as load balancing. The routing protoools (or static routes) are
submitted as "suggestions" to the process that installs routes in the
routing table (or routing information base to be technical). In other
words, if you have multiple sources of routing information, "show IP route"
tells you what was installed in the RIB, which is not necessarily the view
of any specific routing protocol.
GOLR appears at the end of the show IP route display, when a GOLR exists.
Candidates for GOLR can come from any routing protocol or static route.
The selected GOLR is the address to which to send otherwise unroutable
traffic, an address from the most preferred source of default information
based on administrative distance.
Some examples might help.
If you run OSPF (AD 110) and RIP (AD 120), and there is a default route
in each (presumably from another router generating default information),
the GOLR will be the OSPF default.
If you run IGRP (AD 100) with a default network and OSPF, the GOLR will
be the IGRP default network.
If you run RIP but have a static route to 0.0.0.0 with AD less than 120,
the GOLR will be the static route.
If you ran OSPF and IGRP, but overrode the default administrative
distances such that OSPF's were less than 100, an OSPF default route would
be the GOLR if OSPF had one. If there were no default route available to
OSPF, the IGRP default network would become the GOLR even if OSPF's AD were
less than IGRP's
------------------------------------------
Howard C. Berkowitz [EMAIL PROTECTED]
Certified Cisco Systems Instructor (CID), Cisco University Instructor
Author of _Designing Address Architectures for Routing and Switching_,
ISBN 1578700590, Macmillan Technical Publishing (sister group to Cisco
Press), 1998
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----- Original Message -----
From: "Wang, Roger" <[EMAIL PROTECTED]>
To: "'Rizzo Damian'" <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>
Sent: Tuesday, March 27, 2001 3:32 PM
Subject: RE: Gateway of last resort vs. default Gateways
> Default gateway is configured on devices that do not route, so, it must be
> an IP address that belongs to one of the connected routes of the local
> interfaces. If you turn off routing on a cisco box, you would have to
> configure a default gateway for traffic destined for other networks
besides
> the connected ones.
>
> Gateway of last resort tells the router where to route the traffic to. If
> the destination can not be matched with any of the routes in the routing
> table, the router will then forward that to the gateway of last resort.
In
> other words, gateway of last resort doesn't have to on one of router's
local
> networks, it can be anywhere, but the router must have a route to it in
> order for it to show up in the routing table.
>
> HTH,
>
> Rog
>
> > -----Original Message-----
> > From: Rizzo Damian [mailto:[EMAIL PROTECTED]]
> > Sent: Tuesday, March 27, 2001 5:16 PM
> > To: '[EMAIL PROTECTED]'
> > Subject: Gateway of last resort vs. default Gateways
> >
> >
> >
> > Can someone please give me a non-Cisco explanation between the
> > differences of the Gateway of last resort and the Default
> > Gateway, which
> > logically appear to do the same thing?...Thank you!
> >
> >
> > -Rizzo
> >
> > _________________________________
> > FAQ, list archives, and subscription info:
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> >
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