Excellent find !! I never heard or seen that explained so succinctly.
Good Post,
Tony M.
#6172
> Rich,
> Try this brief lab example I got from the CCO. I think this will clear it
> up
> for you:
> ip default-gateway
> The ip default-gateway command differs from the other two commands in that
> it
> should only be used when ip routing is disabled on the Cisco router. For
> instance, if the router is a host in the IP world, you can use this
command
> to define a default gateway for it. You might also use this command when
> your
> low end Cisco router is in boot mode in order to TFTP a Cisco IOS.Software
> image to the router. In boot mode, the router doesn't have ip routing
> enabled.
>
> ip default-network
> Unlike the ip default-gateway command, you can use ip default-network when
> ip
> routing is enabled on the Cisco router. When you configure ip
> default-network
> the router considers routes to that network for installation as the
gateway
> of last resort on the router.
>
> For every network configured with ip default-network, if a router has a
> route
> to that network, that route is flagged as a candidate default route. Let's
> look at the following routing table taken from a Cisco router:
>
> 2513#show ip route
> Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
> D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
> E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
> i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate
> default
> Gateway of last resort is not set
> 161.44.0.0 255.255.255.0 is subnetted, 1 subnets
> C 161.44.192.0 is directly connected, Ethernet0
> S 198.10.1.0 [1/0] via 161.44.192.2
> 131.108.0.0 255.255.255.0 is subnetted, 1 subnets
> C 131.108.99.0 is directly connected, TokenRing0
>
> Note the static route to 198.10.1.0 via 161.44.192.2 and that the gateway
of
> last resort isn't set. If we configure ip default-network 198.10.1.0, the
> routing table changes to the following:
>
> 2513#show ip route
> Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
> D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
> E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
> i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate
> default
>
> Gateway of last resort is 161.44.192.2 to network 198.10.1.0
>
> 161.44.0.0 255.255.255.0 is subnetted, 1 subnets
> C 161.44.192.0 is directly connected, Ethernet0
> S 161.44.0.0 255.255.0.0 [1/0] via 161.44.192.0
> S* 198.10.1.0 [1/0] via 161.44.192.2
> 131.108.0.0 255.255.255.0 is subnetted, 1 subnets
> C 131.108.99.0 is directly connected, TokenRing0
> 2513#show ip protocols
> 2513#
>
> We can see the gateway of last resort has now been set as 161.44.192.2.
This
> result is independent of any routing protocol, as shown by the show ip
> protocols command at the bottom of the output.
>
> We can add another candidate default route simply by configuring another
> instance of ip default-network:
>
> 2513#config terminal
> Enter configuration commands, one per line. End with CNTL/Z.
> 2513(config)#ip route 171.70.24.0 255.255.255.0 131.108.99.2
> 2513(config)#ip default-network 171.70.24.0
> 2513(config)#^Z
>
> 2513#show ip route
> Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
> D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
> E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
> i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate
> default
>
> Gateway of last resort is 161.44.192.2 to network 198.10.1.0
>
> 161.44.0.0 255.255.255.0 is subnetted, 1 subnets
> C 161.44.192.0 is directly connected, Ethernet0
> S 161.44.0.0 255.255.0.0 [1/0] via 161.44.192.0
> S* 198.10.1.0 [1/0] via 161.44.192.2
> 171.70.0.0 is variably subnetted, 2 subnets, 2 masks
> S 171.70.0.0 255.255.0.0 [1/0] via 171.70.24.0
> S 171.70.24.0 255.255.255.0 [1/0] via 131.108.99.2
> 131.108.0.0 255.255.255.0 is subnetted, 1 subnets
> C 131.108.99.0 is directly connected, TokenRing0
>
> Flagging a Default Network
> Note: The ip default-network command is classful, which means if the
router
> has a route to the subnet indicated by this command, it installs the route
> to
> the major net. At this point neither network has been flagged as the
default
> network. The ip default-network command must be issued again, using the
> major
> net, in order to flag the candidate default route.
>
> 2513#config terminal
> Enter configuration commands, one per line. End with CNTL/Z.
> 2513(config)#ip default-network 171.70.0.0
> 2513(config)#^Z
>
> 2513#show ip route
> Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
> D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
> E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
> i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate
> default
>
> Gateway of last resort is 161.44.192.2 to network 198.10.1.0
>
> 161.44.0.0 255.255.255.0 is subnetted, 1 subnets
> C 161.44.192.0 is directly connected, Ethernet0
> S 161.44.0.0 255.255.0.0 [1/0] via 161.44.192.0
> S* 198.10.1.0 [1/0] via 161.44.192.2
> 171.70.0.0 is variably subnetted, 2 subnets, 2 masks
> S* 171.70.0.0 255.255.0.0 [1/0] via 171.70.24.0
> S 171.70.24.0 255.255.255.0 [1/0] via 131.108.99.2
> 131.108.0.0 255.255.255.0 is subnetted, 1 subnets
> C 131.108.99.0 is directly connected, TokenRing0
>
> If the original static route had been to the major network, we wouldn't
have
> needed the extra step of configuring the default network twice.
>
> There are still no IP protocols running here. Without any dynamic
protocols
> running, you can configure your router to choose from a number of
candidate
> default routes based on whether the routing table has routes to networks
> other than 0.0.0.0/0. This command allows you to configure robustness into
> the selection of a gateway of last resort. Rather than configuring static
> routes to specific next-hops, you can have the router choose a default
route
> to a particular network by checking in the routing table.
>
> If you lose the route to a particular network, the router selects the
second
> candidate default, as specified above. You can remove the lost route by
> removing the static route in the configuration as follows:
>
> 2513#config terminal
> Enter configuration commands, one per line. End with CNTL/Z.
> 2513(config)#no ip route 198.10.1.0 255.255.255.0 161.44.192.2
> 2513(config)#^Z
> 2513#
> %SYS-5-CONFIG_I: Configured from console by console
>
> After removing the static route to the original default network, the
routing
> table looks like this:
>
> 2513#show ip route
> Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
> D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
> E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
> i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate
> default
>
> Gateway of last resort is 171.70.24.0 to network 171.70.0.0
>
> 161.44.0.0 255.255.255.0 is subnetted, 1 subnets
> C 161.44.192.0 is directly connected, Ethernet0
> S 161.44.0.0 255.255.0.0 [1/0] via 161.44.192.0
> * 171.70.0.0 is variably subnetted, 2 subnets, 2 masks
> S* 171.70.0.0 255.255.0.0 [1/0] via 171.70.24.0
> S 171.70.24.0 255.255.255.0 [1/0] via 131.108.99.2
> 131.108.0.0 255.255.255.0 is subnetted, 1 subnets
> C 131.108.99.0 is directly connected, TokenRing0
> 2513#
>
> Using Different Routing Protocols
> Gateways of last resort selected using the ip default-network command are
> propagated differently depending on which routing protocol is propagating
> the
> default route. For IGRP and EIGRP to propagate the route, the network
> specified by the ip default-network command must be known to IGRP or
EIGRP.
> This means the network must be an IGRP- or EIGRP-derived network in the
> routing table, or the static route used to generate the route to the
network
> must be redistributed into IGRP or EIGRP.
>
>
> RIP advertises a route to 0.0.0.0. For example, note that the gateway of
> last
> resort on the router below was learned using the combination of the ip
route
> and ip default-network commands. If we enable RIP on this router, RIP
> advertises a route to 0.0.0.0 (although not to the Token Ring network
> because
> of split-horizon):
>
> 2513(config)#router rip
> 2513(config-router)#network 161.44.0.0
> 2513(config-router)#network 131.108.0.0
> 2513(config-router)#^Z
> 2513#
> 2513#
> 2513#
> %SYS-5-CONFIG_I: Configured from console by console
> 2513#debug ip rip
> RIP protocol debugging is on
> 2513#
> RIP: sending update to 255.255.255.255 via Ethernet0 (161.44.192.1)
> default 0.0.0.0, metric 1
> network 131.108.0.0, metric 1
> RIP: sending update to 255.255.255.255 via TokenRing0 (131.108.99.1)
> network 161.44.0.0, metric 1
> 2513#
>
> Note: In IOS release 12.0T and higher, RIP doesn't advertise the default
> router if the route is not learned via RIP. Therefore, it may be necessary
> to
> redistribute the route into RIP, or use the default-information originate
> command.
>
> OSPF, like RIP, advertises a route for 0.0.0.0 0.0.0.0. However, with
OSPF,
> the router originating the default route must be configured with the
> default-information originate command. For more detailed information, see
> How
> Does OSPF Generate Default Routes?.
>
>
>
> Would anybody be so kind to explain me how the hell is this command
working?
> The more I read about it the more I get confused.
>
> Regards.
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