Part V: Transporting Non-Routable Protocols

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OSPF Default Routing

Today, most, and potentially all, networks connect to the Internet. Some routers have registered address space, while others have Internet routers, or firewalls. Being able to generate and propagate a default route is important. Recall from earlier chapters that default routing is a three-step task:

Step 1. Mark or "flag" a network as a default. A route must be "flagged" a default route before OSPF will treat it as a default route. This can be done in one of two ways with the following global commands:

  Router(config)ip default-network   network_address   Router(config)ip route 0.0.0.0 0.0.0.0   ip_address  
This particular static route does not need to be redistributed into OSPF to be propagated. OSPF will recognize the 0.0.0.0 0.0.0.0 route as a default and treat it accordingly . If you use the default-network statement, you need to use the keyword always in Step 2.

Step 2. Propagate the default network. Use the following OSPF router command:

  default-information originate  [  always  ] [  metric   cost  ] [  metric-type OE1   OE2  ] [  route-map   map-name  ] 
Step 3. Enable IP classless. Recall from previous chapters, that for any routing protocol to forward packets to a destination that does not appear in the route table, the router needs ip classless enabled . By default, ip classless in enabled in Cisco IOS Software Release 12.0 and later.

Figure 12-11 adds another connection, 206.191.200.1, to the network that is the gateway to the Internet. To flag the network 206.191.200.0 as a default network, either of the two global configuration commands will work:

Figure 12-11. OSPF Default Routing

graphics/12fig11.gif

  ip default-network 206.191.200.0  

Or point it directly at an address:

  ip route 0.0.0.0 0.0.0.0 206.191.200.1  

To propagate the default network, use the default-information originate always command. The always keyword always propagates a default route if the router flags the default network with the default-network command.

Example 12-27 lists the configuration of paul and shows how the default route is configured.

Example 12-27 Default Route Configuration on paul
  router ospf 7   router-id 192.168.250.9   redistribute connected subnets tag 9   network 172.16.9.0 0.0.0.255 area 0    default-information originate always  graphics/u2190.gif Propagate the default route   default-metric 10   !    ip classless  graphics/u2190.gif IP classless is always needed    ip route 0.0.0.0 0.0.0.0 206.191.200.1  graphics/u2190.gif "Flag" the route as default   !  

Example 12-28 lists how the route is propagated into the NSSA area on the router mathew. Notice that the gateway of last resort now is set.

Example 12-28 Route Table of mathew
 mathew#  show ip route  <<<text omitted>>>  Gateway of last resort is 172.16.1.5 to network 0.0.0.0  100.0.0.0/24 is subnetted, 3 subnets C       100.10.2.0 is directly connected, Loopback21 C       100.10.3.0 is directly connected, Ethernet2 C       100.10.1.0 is directly connected, Loopback20      172.16.0.0/24 is subnetted, 8 subnets C       172.16.250.0 is directly connected, Loopback0 O IA    172.16.9.0 [110/74] via 172.16.1.5, 1d23h, Serial0.1 O       172.16.10.0 is a summary, 2d00h, Null0 O IA    172.16.6.0 [110/128] via 172.16.1.5, 1d23h, Serial0.1 C       172.16.1.0 is directly connected, Serial0.1 O       172.16.2.0 is a summary, 2d00h, Null0 O IA    172.16.3.0 [110/134] via 172.16.1.5, 1d23h, Serial0.1 O E2    172.16.100.0 [110/10] via 172.16.1.5, 02:54:58, Serial0.1 O E2 206.191.200.0/24 [110/20] via 172.16.1.5, 00:09:56, Serial0.1      12.0.0.0/16 is subnetted, 1 subnets O E2    12.16.0.0 [110/10] via 172.16.1.5, 1d23h, Serial0.1  O*E2 0.0.0.0/0 [110/1] via 172.16.1.5, 01:04:36, Serial0.1  O E2 192.168.0.0/16 [110/10] via 172.16.1.5, 01:36:57, Serial0.1 mathew# 
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CCIE Practical Studies, Volume I
CCIE Practical Studies, Volume I
ISBN: 1587200023
EAN: 2147483647
Year: 2001
Pages: 283
Authors: Karl Solie

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