mynetworkjourney

Using the Hello protocol, there is a series of exchanges that routers go through in order to establish relationship when OSPF is initilized. I’d like to go through some of this steps using examples from a lab environment, and watching some debug output in the process. To start, here’s the setup for the exercise: Figure 1: A simple topology Dynamips .net Config: # OSPF Neighbor Exchange Lab Topology autostart = False ghostios = true sparsemem = true [localhost]    [[7200]]         image = \Program Files\Dynamips\images\C7200-JK.BIN         # On Linux / Unix use forward slashes:         # image = /opt/7200-images/c7200-jk9o3s-mz.124-7a.image         npe = npe-400         ram = 96         ghostios = True         [[ROUTER A]]         Fa0/0 = B Fa0/0         model = 7200         console = 2001     [[router B]]         model = 7200         console = 2002 Down State Figure 2: Router A – interface added to OSPF When the router is enabled on the LAN, it starts in the Down state and sta

Type LSA Functionality 1 Router Defines the state and cost of the link to the neighbor and IP prefix associated with the point-to-point link. 2 Network Defines the number of routers attached to the segment. It gives information about the subnet mask on that segment. 3 Summary network Describes the destination outside an area but within the OSPF domain. The summary for one area is flooded into other areas, and vice versa. 4 Summary ASBR Describes the information about the ASBR. In a single area, there will be no summary Type 4 LSA. 5 External Defines routes to destination external to OSPF domain. Every subnet is represented by a single external LSA. 6 [*] Group membership 7 NSSA Defines routes to an external destination, but in a separate LSA format known as Type 7. 8 [*] Unuse

RFC 1597 provides for a group of Internet Networks that will never be assigned. Furthermore, these addresses will not route through the Internet. This makes them the proper choice for use in your home or company Intranet. We recommend non-routable IP for most networks. These IP addresses have specifically been set aside to use when it isn’t necessary (or desirable) for anyone on the Internet to be able to instantly navigate to your computer. It is especially useful to the DSL and Cable Modem users who have multiple computers connected to a single proxy server, firewall or router. These non-routable IP addresses also provide additional security on the “Internal” side of your network. Typically a firewall, Cable Modem or DSL router will provide Network Address Translation (NAT) to translate the these nonroutable address to a real Internet routable IP address. Reserved IP addresses for private networks 10.0.0.0 – 10.255.255.255                     /8 172.16.0.0 – 172.31.255.255        

When you work with classical subnetting, you always have to eliminate the subnets that contain either all zeros or all ones in the subnet portion. Hence, you always used the formula 2 N  2 to define the number of valid subnets created. However, Cisco devices can use those subnets, as long as the command ip subnet-zero is in the configuration. This command is on by default in Cisco IOS Software Release 12.0 and later; if it was turned off for some reason, however, you can re-enable it by using the following command: Router(config)#ip subnet-zero  Now you can use the formula 2 N  rather than 2 N  - 2. 2 N Number of total subnets created   2 N  2 Number of valid subnets created No longer needed because you have the ip subnet-zero command enabled 2 H Number of total hosts per subnet   2 H  2 Number of valid hosts per subnet