Single Area OSPF

OSPF: Mandatory Commands




Using Wildcard Masks with OSPF Areas

When compared to an IP address, a wildcard mask identifies which addresses get matched for placement into an area:

• A 0 (zero) in a wildcard mask means to check the corresponding bit in the address for an exact match.

• A 1 (one) in a wildcard mask means to ignore the corresponding bit in the address—can be either 1 or 0.

TIP: An octet of all 0s means that the octet has to match exactly to the address. An octet of all 1s means that the octet can be ignored.




Configuring OSPF: Optional Commands

The following commands, although not mandatory, enable you to have a more controlled and efficient deployment of OSPF in your network.


Loopback Interfaces


Router ID


DR/BDR Elections


Modifying Cost Metrics


Authentication: Simple


Authentication: Using MD5 Encryption


Timers


Propagating a Default Route



Verifying OSPF Configuration


Troubleshooting OSPF

Configuration Example: EIGRP

Austin Router




Houston Router

EIGRP

Configuring Enhanced Interior Gateway Routing Protocol (EIGRP)



NOTE: tos is a reference to the original Interior Gateway Routing Protocol (IGRP) intention to have IGRP perform type-of-service routing. Because this was never adopted into practice, the tos field in this command is always set to zero (0).

NOTE: With default settings in place, the metric of EIGRP is reduced to the slowest bandwidth plus the sum of all the delays of the exit interfaces from the local router to the destination network.

TIP: For two routers to form a neighbor relationship in EIGRP, the k values must match.

CAUTION: Unless you are very familiar with what is occurring in your network, it is recommended that you do not change the k values.


EIGRP Auto-Summarization



CAUTION: EIGRP automatically summarizes networks at the classful boundary. A poorly designed network with discontiguous subnets could have problems with connectivity if the summarization feature is left on. For instance, you could have two routers advertise the same network—172.16.0.0/16—when in fact they wanted to advertise two different networks—172.16.10.0/24 and 172.16.20.0/24.

Recommended practice is that you turn off automatic summarization if necessary, use the ip summary-address command, and summarize manually what you need to.


Load Balancing: variance


NOTE: If a path is not a feasible successor, it is not used in load balancing.

NOTE: EIGRP supports up to six unequal-cost paths.


Bandwidth Use


NOTE: By default, EIGRP is set to use only up to 50 percent of the bandwidth of an interface to exchange routing information. Values greater than 100 percent can be configured. This configuration option might prove useful if the bandwidth is set artificially low for other reasons, such as manipulation of the routing metric or to accommodate an oversubscribed multipoint Frame Relay configuration.

NOTE: The ip bandwidth-percent command relies on the value set by the bandwidth command.


Authentication



NOTE: For the start time and the end time to have relevance, ensure that the router knows the correct time. Recommended practice dictates that you run Network Time Protocol (NTP) or some other time-synchronization method if you intend to set lifetimes on keys.


Verifying EIGRP


Troubleshooting EIGRP

RIP

The ip classless Command


NOTE: A supernet route is a route that covers a range of subnets with a single entry.

NOTE: The ip classless command is enabled by default in Cisco IOS Software Release 11.3 and later.


RIP Routing: Mandatory Commands


NOTE: You need to advertise only the classful network number, not a subnet:

Router(config-router)#network 172.16.0.0
not
Router(config-router)#network 172.16.10.0

If you advertise a subnet, you will not receive an error message, because the router will automatically convert the subnet to the classful network address.


RIP Routing: Optional Commands



Troubleshooting RIP Issues



Configuration Example: RIPv2 Routing


Cancun Router



Acapulco Router


Mazatlan Router

Static Routing

Configuring a Static Route on a Router

When using the ip route command, you can identify where packets should be routed in two ways:

• The next-hop address

• The exit interface

Both ways are shown in the “Configuration Example: Static Routes” and the “Configuring a Default Route on a Router” sections.



The permanent Keyword (Optional)

Without the permanent keyword in a static route statement, a static route will be removed if an interface goes down. A downed interface will cause the directly connected network and any associated static routes to be removed from the routing table. If the interface comes back up, the routes are returned.

Adding the permanent keyword to a static route statement will keep the static routes in the routing table even if the interface goes down and the directly connected networks are removed. You cannot get to these routes—the interface is down—but the routes remain in the table. The advantage to this is that when the interface comes back up, the static routes do not need to be reprocessed and placed back into the routing table, thus saving time and processing power.

When a static route is added or deleted, this route, along with all other static routes, is processed in one second. Before Cisco IOS Software Release 12.0, this processing time was
five seconds.

The routing table processes static routes every minute to install or remove static routes according to the changing routing table.

To specify that the route will not be removed, even if the interface shuts down, enter the following command, for example:

Router(config)#ip route 172.16.20.0 255.255.255.0 172.16.10.2 permanent


Static Routes and Administrative Distance (Optional)

To specify that an administrative distance of 200 has been assigned to a given route, enter the following command, for example:

Router(config)#ip route 172.16.20.0 255.255.255.0 172.16.10.2 200

By default, a static route is assigned an administrative distance (AD) of 1. Administrative distance rates the “trustworthiness” of a route. AD is a number from 0 through 255, where 0 is absolutely trusted and 255 cannot be trusted at all. Therefore, an AD of 1 is an extremely reliable rating, with only an AD of 0 being better. An AD of 0 is assigned to a directly connected route. The following table lists the administrative distance for each type of route.



By default, a static route is always used rather than a routing protocol. By adding an AD number to your ip route statement, however, you can effectively create a backup route to your routing protocol. If your network is using EIGRP, and you need a backup route, add a static route with an AD greater than 90. EIGRP will be used because its AD is better (lower) than the static route. If EIGRP goes down, however, the static route will be used in its place. This is known as a floating static route.

If a static route refers to an exit interface rather than a next-hop address, the destination is considered to be directly connected and is therefore given an AD of 0 rather than 1.


Configuring a Default Route on a Router



Verifying Static Routes

To display the contents of the IP routing table, enter the following command:

Router#show ip route

NOTE: The codes to the left of the routes in the table tell you from where the router learned the routes. A static route is described by the letter S.


Configuration Example: Static Routes

Boston Router



Buffalo Router


Bangor Router