Advanced OSPF Design Concepts | OSPF Network Design Solutions

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Syntax Description:

weight. Administrative distance. This can be an integer from 10 to 255. (The values 0 through 9 are reserved for internal use.) Used alone, the argument weight specifies a default administrative distance that the Cisco IOS software uses when no other specification exists for a routing information source. Routes with a distance of 255 are not installed in the routing table.

address (optional). IP address in four-part dotted notation.

mask (optional). IP address mask in four-part dotted-decimal format. A bit set to 1 in the mask argument instructs the software to ignore the corresponding bit in the address value.

access-list-number (optional). Number of a standard IP access list to be applied to incoming routing updates.

ip (optional). IP-derived routes for IS-IS. It can be applied independently for IP routes and ISO CLNS routes.

Default Distance Weights: Table 6-1 provides you with the accepted default values for weights.

Command Mode: Router configuration.

**Table 6-1** Default distance weights
Route Source	Default Distance

Connected interface	0
Static route	1
Enhanced IGRP summary route	5
External BGP	20
Internal Enhanced IGRP	90
IGRP	100
OSPF	110
IS-IS	115
RIP	120
EGP	140
Internal BGP	200
Unknown	255

Usage Guidelines: Numerically, an administrative distance is an integer between 0 and 255. In general, the higher the value, the lower the trust rating. An administrative distance of 255 means the routing information source cannot be trusted at all and should be ignored.

When the optional access list number is used with this command, it is applied when a network is being inserted into the routing table. This behavior allows filtering of networks according to the IP address of the router supplying the routing information. This could be used, as an example, to filter out possibly incorrect routing information from routers not under your administrative control.

The order in which you enter distance commands can affect the assigned administrative distances in unexpected ways (see “Example” for further clarification). Weight values are also subjective; there is no quantitative method for choosing weight values.

For BGP, the distance command sets the administrative distance of the External BGP route. The show ip protocols EXEC command displays the default administrative distance for a specified routing process.

Example: In the following example, the router igrp global configuration command sets up IGRP routing in autonomous system number 109. The network router configuration commands specify IGRP routing on networks 192.31.7.0 and 128.88.0.0. The first distance router configuration command sets the default administrative distance to 255, which instructs the Cisco IOS software to ignore all routing updates from routers for which an explicit distance has not been set. The second distance command sets the administrative distance for all routers on the Class C network 192.31.7.0 to 90. The third distance command sets the administrative distance for the router with the address 128.88.1.3 to 120.

    router igrp 109     network 192.31.7.0     network 128.88.0.0     distance 255     distance 90 192.31.7.0 0.0.0.255     distance 120 128.88.1.3 0.0.0.0

distribute-list out

To suppress networks from being advertised in updates, use the distribute-list out router configuration command. To cancel this function, use the no form of this command. The syntax for this command (and the no form) is as follows:

    distribute-list access-list-number out [interface-name |    routing-process | autonomous-system-number]    no distribute-list access-list-number out [interface-name |    routing-process | autonomous-system-number]

Syntax Description:

access-list-number. Standard IP access list number. The list defines which networks are to be sent and which are to be suppressed in routing updates.

out. Applies the access list to outgoing routing updates.

interface-name (optional). Name of a particular interface.

routing-process (optional). Name of a particular routing process, or the keyword static or connected.

autonomous-system-number (optional). Autonomous system number.

Default: Disabled.

Command Mode: Router configuration.

Usage Guidelines: When redistributing networks, a routing process name can be specified as an optional trailing argument to the distribute-list command. This causes the access list to be applied to only those routes derived from the specified routing process. After the process-specific access list is applied, any access list specified by a distribute-list command without a process name argument will be applied. Addresses not specified in the distribute-list command will not be advertised in outgoing routing updates.

Notes:
To filter networks received in updates, use the distribute-list in command.

Example: The following example would cause only one network to be advertised by a RIP router and will not work for OSPF.

    process: network 131.108.0.0.     access-list 1 permit 131.108.0.0     access-list 1 deny 0.0.0.0 255.255.255.255    router rip     network 131.108.0.0     distribute-list 1 out

ip ospf authentication-key

To assign a password to be used by neighboring routers that are using OSPF’s simple password authentication, use the ip ospf authentication-key interface configuration command. To remove a previously assigned OSPF password, use the no form of this command. The syntax for this command (and the no form) is as follows:

    ip ospf authentication-key password    no ip ospf authentication-key

Syntax Description:

password. Any continuous string of characters that can be entered from the keyboard up to eight bytes in length.

Default: No password is specified.

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