Advanced OSPF Design Concepts

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Usage Guidelines: You must configure the area stub command on all routers and access servers in the stub area.

Use the area router configuration command with the default-cost option to specify the cost of a default internal router sent into a stub area by an ABR.

There are two stub area router configuration commands: the stub and default-cost options of the area router configuration command. In all routers attached to the stub area, the area should be configured as a stub area using the stub option of the area command. Use the default-cost option only on an ABR attached to the stub area. The default-cost option provides the metric for the summary default route generated by the ABR into the stub area.

To further reduce the number of link-state advertisements (LSAs) sent into a stub area, you can configure no-summary on the ABR to prevent it from sending summary LSAs (LSA type 3) into the stub area.

Example: The following example assigns a default cost of 20 to stub network 36.0.0.0:

    interface ethernet 0    ip address 36.56.0.201 255.255.0.0    !    router ospf 201    network 36.0.0.0 0.255.255.255 area 36.0.0.0    area 36.0.0.0 stub    area 36.0.0.0 default-cost 20 

Related Commands: area authentication, area default-cost

area virtual-link

To define an OSPF virtual link, use the area virtual-link router configuration command with the optional parameters. To remove a virtual link, use the no form of this command. The syntax for this command (and the no form) is as follows:

    area area-id virtual-link router-id [hello-interval seconds]    [retransmit-interval seconds] [transmit-delay seconds] [dead-interval seconds] [[authentication-key key] |    [message-digest-key keyid md5 key]]    no area area-id virtual-link router-id [hello-interval seconds]    [retransmit-interval seconds] [transmit-delay seconds]    [dead-interval seconds] [[authentication-key key] |    [message-digest-key keyid md5 key]] 

Syntax Description:

area-id. Area ID assigned to the transit area for the virtual link. This can be either a decimal value or a valid IP address. There is no default.
router-id. Router ID associated with the virtual link neighbor. The router ID appears in the show ip ospf display. It is internally derived by each router from the router’s interface IP addresses. This value must be entered in the format of an IP address. There is no default.
hello-interval seconds (optional). Time in seconds between the hello packets that the Cisco IOS software sends on an interface. Unsigned integer value to be advertised in the software’s hello packets. The value must be the same for all routers and access servers attached to a common network. The default is 10 seconds.
retransmit-interval seconds (optional). Time in seconds between LSA retransmissions for adjacencies belonging to the interface. Expected round-trip delay between any two routers on the attached network. The value must be greater than the expected round-trip delay. The default is 5 seconds.
transmit-delay seconds (optional). Estimated time in seconds it takes to transmit a link-state update packet on the interface. Integer value that must be greater than zero. LSAs in the update packet have their age incremented by this amount before transmission. The default value is 1 second.
dead-interval seconds (optional). Time in seconds that a software’s hello packets are not seen before its neighbors declare the router down. Unsigned integer value. The default is four times the hello interval, or 40 seconds. As with the hello interval, this value must be the same for all routers and access servers attached to a common network.
authentication-key key (optional). Password to be used by neighboring routers. The password can be any continuous string of characters that you can enter from the keyboard up to eight bytes long. This string acts as a key that will allow the authentication procedure to generate or verify the authentication field in the OSPF header. This key is inserted directly into the OSPF header when originating routing protocol packets. A separate password can be assigned to each network on a per-interface basis.
All neighboring routers on the same network must have the same password to be able to route OSPF traffic. The password is encrypted in the configuration file if the service password-encryption command is enabled. There is no default value.
message-digest-key, keyid md5 key (optional). Key identifier and password to be used by neighboring routers and this router for MD5 authentication. The keyid is a number in the range 1 through 255. The key is an alphanumeric string of up to 16 characters. All neighboring routers on the same network must have the same key identifier and key to be able to route OSPF traffic. There is no default value.

Default Values

area-id. No area ID is predefined.
router-id. No router ID is predefined.
hello-interval seconds. 10 seconds.
retransmit-interval seconds. 5 seconds.
transmit-delay seconds. 1 second.
dead-interval seconds. 40 seconds.
authentication-key key. No key is predefined.
message-digest-key, keyid md5 key. No key is predefined.


TIPS:  
If area authentication is configured, but no key is specified, an empty key is used. For simple-text key, it is the NULL string. For MD5, Cisco uses key 0 with NULL string.

Command Mode: Router configuration.

Usage Guidelines: In OSPF, all areas must be connected to a backbone area. If the connection to the backbone is lost, it can be repaired by establishing a virtual link. The smaller the hello interval, the faster topological changes will be detected, but more routing traffic will ensue. The setting of the retransmit interval should be conservative, or needless retransmissions will result. The value should be larger for serial lines and virtual links. The transmit delay value should take into account the transmission and propagation delays for the interface.


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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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