Alteon Application Switch Operating System Application Guide
Open Shortest Path First (OSPF)
Document ID: RDWR-ALOS-V2900_AG1302
143
Interface Cost
The OSPF link-state algorithm (Dijkstra's algorithm) places each routing device at the root of a tree
and determines the cumulative cost required to reach each destination. Usually, the cost is inversely
proportional to the bandwidth of the interface. A low cost indicates high bandwidth. You can
manually enter the cost for the output route with the following command:
Electing the Designated Router and Backup
In any area with more than two routing devices, a Designated Router (DR) is elected as the central
contact for database exchanges among neighbors, and a Backup Designated Router (BDR) is elected
in case the DR fails.
DR and BDR elections are made through the hello process. The election can be influenced by
assigning a priority value to the OSPF interfaces with the following command:
A priority value of 255 is the highest, and 1 is the lowest. A priority value of 0 specifies that the
interface cannot be used as a DR or BDR. In case of a tie, the routing device with the highest router
ID wins.
Summarizing Routes
Route summarization condenses routing information. Without summarization, each routing device in
an OSPF network would retain a route to every subnet in the network. With summarization, routing
devices can reduce some sets of routes to a single advertisement, reducing both the load on the
routing device and the perceived complexity of the network. The importance of route summarization
increases with network size.
Summary routes can be defined for up to 16 IP address ranges using the following command:
•
range number is a number 1 to 16
•
IP address is the base IP address for the range
•
mask is the IP address mask for the range
For a detailed configuration example, see
Example 3: Summarizing Routes, page 156
.
Default Routes
When an OSPF routing device encounters traffic for a destination address it does not recognize, it
forwards that traffic along the default route. Typically, the default route leads upstream toward the
backbone until it reaches the intended area or an external router.
Each Alteon acting as an ABR inserts a default route into each attached area. In simple OSPF stub
areas or NSSAs with only one ABR leading upstream (see Area 1 in
), any traffic for IP address destinations outside the area is forwarded to Alteon's
IP interface, and then into the connected transit area (usually the backbone). Since this is
automatic, no further configuration is required for such areas.
>> OSPF Interface 14# aindex 1
(Attach area to network on interface
14)
>> OSPF Interface 14# enable
(Enable interface 14 for area index 1)
>> # /cfg/l3/0spf/if <OSPF interface number> /cost <cost value (1-65535)
>> # /cfg/l3/ospf/if <OSPF interface number> /prio <priority value (0-255)>
>> # /cfg/l3/ospf/range <
range number
> /addr <
IP address
> /mask <
mask
>