Handbook

N8406-023 1Gb Intelligent L3 Switch Application Guide 114
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. Low cost indicates high bandwidth. You can manually enter the cost for the output route with the
following command:
>> # /cfg/l3/ospf/if <OSPF interface number>/cost <cost value (1-65535)>
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 on the switch. The command is as follows:
>>#/cfg/l3/ospf/if <OSPF interface number>/prio <priority value (0-255)>
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 lowest 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:
>> # /cfg/l3/ospf/range <range number>/addr <IP address>/mask <mask>
where <range number> is a number 1 to 16, <IP address> is the base IP address for the range, and <mask>
is the IP address mask for the range.
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 switch acting as an ABR automatically 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 the figure below), any traffic for IP address
destinations outside the area is forwarded to the switch‘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.
Figure 18 Injecting default routes
In more complex OSPF areas with multiple ABRs or ASBRs (such as area 0 and area 2 in the figure), there are
multiple routes leading from the area. In such areas, traffic for unrecognized destinations cannot tell which route
leads upstream without further configuration.
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