Summarizing Routes; Default Routes - Lenovo Flex System Fabric CN4093 Application Manual

Summarizing Routes

Default Routes

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CN4093 Application Guide for N/OS 8.2
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 CN4093. The
command is as follows:
CN4093(config­ip­if)# ip ospf priority <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 highest router ID wins. Interfaces configured as passive do
not participate in the DR or BDR election process:
CN4093(config­ip­if)# ip ospf passive­interface
CN4093(config­ip­if)# exit
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:
CN4093(config)# router ospf
CN4093(config­router­ospf)# area­range
<mask>
where <range number> is a number 1 to 16, <IPv4 address> is the base IP address for
the range, and <subnet mask> is the IPv4 address mask for the range. For a detailed
configuration example, see "Example 3: Summarizing Routes" on page
Note: OSPFv2 supports IPv4 only. IPv6 is supported in OSPFv3 (see
Implementation in Lenovo N/OS" on page
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 CN4093 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 Figure
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.
<range number>
444).
48), any traffic for IP address destinations outside
address
<IP address>
442.
"OSPFv3