Westermo RedFox Series - Manual (Page 605)

Westermo OS Management Guide
Version 4.17.0-0
Each IP subnet can only be part of one OSPF area, and when configuring OSPF
networks you should also define which area it belongs to. The area identifier is a
32 bit value, which can be stated as a decimal value, but is commonly written in
dotted decimal form. E.g., ”network 10.0.1.0/24 area 0.0.0.0” is equivalent
to writing ”network 10.0.1.0/24 area 0”.
A router which have networks in different areas is called an area border router
(ABR). An example is given below.
Example
router
ospf
router-id 192.168.5.11
network 192.168.5.0/24 area 0.0.0.0
network 192.168.11.0/24 area 0.0.0.1
end
In OSPF, areas are organised in a two-level hierarchy. At the top we have area
0, which is referred to as the backbone area. As the hierarchy is limited to two
levels, every ABR must be connected to the backbone area. Direct connections
between areas at lower level is prohibited; all inter-area traffic should go via the
backbone area
2 .
To allow for a more flexible area hierarchy, OSPF provides a feature referred to as
virtual links, however, OSPF virtual links are not supported in WeOS v4.17.0.
27.1.1.3 Route redistribution and default route
Route information learnt from other routing protocols (RIP, BGP 3 , etc.) can be
redistributed (i.e., imported) into the OSPF domain. The same goes for static
routes, and directly connected networks.
To let a router redistribute routing information into the OSPF domain, the
”redistribute” command is used, e.g., ”redistribute rip” to import routes
learnt via RIP. An OSPF router performing route distribution into the OSPF domain
is referred to as an administrative system border router (ASBR).
Routers can inject a default route (0.0.0.0/0) into the OSPF domain. This is done
using the ”distribute-default [always]” command. Without the ”always”
keyword, the router will only inject the default route if it itself has a default route.
2 The reason for introducing these topology limitations is to avoid the ”counting to infinity” seen
in distance vector protocols (see chapter 28 ) problem to occur for OSPF inter-area routing.)
3 As of WeOS v4.17.0 BGP is not supported.
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