Exporting Interface routes
The current ipsrd treats the directly attached networks differently
than the learned networks. Ipsrd advertise specific host address of its
directly attached(point-to-point interface), but does not advertise any
attached network address to the dynamic routing domain by default. In order
for routing domain, RIP/OSPF, to know about the directly attached networks,
one needs to export interface routes.
Example: network is as shown in Figure-1
On GW-1
Now, other routers, like SW-1 and SW-2 know the route to network of
205.226.22.0 through RIP.
Exporting Aggregate Routes
There are two situation one wants to do Exporting Aggregate Routes.
I. The current Ipsrd does not like to do any auto (implicit)
aggregation since in the global state with routing this can lead to *many*
situations where the wrong thing is happening. For RIP version 1, since
it does not have subnet information, ipsrd will not export subnetted network
to another networks. For OSPF and RIP version 2, there is no need to do
this.
Example 1: Export natural network in RIP 1 using aggregate exporting.
In Figure-1, RIP version 1 is enabled on Interface E1 which has network
205.226.26.0, Interface E2 has network 205.226.24.0 but it is subnetted
with 30 bits to be network 205.226.24.100(the type of routing protocol
enabled on E2 is irrelevant). Interface E1 will not advertise 205.226.24.100
to RIP 1 because it is not a valid network address in network 205.226.26.0.
Interface E1 will not advertise 205.226.24.0 to RIP version 1 either because
the current ipsrd can not do auto implicit aggregation. The future release
will correct this behavior to compatible with RIP spec which requires a
router to filter out subnet information by default when it is a border
router of two natural networks. Therefore, a route aggregation export is
required in the current release.
On GW-3,
Now, 205.226.24.0 network will be advertised on the interface of 205.226.26.0
as well as on the interface of 205.226.25.0. Assuming both interfaces are
enabled for RIP version 1.
II When one router is on the border of several networks that
can aggregate to one network number with a super netmask, one can do exporting
aggregate routes to reduce routing table size. This applies to any protocols,
including RIP version 1 and 2, OSPF and IGRP and it also applies to subnetted
environment.
Example2: Export aggregate route in OSPF to reduce routing table size
In Figure-1, if IP networks of 205.226.24.0 to 205.226.27.0 are all
located on the right hand of the network diagram. Since 205.226.24.0 to
205.226.27.0 can be grouped as 205.226.24.0 with mask bit of 22(a super
net mask), interface can advertise an aggregate route of 205.226.24.0/22
for networks 205.226.24.0, 205.226.25.0. 225.226.26.0 and 225.226.27.0
that resides on the right side of the map.
Select on Export all Aggregates into OSPF. Apply