Default Routing Policy in Junos

Published by Keyboard Banger on

import = receive into routing table
export = send to neighbors, or, redistribute (or not).OSPF
——
Default import policy: accept all OSPF routes from neighbors
Default export policy: advertise all direct routes (including loopback interfaces).Here’s an example to illustrate it.
Dubai learned the 15.15.15.15/32 route through OSPF from Tokyo. In fact, the default export policy in Tokyo is to advertise its direct routes, one of which is 15.15.15.15/32.

The default export policy on Dubai is to export all direct routes.

Then MontReal learned 15.15.15.15/32 from Dubai, because the default import policy is to accept any OSPF routes.

Wait a minute, 15.15.15.15/32 is not a direct route in Dubai. So how does Dubai advertise it to MontReal?

I found the answer to it in JUNOS Enterprise Routing book. It says :

The default LS export policy is to reject everything. LSA flooding is not affected by
export policy, and it is used to convey routing in an indirect manner in an LS proto-
col. The result of this flooding is the advertisement of local interfaces that are
enabled to run OSPF, as well as the readvertisement (flooding) of LSAs received from
other routers.

“As well as the readvertisement of LSAs received from other routers”. This is the explanation.
So Dubai learned te 15.15.15.15/32 route from Tokyo, then reflooded it (as an LSA) to MontReal. And the default policy couldn’t stop that :)

So in the figure, 15.15.15.15/32 along with other routes, are flooded to MontReal despite the default policy.

To sum up:
OSPF inbound behaviour: accept all OSPF routes from neighbors
OSPF outbound behaviour: advertise direct routes(=default policy)+advertise OSPF-learned routes

Categories: Juniper

Keyboard Banger

Keyboard Banger is a network engineer from Africa. He has been working in network support and administration since 2008. He started writing study notes about certification exams and technology topics a couple of years ago. When he's not writing articles, he can be found wandering on technical forums.

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