IS-IS allows a two-level hierarchy to route PDUs. Level 1 areas can be interconnected by a contiguous Level 2 backbone.
The Level 1 link-state database contains information about only that area. The Level 2 link-state database contains information about the Level 2 system and each of the Level 1 systems in the area. A Level 1/2 router contains information about both Level 1 and Level 2 databases. A Level 1/2 router advertises information about its Level 1 area toward the other Level 1/2 or Level 2 (only) routers.
Packets with destinations outside the Level 1 area are forwarded toward the closest Level 1/2 router which, in turn, forwards the packets to the destination area.
Sometimes, the shortest path to an outside destination is not through the closest Level 1/2 router, or, the only Level 1/2 system to forward packets out of an area is not operational. Route leaking provides a mechanism to leak Level 2 information to Level 1 systems to provide routing information about inter-area routes. Then, a Level 1 router has more options to forward packets.
Configure a route policy to leak routers from Level 2 into Level 1 areas in the config>router>policy-options>policy-statement context.
The following shows the command usage to configure prefix list and policy statement parameters in the config>router context.
config>router>policy-options# prefix-list loops
..>policy-options>prefix-list# prefix 10.1.1.0/24 longer
..>policy-options>prefix-list# exit
..>policy-options# policy-statement leak
..>policy-options>policy-statement# entry 10
..>policy-options>policy-statement>entry# from
..>policy-options>policy-statement>entry>from# prefix-list loops
..>policy-options>policy-statement>entry>from# level 2
..>policy-options>policy-statement>entry>from# exit
..>policy-options>policy-statement>entry# to
..>policy-options>policy-statement>entry>to# level 1
..>policy-options>policy-statement>entry>to# exit
..>policy-options>policy-statement>entry# action accept
..>policy-options>policy-statement>entry>action# exit
..>policy-options>policy-statement>entry# exit
..>policy-options>policy-statement#exit
..>policy-options# commit
..>policy-options#A:ALA-A>config>router>policy-options# info
----------------------------------------------
prefix-list "loops"
prefix 10.1.1.0/24 longer
exit
policy-statement "leak"
entry 10
from
prefix-list "loop"
level 2
exit
to
level 1
exit
action accept
exit
exit
exit
----------------------------------------------
A:ALA-A>config>router>policy-options#
Next, apply the policy to leak routes from Level 2 into Level 1 systems on ALA-A.
config>router#isis
config>router>isis# export leak
A:ALA-A>config>router>isis# info
----------------------------------------------
area-id 49.0180.0001
area-id 49.0180.0002
area-id 49.0180.0003
authentication-key "//oZrvtvFPn06S42lRIJsE" hash
authentication-type password
no authentication-check
export "leak"
...
----------------------------------------------
A:ALA-A>config>router>isis#
After the policy is applied, create a policy to redistribute external IS-IS routes from Level 1 systems into the Level 2 backbone (see Redistributing external IS-IS routers). In the config>router context, configure the following policy statement parameters:
config>router>policy-options# begin
..>policy-options# policy-statement "isis-ext"
..>policy-options>policy-statement# entry 10
..>policy-options>policy-statement>entry$ from
..>policy-options>policy-statement>entry>from$ external
..>policy-options>policy-statement>entry>from# exit
..>policy-options>policy-statement>entry# to
..>policy-options>policy-statement>entry>to$ level 2
..>policy-options>policy-statement>entry>to# exit
..>policy-options>policy-statement>entry# action accept
..>policy-options>policy-statement>entry>action# exit
..>policy-options>policy-statement>entry# exit
..>policy-options>policy-statement# exit
..>policy-options# commitA:ALA-A>config>router>policy-options# info
----------------------------------------------
prefix-list "loops"
prefix 10.1.1.0/24 longer
exit
policy-statement "leak"
entry 10
from
prefix-list "loop"
level 2
exit
to
level 1
exit
action accept
exit
exit
exit
policy-statement "isis-ext"
entry 10
from
external
exit
to
level 2
exit
action accept
exit
exit
exit
----------------------------------------------
A:ALA-A>config>router>policy-options#