# IS-IS Filtering **URL:** \ **Category:** Lessons Discussion\ **Created:** [June 2, 2017, 10:20am UTC](https://forum.networklessons.com/t/is-is-filtering/1784 "2017-06-02T10:20:32Z")\ **Posts on this page:** 1\ **Showing post:** 5
**Author:** ![lagapidis](https://cdn-forum.networklessons.com/user_avatar/forum.networklessons.com/lagapidis/32/4949_2.png) [@lagapidis](https://forum.networklessons.com/u/lagapidis)\ **Post date:** [August 14, 2023, 6:42am UTC](https://forum.networklessons.com/t/is-is-filtering/1784/5 "2023-08-14T06:42:26Z")
Hello Luca IS-IS uses a hierarchical structure using areas in a similar manner to how OSPF uses areas. Whenever you use areas, you separate the IS-IS topology into logical units that operate independently but are connected using Level 2 routers. The `redistribute isis ip level-1 into level-2` command is used to redistribute L1 routes into L2 in an IS-IS network. Similarly, the `redistribute isis ip level-2 into level-1` command is used to redistribute routes back from L2 to L1. These commands are typically used on L1-L2 routers, which are boundary routers between L1 and L2 areas for the purpose of redistributing routes between IS-IS areas. In other words, **redistribution is used to inject routes in one IS-IS area into another**. Now if you have only L1 routers, that means **you only have a single area** , which in turn means that all routes are already exchanged between routers through the normal IS-IS routing protocol operation. Thus, there’s no need to redistribute routes within the same level because they are already known to each other. For this reason, the `redistribute isis ip level-1 into level-1` is not valid. The [Cisco command reference for this command](https://www.cisco.com/c/en/us/td/docs/ios/iproute_isis/command/reference/irs_book/irs_is2.html#wp1012723) states that you must: > Specify either level-l into level-2 or level-2 into level-1 I hope this has been helpful! Laz --- _[View the full topic](https://forum.networklessons.com/t/is-is-filtering/1784)._