> For the complete documentation index, see [llms.txt](https://jncia-workbook.gitbook.io/workbook/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jncia-workbook.gitbook.io/workbook/junos-routing-policy/4-ospf-inter-area-filtering.md).
# 4) OSPF Inter-Area Filtering
### Pre-Work
Load the config called **policy.cfg** using the **load\_config\_on\_nodes.py** script.
```
python3 load_config_on_nodes.py --lab_dir three-routers --config_filename policy.cfg
```
### Lab
OSPF is already setup between R1 and R2.
Configure R2 to filter all prefixes in area 1 from being advertised into area 0. R1 should no longer have a route for `2.2.2.2/32`.
### Answer
Expand to reveal
```
set policy-options policy-statement block-inter-area from area 1
set policy-options policy-statement block-inter-area then reject
set protocols ospf area 0 network-summary-export block-inter-area
```
### Explanation
Expand to reveal
OSPF and IS-IS behave like distance vector protocols for inter-area routing. Within the area/level we cannot filter routes, as database synchronization is required. However, between area/level boundaries, we can filter and summarize routes. The same route policies we use for redistribution are used to accomplish IGP inter-area filtering.
\
In this lab, R2's Lo0 is in area 1. R2 creates a type 3 LSA for the `2.2.2.2/32` prefix in area 0:
```
admin@R1> show ospf database
OSPF database, Area 0.0.0.0
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *10.0.0.1 10.0.0.1 0x80000003 41 0x22 0xbc3a 84
Router 10.0.0.2 10.0.0.2 0x80000003 42 0x22 0xf80f 72
Summary 2.2.2.2 10.0.0.2 0x80000002 47 0x22 0xc463 28
```
\
As a consequence of OSPF's distance vector nature for inter-area routing, we have control over how routes are advertised into, or out of, areas. We can apply a basic route policy that filters all routes from area 1 as they are advertised into area 0. We match prefixes from area 1 simply using `from area 1`. We apply this policy to area 0, as routes are exported from other areas into area 0.
```
set policy-options policy-statement block-inter-area from area 1
set policy-options policy-statement block-inter-area then reject
set protocols ospf area 0 network-summary-export block-inter-area
```
\
Note that if you applied this to area 1, you would be filtering routes as they are exported from area 0 into area 1.
\
We should see that R1 no longer has the type 3 LSA in its area 0 database:
```
admin@R1> show ospf database
OSPF database, Area 0.0.0.0
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *10.0.0.1 10.0.0.1 0x80000003 199 0x22 0xbc3a 84
Router 10.0.0.2 10.0.0.2 0x80000003 200 0x22 0xf80f 72
admin@R1> show route protocol ospf
inet.0: 9 destinations, 9 routes (9 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
224.0.0.5/32 *[OSPF/10] 00:03:35, metric 1
MultiRecv
```
### Further Reading