# IS-IS Route Leaking
**URL:** https://forum.networklessons.com/t/is-is-route-leaking/1796
**Category:** Lessons Discussion
**Created:** [June 9, 2017, 9:51am UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796 "2017-06-09T09:51:30Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@ReneMolenaar](https://forum.networklessons.com/u/ReneMolenaar)
#### Post date: [June 9, 2017, 9:51am UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/1 "2017-06-09T09:51:30Z")
This topic is to discuss the following lesson:
> **[IS-IS Route Leaking](https://networklessons.com/is-is/is-is-route-leaking)**
>
> IS-IS route leaking allows you to redistribute level 2 prefixes into level 1 so that level 1 routes can choose the most optimal path in the network.
---
### Author:  [@walidabdullah83](https://forum.networklessons.com/u/walidabdullah83)
#### Post date: [April 10, 2018, 5:26am UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/2 "2018-04-10T05:26:43Z")
Hi Rene,
thanks a lot for your helpful explanation, I simulated your lab and found something that I need your explanation for it.
when I leaked 8.8.8.8 at R4, R2 calculated the metric of 8.8.8.8 with 168 as below:
```
R2#sh ip route isis 1
8.0.0.0/32 is subnetted, 1 subnets
i ia 8.8.8.8 [115/168] via 192.168.24.4, 00:14:47, Ethernet0/2
```
although R4 calculated it with 30 :
```
R4#sh ip route isis
8.0.0.0/32 is subnetted, 1 subnets
i L2 8.8.8.8 [115/30] via 192.168.47.7, 00:50:55, Ethernet0/1
```
the metric calculated with 40 at R2 when I configured the metric-style wide on all routers, I need your explanation for this please.
Thanks,
Walid
---
### Author:  [@ReneMolenaar](https://forum.networklessons.com/u/ReneMolenaar)
#### Post date: [April 13, 2018, 7:10pm UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/3 "2018-04-13T19:10:59Z")
Hi Walid,
That’s a good question. I just booted this topology again and I see a metric of 40 on R2:
```
R2#show ip route isis
Gateway of last resort is 192.168.24.4 to network 0.0.0.0
i*L1 0.0.0.0/0 [115/10] via 192.168.24.4, 00:14:23, GigabitEthernet0/2
8.0.0.0/32 is subnetted, 1 subnets
i ia 8.8.8.8 [115/40] via 192.168.24.4, 00:14:23, GigabitEthernet0/2
i L1 192.168.13.0/24 [115/20] via 192.168.12.1, 00:15:16, GigabitEthernet0/1
i L1 192.168.35.0/24 [115/30] via 192.168.12.1, 00:15:10, GigabitEthernet0/1
i L1 192.168.47.0/24 [115/20] via 192.168.24.4, 00:15:16, GigabitEthernet0/2
```
That’s without wide metrics:
R2:
```
router isis
net 49.1234.0000.0000.0002.00
is-type level-1
log-adjacency-changes
```
R4:
```
router isis
net 49.1234.0000.0000.0004.00
log-adjacency-changes
redistribute isis ip level-2 into level-1 route-map ROUTE_LEAKING
```
What do you see in the database?
```
R2#show isis database level-1 R4.00-00 detail
IS-IS Level-1 LSP R4.00-00
LSPID LSP Seq Num LSP Checksum LSP Holdtime/Rcvd ATT/P/OL
R4.00-00 0x00000005 0xC9D0 976/1199 1/0/0
Area Address: 49.1234
NLPID: 0xCC
Hostname: R4
Metric: 10 IS R4.01
IP Address: 192.168.47.4
Metric: 10 IP 192.168.24.0 255.255.255.0
Metric: 10 IP 192.168.47.0 255.255.255.0
Metric: 30 IP-Interarea 8.8.8.8 255.255.255.255
```
Rene
---
### Author:  [@bahriaygordu](https://forum.networklessons.com/u/bahriaygordu)
#### Post date: [August 13, 2018, 7:08pm UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/4 "2018-08-13T19:08:06Z")
Hi Rene,
```
R1#traceroute 8.8.8.8
```
this command result:
R1 uses R3
Why do you write:
R1 uses R2
---
### Author:  [@bahriaygordu](https://forum.networklessons.com/u/bahriaygordu)
#### Post date: [August 13, 2018, 7:26pm UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/5 "2018-08-13T19:26:05Z")
Hi Rene
why we will configure on R4 , not on R2 , is there a reason or first level-1-2 router
---
### Author:  [@ReneMolenaar](https://forum.networklessons.com/u/ReneMolenaar)
#### Post date: [August 16, 2018, 4:49am UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/6 "2018-08-16T04:49:04Z")
Hi Bahri,
Thanks, this is a typo yes. Just fixed it.
Rene
---
### Author:  [@ReneMolenaar](https://forum.networklessons.com/u/ReneMolenaar)
#### Post date: [August 16, 2018, 4:50am UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/7 "2018-08-16T04:50:44Z")
We leak from level 2 into level 1 so that’s why you have to do it on a level 1-2 router (R4 in this case).
---
### Author:  [@swaps1225](https://forum.networklessons.com/u/swaps1225)
#### Post date: [February 26, 2019, 4:10pm UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/8 "2019-02-26T16:10:28Z")
When do we use route map and distribute list ?? What are the peculiarities of both?? Can you please distinguish between access list and prefix list also??
---
### Author:  [@lagapidis](https://forum.networklessons.com/u/lagapidis)
#### Post date: [February 28, 2019, 2:08pm UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/9 "2019-02-28T14:08:13Z")
Hello Swapnil
Specifically for IS-IS route leaking, you can use either a distribute list or a route-map. Of course, each one has different capabilities. This distinction is not specific to IS-IS route leaking, but is the case for most applications in which both can be used. In general, a distribute list is a more quick and dirty, more simplistic way of implementing route leaking and other similar operations. A route map will provide you with more flexibility and capabilities in matching and taking action. Both can be equally effective for IS-IS route leaking however.
As for access lists and prefix lists, the following lesson explains the difference between the two quite eloquently and should cover most if not all of your questions.
I hope this has been helpful!
Laz
---
### Author:  [@staut\_stefanita](https://forum.networklessons.com/u/staut_stefanita)
#### Post date: [June 16, 2019, 5:24pm UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/10 "2019-06-16T17:24:05Z")
Just to be picky: at the beginning there is a “We can deal with this by” that repeats.
Thank you for all these lessons, they are really great!
Stefanita
---
### Author:  [@lagapidis](https://forum.networklessons.com/u/lagapidis)
#### Post date: [June 18, 2019, 7:25am UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/11 "2019-06-18T07:25:08Z")
Hello Stuat
Thanks for picking this up. You’re not being picky at all, we appreciate you bringing these things to our attention!
Laz
---
### Author:  [@ReneMolenaar](https://forum.networklessons.com/u/ReneMolenaar)
#### Post date: [June 20, 2019, 1:47pm UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/12 "2019-06-20T13:47:27Z")
Hi Stefanita,
I always appreciate it when people report errors 👍 Just fixed this.
Rene
---
### Author:  [@luis.fel.ortiz](https://forum.networklessons.com/u/luis.fel.ortiz)
#### Post date: [November 28, 2020, 12:51pm UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/13 "2020-11-28T12:51:10Z")
Hello Rene,
Just getting familiar with isis.
About your filtrering with acls my understanding was that we follow a logic netwok mask. When you filter the prefix 8.8.8.8 with any logic, the filtering shouldn’t be instead permit ip host 8.8.8.8 255.255.255.255 0.0.0.0
---
### Author:  [@lagapidis](https://forum.networklessons.com/u/lagapidis)
#### Post date: [November 30, 2020, 8:29am UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/14 "2020-11-30T08:29:33Z")
Hello Luis
The access list that is defined in the lesson is an extended access list that specifies a host address as the source, and any address as the destination. When you specify a host address using the `host` keyword, the access list automatically assumes a 255.255.255.255 subnet mask. If you use the `host` keyword it doesn’t allow you to enter a subnet mask at all, as seen below:
```
Router(config)#ip access-list extended My_ACL
Router(config-ext-nacl)#permit ip host 8.8.8.8 ?
A.B.C.D Destination address
any Any destination host
host A single destination host
object-group Destination network object group
Router(config-ext-nacl)#permit ip host 8.8.8.8
```
After the `host 8.8.8.8`, it expects **the destination address** and not the subnet mask, so the syntax you use in your post will not work.
The result of the configuration in the lesson is that packets destined for the 8.8.8.8/32 host address will be matched exactly by the ACL, and will thus be leaked into area 1234. This way, routing only for this host address will use a different next hop address.
I hope this has been helpful!
Laz
---
### Author:  [@luis.fel.ortiz](https://forum.networklessons.com/u/luis.fel.ortiz)
#### Post date: [November 30, 2020, 5:35pm UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/15 "2020-11-30T17:35:45Z")
Hello Laz,
Thanks for tour feedback, but I might explain myself wrong.
As per my understanding when we use extended acls for redistribution we can match the network part + subnet part.
In the particular exemple the acls below will accomplish the exact same thing isn’t it?
```
permit ip host 8.8.8.8 any
permit ip 8.8.8.8 0.0.0.0 255.255.255.255 0.0.0.0
```
Thanks for your support.
---
### Author:  [@lagapidis](https://forum.networklessons.com/u/lagapidis)
#### Post date: [December 2, 2020, 6:07am UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/16 "2020-12-02T06:07:22Z")
Hello Luis
Sorry I misunderstood. Actually, the following two commands are identical:
```
permit ip host 8.8.8.8 any
permit ip 8.8.8.8 0.0.0.0 0.0.0.0 255.255.255.255
```
If you put in the second command and then look at the configuration file, you will find that it has been converted into the first:
```
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#ip access-list extended my_list
Router(config-ext-nacl)#permit ip 8.8.8.8 0.0.0.0 0.0.0.0 255.255.255.255
Router(config-ext-nacl)#exit
Router(config)#exit
Router#show ip access-lists
Extended IP access list my_list
10 permit ip host 8.8.8.8 any
Router#
```
Notice the entry in the access list has been converted simply to `ip host 8.8.8.8 any`.
I hope this has been helpful!
Laz
---
### Author:  [@luis.fel.ortiz](https://forum.networklessons.com/u/luis.fel.ortiz)
#### Post date: [December 2, 2020, 2:18pm UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/17 "2020-12-02T14:18:46Z")
Perfect thanks a lot
---
### Author:  [@vignesh4832](https://forum.networklessons.com/u/vignesh4832)
#### Post date: [April 21, 2021, 10:54am UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/18 "2021-04-21T10:54:47Z")
Won’t there be a disadvantage on R3, as it has to follow the path towards R1 to reach 8.8.8.8 which is a longest route? Can you please let me know how to overcome this issue?
---
### Author:  [@lagapidis](https://forum.networklessons.com/u/lagapidis)
#### Post date: [April 23, 2021, 5:24am UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/19 "2021-04-23T05:24:24Z")
Hello Vigneshwar
You are correct in your thinking. However, IS-IS has a failsafe built-in for such situations. Although L1 routes are always preferred to L2 routes, in this case, the R3 router will have learned about the 8.8.8.8 destination as an L2 route, but it will also have learned it from the leaked route via R4. Because the leaked route has a designation of “inter-area” shown as “ia” in the routing table, it will actually prefer the L2 route to that destination rather than the leaked route.
I tried to confirm this using a lab, and to find this clearly stated in documentation, but I was unable to do so. But if you put this lab together, I suggest you take a look at the routing table of R3 after the route is leaked to see what path has been installed in the routing table.
I hope this has been helpful!
Laz
---
### Author:  [@antonyac1707](https://forum.networklessons.com/u/antonyac1707)
#### Post date: [December 12, 2022, 1:56pm UTC](https://forum.networklessons.com/t/is-is-route-leaking/1796/20 "2022-12-12T13:56:01Z")
Hello René, I just finished the ISIS topics, however, I would like to ask you if you have reference material to configure ISIS with IPv6. I also want to ask you: What does Single/Multi-topology and multi-topology refer to in ISIS for IPv6.
Thank you
[Next page](https://forum.networklessons.com/t/is-is-route-leaking/1796.md?page=2)