IS-IS Route Leaking

This topic is to discuss the following lesson:

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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 at R4, R2 calculated the metric of with 168 as below:

R2#sh ip route isis 1 is subnetted, 1 subnets
i ia [115/168] via, 00:14:47, Ethernet0/2

although R4 calculated it with 30 :

R4#sh ip route isis is subnetted, 1 subnets
i L2 [115/30] via, 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.


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 to network

i*L1 [115/10] via, 00:14:23, GigabitEthernet0/2 is subnetted, 1 subnets
i ia [115/40] via, 00:14:23, GigabitEthernet0/2
i L1 [115/20] via, 00:15:16, GigabitEthernet0/1
i L1 [115/30] via, 00:15:10, GigabitEthernet0/1
i L1 [115/20] via, 00:15:16, GigabitEthernet0/2

That’s without wide metrics:


router isis
 net 49.1234.0000.0000.0002.00
 is-type level-1


router isis
 net 49.1234.0000.0000.0004.00
 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:
  Metric: 10         IP
  Metric: 10         IP
  Metric: 30         IP-Interarea


Hi Rene,


this command result:

R1 uses R3

Why do you write:

R1 uses R2

Hi Rene
why we will configure on R4 , not on R2 , is there a reason or first level-1-2 router

Hi Bahri,

Thanks, this is a typo yes. Just fixed it.


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).

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??

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!


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!

Hello Stuat

Thanks for picking this up. You’re not being picky at all, we appreciate you bringing these things to our attention!


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Hi Stefanita,

I always appreciate it when people report errors :+1: Just fixed this.


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