Solved: Re: WHY ISIS LEVEL 1 NOT RECEIVING ROUTERS FROM LEVEL 2 ?

SubnetWarrior · ‎07-05-2021

Hello experts,

So i begin to study isis but i stuck and need your help.

This is my topology :

In shorts, on R4 and R1, they are not receiveng 192.168.1.12/32 prefix form R3. But in R2, it is receiving 192.168.1.12/32 from R3. I'm sory for the very basic question, but any enlighment will help me to improve. Here is the config :

R4
interface Loopback0
 ip address 192.168.2.1 255.255.255.255
 ip router isis
!
interface FastEthernet0/0
 ip address 172.16.1.1 255.255.255.252
 ip router isis
 duplex auto
 speed auto
!
router isis
 net 49.0001.1921.6800.1020.00
 is-type level-1
!

R1
interface Loopback0
 ip address 192.168.1.1 255.255.255.255
 ip router isis
!
interface FastEthernet0/0
 ip address 172.16.1.2 255.255.255.252
 ip router isis
 duplex auto
 speed auto
!
interface FastEthernet0/1
 ip address 172.16.2.1 255.255.255.252
 ip router isis
 duplex auto
 speed auto
router isis
 net 49.0001.1921.6800.1001.00
 is-type level-1
!

R2
interface Loopback0
 ip address 192.168.1.11 255.255.255.255
 ip router isis
!
interface FastEthernet0/0
 ip address 172.16.3.1 255.255.255.252
 ip router isis
 duplex auto
 speed auto
 isis circuit-type level-2-only
!
interface FastEthernet0/1
 ip address 172.16.2.2 255.255.255.252
 ip router isis
 duplex auto
 speed auto
 isis circuit-type level-1
!
router isis
 net 49.0001.1921.6800.1011.00
!

R3
interface Loopback0
 ip address 192.168.1.12 255.255.255.255
 ip router isis
!
interface FastEthernet0/0
 ip address 172.16.3.2 255.255.255.252
 ip router isis
 duplex auto
 speed auto
!
router isis
 net 49.0001.1921.6800.1012.00
 is-type level-2-only
!

Here is the sh ip route and isis neighbor :

R4
R4#sh isis neigh

System Id      Type Interface   IP Address      State Holdtime Circuit Id
R1             L1   Fa0/0       172.16.1.2      UP    29       R4.02
R4#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     172.16.0.0/30 is subnetted, 2 subnets
C       172.16.1.0 is directly connected, FastEthernet0/0
i L1    172.16.2.0 [115/20] via 172.16.1.2, FastEthernet0/0
     192.168.1.0/32 is subnetted, 2 subnets
i L1    192.168.1.11 [115/30] via 172.16.1.2, FastEthernet0/0
i L1    192.168.1.1 [115/20] via 172.16.1.2, FastEthernet0/0
     192.168.2.0/32 is subnetted, 1 subnets
C       192.168.2.1 is directly connected, Loopback0
R4#
R4#
R1#SH IP ROUTE
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     172.16.0.0/30 is subnetted, 2 subnets
C       172.16.1.0 is directly connected, FastEthernet0/0
C       172.16.2.0 is directly connected, FastEthernet0/1
     192.168.1.0/32 is subnetted, 2 subnets
i L1    192.168.1.11 [115/20] via 172.16.2.2, FastEthernet0/1
C       192.168.1.1 is directly connected, Loopback0
     192.168.2.0/32 is subnetted, 1 subnets
i L1    192.168.2.1 [115/20] via 172.16.1.1, FastEthernet0/0
R1#
R1#sh isis neigh

System Id      Type Interface   IP Address      State Holdtime Circuit Id
R4             L1   Fa0/0       172.16.1.1      UP    7        R4.02
R2             L1   Fa0/1       172.16.2.2      UP    9        R2.03
R1#
R1#

R2
R2#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     172.16.0.0/30 is subnetted, 3 subnets
i L1    172.16.1.0 [115/20] via 172.16.2.1, FastEthernet0/1
C       172.16.2.0 is directly connected, FastEthernet0/1
C       172.16.3.0 is directly connected, FastEthernet0/0
     192.168.1.0/32 is subnetted, 3 subnets
C       192.168.1.11 is directly connected, Loopback0
i L2    192.168.1.12 [115/20] via 172.16.3.2, FastEthernet0/0
i L1    192.168.1.1 [115/20] via 172.16.2.1, FastEthernet0/1
     192.168.2.0/32 is subnetted, 1 subnets
i L1    192.168.2.1 [115/30] via 172.16.2.1, FastEthernet0/1
R2#
R2#
R2#sh isis neigh

System Id      Type Interface   IP Address      State Holdtime Circuit Id
R1             L1   Fa0/1       172.16.2.1      UP    22       R2.03
R3             L2   Fa0/0       172.16.3.2      UP    7        R3.02
R2#
R2#

R3
R3#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     172.16.0.0/30 is subnetted, 3 subnets
i L2    172.16.1.0 [115/30] via 172.16.3.1, FastEthernet0/0
i L2    172.16.2.0 [115/20] via 172.16.3.1, FastEthernet0/0
C       172.16.3.0 is directly connected, FastEthernet0/0
     192.168.1.0/32 is subnetted, 3 subnets
i L2    192.168.1.11 [115/20] via 172.16.3.1, FastEthernet0/0
C       192.168.1.12 is directly connected, Loopback0
i L2    192.168.1.1 [115/30] via 172.16.3.1, FastEthernet0/0
     192.168.2.0/32 is subnetted, 1 subnets
i L2    192.168.2.1 [115/40] via 172.16.3.1, FastEthernet0/0
R3#
R3#sh isis neigh

System Id      Type Interface   IP Address      State Holdtime Circuit Id
R2             L2   Fa0/0       172.16

Giuseppe Larosa · ‎07-05-2021

Hello @SubnetWarrior ,

what you see it's normal and it is represents the jhierarchical capabilities of IS-IS.

both R1 and R4 are L1 only and they do know nothing about other IS-IS areas no IS-IS L2 routes are leaked by default to Level 1.

route leakage from Level 2 to Level 1 needs to be explicitly configured and it is used only to avoid sub optimal routing .

To be noted R2 as L1L2 device will propagate L1 routes as L2 routes automatically in the backbone that is the collection of L2 links.

R2 should have the ATT bit set in its L1 LSP, this is to inform R1 and R4 that R2 is connected to the backbone and it can route to other areas.

You can see this as an implicit default route that can be used by L1 IS nodes.

This also explains why leakage of L2 to L1 is not necessary by default.

Hope to help

Giuseppe

View solution in original post

Giuseppe Larosa · ‎07-05-2021

Hello @SubnetWarrior ,

what you see it's normal and it is represents the jhierarchical capabilities of IS-IS.

both R1 and R4 are L1 only and they do know nothing about other IS-IS areas no IS-IS L2 routes are leaked by default to Level 1.

route leakage from Level 2 to Level 1 needs to be explicitly configured and it is used only to avoid sub optimal routing .

To be noted R2 as L1L2 device will propagate L1 routes as L2 routes automatically in the backbone that is the collection of L2 links.

R2 should have the ATT bit set in its L1 LSP, this is to inform R1 and R4 that R2 is connected to the backbone and it can route to other areas.

You can see this as an implicit default route that can be used by L1 IS nodes.

This also explains why leakage of L2 to L1 is not necessary by default.

Hope to help

Giuseppe

SubnetWarrior · ‎07-10-2021

Hello sir,

Thank you very very much for your crystal clear explanation, i'm labbing from your advice and yes L2 roututes can be distributed to L1 via route-map.

May i ask one more question sir? By default L1 has default route to L1/L2 routers in its area, how can i prevent L1 has default route 0.0.0.0? Because when i leak L2 to L1, the L2 routes are in L1, but the default route of 0.0.0.0 is still exist in L1, how can i delete this default route sir?

Giuseppe Larosa · ‎07-11-2021

Hello @SubnetWarrior ,

>> May i ask one more question sir? By default L1 has default route to L1/L2 routers in its area, how can i prevent L1 has default route 0.0.0.0? Because when i leak L2 to L1, the L2 routes are in L1, but the default route of 0.0.0.0 is still exist in L1, how can i delete this default route sir?

Most specific routes are used first so the default route is used only for destinations not matching any specific route.

The existence of the default route in L1 is related to the ATT bit in the LSP level 1 generated by the L1L2 Intermediate nodes.

The ATT bit would be cleared only when the L1L2 node loses all its L2 neighbors and it becomes not able to route outside the area.

Suppressing this default route is not possible and it is not necessary because with leakage of L2 to L1 you can optimize the routing towards specific prefixes when having multiple L1L2 nodes.. The default route will not disturb this ( most specific prefix is used first )

Hope to help

Giuseppe

SubnetWarrior · ‎07-11-2021

Wow, very clear answer as usual sir, thanks a lot for your inisight and advice.