ATT Bit Not Being Set in ISIS

Hello,

I've been working through the INE SP workbook, and found that any time there's a task that has one or more devices participate in Level 1 and peer with a L1/L2 device, I never see the ATT bit being set in the ISIS database, and my L1 peer does not receive a default route from it's L1/L2 neighbor.  Any ideas?


It's been quite a while since starting SP preparations since I've worked with ISIS...

Thanks,
Jason

Comments

  • Hey! did you solve the issue? 

    I assume your L1/L2 router was connected to L2 router in the same area, that's why ATT bit was not configured. So you might configure another area address on L2 router and see what happens. 

     

  • I've been working through the INE SP workbook, and found that any time there's a task that has one or more devices participate in Level 1 and peer with a L1/L2 device, I never see the ATT bit being set in the ISIS database, and my L1 peer does not receive a default route from it's L1/L2 neighbor.  Any ideas?

    Hi Jason,

    You might missing some configuration that's why default route hasn't installed on L1 router. If you post your config, would be nice but here is simple example:

    R1(L1)-R2(L1/L2)-R3(L2)

    R1#show running-config | section router isis
     ip router isis 1
     ip router isis 1
    router isis 1
     net 49.0001.0000.0000.0001.00
     is-type level-1
    R1#

    R2#show running-config | section router isis
     ip router isis 1
     ip router isis 1
     ip router isis 1
    router isis 1
     net 49.0001.0000.0000.0002.00
    R2#show run int fa1/0
    Building configuration...

    Current configuration : 140 bytes
    !
    interface FastEthernet1/0
     ip address 12.12.12.2 255.255.255.0
     ip router isis 1
     duplex auto
     speed auto
     isis circuit-type level-1
    end

    R2#show run int fa1/1
    Building configuration...

    Current configuration : 145 bytes
    !
    interface FastEthernet1/1
     ip address 23.23.23.2 255.255.255.0
     ip router isis 1
     duplex auto
     speed auto
     isis circuit-type level-2-only
    end

    and R3 L2 router:

    R3#show running-config | section router isis
     ip router isis 1
     ip router isis 1
    router isis 1
     net 49.0002.0000.0000.0003.00
     is-type level-2-only

    Now I have default route on R1(L1) router:

    R1#show ip route
    <removed>

    Gateway of last resort is 12.12.12.2 to network 0.0.0.0

    i*L1  0.0.0.0/0 [115/10] via 12.12.12.2, 00:16:27, FastEthernet1/0
          1.0.0.0/32 is subnetted, 1 subnets
    C        1.1.1.1 is directly connected, Loopback0
          12.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
    C        12.12.12.0/24 is directly connected, FastEthernet1/0
    L        12.12.12.1/32 is directly connected, FastEthernet1/0

    Good Luck

  • Yeah, you need to have different Area addresses on L1/L2 and L2 routers for the L1/L2 router to set the ATT bit in the LSPs sent to L1 routers. I think the specification says that the L1/L2 router only needs to have an L2 adjacency with an L2 router but Cisco (and may be other vendors) require different area addresses.

     

    Regards,

    Amit.

  • The ATT bit is set when an L1/L2 router signals a L1 router of a default route to optimize the routing table, as per nnn's example. So bottom line, is that the L1/L2 router will be on the same area as the L1 router, but in a different area of the L2 router:

     

    R1(config-router)#do sh ip route isis

         2.0.0.0/32 is subnetted, 1 subnets

    i L1    2.2.2.2 [115/10] via 12.12.12.2, FastEthernet0/0

    i*L1 0.0.0.0/0 [115/10] via 12.12.12.2, FastEthernet0/0


    R2.00-00              0x00000003   0xD16D        1192              1/0/0


  • So bottom line, is that the L1/L2 router will be on the same area as the L1 router, but in a different area of the L2 router:

     

    That's correct. The L2 router and the L1/L2 router must be in different Areas for L1/L2 router to generate an LSP with ATT bit set. If they are in the same Area, the L1/L2 router wont set the ATT bit.


    Regards,

    Amit.

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