JNCIS-SP: IS-IS DIS and the ATT bit.

A brief rundown on the IS-IS DIS and the attached bit (ATT-bit). The setup is as follows;


The DIS.

DIS stands for Designated Intermediate Router. When there are multiple IS-IS routers active on a single LAN-segment, they will elect a DIS. The router with the highest interface priority or, in case of a tie, the highest MAC-address will be elected as DIS. The DIS-elected router will create a Pseudo Node that represents the LAN-segment. All LAN-connected routers will report their links to the pseudonode. The DIS, on behalf of the pseudonode, will flood the LSPs on the LAN.

The DIS doesn’t have a backup router and it can be preempted if a router with a higher priority joins the LAN. The DIS also has an increased hello interval of 3 seconds, versus 9 seconds for non-DIS routers. In our case, R15 will be configured as DIS.

The ATT bit.

The ATT bit is set by routers who are connected to both a level 1 and a level 2 area. The bit is set to indicate to other routers inside the level 1 area that they know the backbone. The ATT bit will be set in the LSPs that are distributed in the level 1 area. When a level 1 router sees that the ATT-bit is set, they can create and install a default route in their routing table. The router that sets the ATT bit in the level 1 LSPs does not need to have a default route itself. In our case, R15 will be the router that will set the ATT bit in the LSPs distributed to S1 and 45_1.

Let’s start with the configuration of R12. What needs to be done is the following:
    • IS-IS enabled interfaces have to be configured for family iso
    • the loopback needs to have an iso address
    • the interfaces towards R15 need to be configured under IS-IS with level 2 disabled
This can be done like this:


This will enable R12 as a level 2 router in area 60 towards R15. Moving on to R15, what needs to happen there is the following:
    • IS-IS enabled interfaces have to to be configured for family iso
    • the loopback needs to have an iso address
    • the IS-IS interface towards R12 needs to have level 1 disabled
    • the IS-IS interfaces towards S1 and 45_1 need to have level 1 disabled and the priority set to 127 (to ensure the router will become the DIS)


S1’s configuration:


The configuration for 45_1 is similar.
Let’s examine the outcome of this configuration. Let’s verify that R15 is in fact in control of the pseudo node:


Here we can see the following:
    • there are 2 level 1 adjacencies
    • the hello time is set to 3 seconds with a hold time of 9 seconds
    • the Designated Router is MX480-TEST-R15.02 (which is R15)

Let’s move to S1 and verify if R15 set the ATT bit:


Here we see the default route installed into the routing table, with R15 being the next-hop and the 'show isis database' tells us R15 has the ATT bit set.

That’s it.