Which two statements are true regarding a designated intermediate system (DIS) in an IS-IS network? (Choose two.)

Which two statements are true regarding a designated intermediate system (DIS) in an IS-IS network? (Choose two.)

Which two statements are true regarding a designated intermediate system (DIS) in an IS-IS network? (Choose two.)

A.
It performs a similar role to the designated router (DR) in OSPF.

B.
It is not allowed to send any PDUs on a designated intermediate system (DIS).

C.
A backup DIS is not elected.

D.
DIS selection uses the router’s IP address as a tiebreaker.

Explanation:
Routing devices send hello packets at a fixed interval on all interfaces to establish and maintain neighbor relationships. This interval is advertised in the hello interval field in the hello packet. By default, a designated intermediate system (DIS) router sends hello packets every 3 seconds, and a non-DIS router sends hello packets every 9 seconds.

To modify how often the routing device sends hello packets out of an interface, include the hello-interval statement:

hello-interval seconds;

The hello interval range is from 1 through 20,000 seconds.

You can send out hello packets in subsecond intervals. To send out hello packets every 333 milliseconds, set the hello-interval value to 1.

For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.

IS-IS Designated Intermediate Systems

IS-IS designated routersor, in IS-IS parlance, designated intermediate systems (DIS)serve the same purpose as OSPF DRs. They are elected on all broadcast networks (IS-IS does not recognize NBMA network types), originate a pseudonode LSP to represent the pseudonode, and all other routers sharing the broadcast link synchronize their LS databases with that of the DIS. Although multicast is used for communication between the DIS and other routers on the broadcast network, there is not, as with OSPF, a special multicast address for the DIS. But underlying these functional similarities are some distinct differences in how the functions are performed.

IS-IS DISs differ from OSPF DRs in several significant ways:

* There is no backup DIS.
* An IS with a higher priority will preempt the existing DIS.
* Setting a priority of 0 does not mean the router is ineligible to become the DIS.
* A full mesh of adjacencies is established on a broadcast network, rather than just with the DIS.
* On a given broadcast network, separate DISs can exist for L1 and L2 adjacencies.

As with OSPF, IS-IS broadcast interfaces are assigned a priority. You can see in the format of the IS-IS LAN Hello in Figure 4.11 that the Priority field is 7 bits, so the priority value can be any number between 0 and 127. (The default for both Cisco Systems and Juniper Networks routers is 64.) The priority value is set separately for level 1 and level 2, so it is possible for one router to be elected as the level 1 DIS and a different router on the same broadcast network to be elected as the level 2 DIS. If a router’s interface is L1 only, then it participates only in the election of an L1 DIS, and likewise if the interface is L2 only, it participates only in the election of the L2 DIS. If the interface is L1/L2, the router participates in both election processes.

If a router is attached to a broadcast network but does not find any adjacent neighbors, it does not consider itself the DIS. This eliminates the possibility of a router that can transmit packets but cannot receive them either because of a network component defect or some network configuration from erroneously declaring itself the DIS.

The election process is simple: Select the router within the L1 or L2 set with the highest priority to be the DIS. If the priority values are all the same, select the router whose interface connecting to the network has the numerically highest SNPA (MAC address). Unlike OSPF, a priority of 0 does not eliminate a router from the election process; it only means that the router will not be the DIS as long as another router has a higher priority. It also means that unlike OSPF you cannot accidentally create a situation in which no router on the link is eligible to become DIS.

Another dissimilarity from OSPF is that there is no wait timer and no association with neighbor state changes. An IS-IS router runs this election process every time a LAN Hello is received from an adjacent neighbor and every time it transmits its own LAN Hello as long as there is at least one adjacent neighbor. The need to ensure an orderly transition from the BDR to DR contributes to much of the complexity of the OSPF DR election process. The absence of any sort of “backup” DIS is a major factor in the simplicity of the IS-IS DIS election process, and is a reasonable tradeoff: The election process happens so fast that the benefit of a backup is negligible.

Running the election process whenever a LAN Hello is received means that whenever a Hello is received that has a higher priority than the existing DIS (or higher MAC address if the priority is equal to the DIS priority) the originator of the Hello becomes the DIS. Whenever the DIS is preempted (or resigns by lowering its priority to below that of another router on the link) the new DIS purges the pseudonode LSP generated by the old DIS and originates its own, and all other routers synchronize to the new DIS’s LS database. At first look, this might seem to be a source of network instabilitysomething good network engineers constantly strive to minimize. But you can answer that concern yourself. How often is a router likely to be added or removed from a broadcast network shared with other routers? The answer is “once in a while, at the most.” Even during transition projects, DIS preemption is not going to have a noticeable impact on network stability.



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