On a broadcast network, which three statements are true about IS-IS adjacencies? (Choose three.)

A.
Peers send full copies of databases at regular intervals.

B.
IIH PDUs are used as a keep alive mechanism.

C.
Peers can request missing link-state information.

D.
An elected DIS retains its status as long as the device remains up.

E.
Peers acknowledge receipt of link-state updates.

Explanation:
Neighbors and Adjacencies

IS-IS routers discover neighbors and form adjacencies by exchanging IS-IS Hello PDUs. Hellos are transmitted every 10 seconds, and on Cisco routers this interval can be changed on a per interface basis with the command isis hello-interval. Although IS-IS Hellos are slightly different for broadcast and point-to-point subnetworks, the Hellos include the same essential information, described in the section “IS-IS PDU Formats.” An IS-IS router uses its Hello PDUs to identify itself and its capabilities and to describe the parameters of the interface on which the Hellos are sent. If two neighbors are in agreement about their respective capabilities and interface parameters, they become adjacent. IS-IS is, however, less strict than OSPF in accepting adjacencies. In most cases, capabilities advertised by one neighbor but not supported by the other neighbor will not prevent an adjacency from forming; the capability is just ignored. The neighbors can even advertise different Hello intervals.

IS-IS forms separate adjacencies for L1 and L2 neighbors. L1-only routers form L1 adjacencies with L1 and L1/L2 neighbors, and L2-only routers form L2 adjacencies with L2 and L1/L2 neighbors. Neighboring L1/L2 routers can form both an L1 adjacency and an L2 adjacency. An L1-only router and an L2-only router will not establish an adjacency. As mentioned previously, Cisco routers are by default L1/L2.

While the type of router (L1-only, L2-only, or L1/L2) influences the type of adjacency that is formed, so do the area IDs configured on the two neighbors in question. The following rules apply:

* Two L1-only routers form an L1 adjacency only if their AIDs match.
* Two L2-only routers form an L2 adjacency, even if their AIDs are different.
* An L1-only router forms an L1 adjacency with an L1/L2 router only if their AIDs match.
* An L2-only router forms an L2 adjacency with an L1/L2 router even if their AIDs are different.
* Two L1/L2 routers form both L1 and L2 adjacencies if their AIDs match.
* Two L1/L2 routers form only an L2 adjacency if their AIDs do not match.

Once an adjacency is established, the Hellos act as keepalives. Each router sends a hold time in its Hellos, informing its neighbors how long they should wait to hear the next Hello before declaring the router dead. The default hold time on Cisco routers is three times the Hello interval and can be changed on a per interface basis with the command isis hello-multiplier. A significant difference from OSPF is that the hello intervals and hold times between two IS-IS neighbors do not have to match. Each router honors the hold time advertised by its neighbor.

Another interesting difference between OSPF and IS-IS adjacencies is when two routers become adjacent. OSPF can be a bit confusing here; two routers are considered adjacent as soon as two-way communication is established, but not fully adjacent until database synchronization is completed. IS-IS considers routers adjacent as soon as they have exchanged Hellos.

The IS-IS neighbor table can be observed with the command show clns is-neighbors (Example 10-1). The first four columns of the display show the System ID of each neighbor, the interface on which the neighbor is located, the state of the adjacency, and the adjacency type. The state will be either Init, indicating that the neighbor is known, but is not adjacent, or Up, indicating that the neighbor is adjacent. The priority is the router priority used for electing a Designated Router on a broadcast network, as described in the next section.