Helping Hand Questions for JN0-664

TLVs are tuples of (Type, Length, Value) that can be advertised in IS-IS packets. TLVs can carry different kinds of information in the Link State Packets (LSPs). IS-IS supports both narrow and wide metrics for link costs. Narrow metrics use a single octet to encode the link cost, while wide metrics use three octets. Narrow metrics have a maximum value of 63, while wide metrics have a maximum value of 16777215. If there are both narrow and wide metric devices in the topology, IS-IS will advertise two TLVs for each link: one with the narrow metric and one with the wide metric. This allows backward compatibility with older devices that only support narrow metrics12.

IS-IS routers can form Level 1 or Level 2 adjacencies depending on their configuration and network topology. Level 1 routers are intra-area routers that share the same area address with their neighbors. Level 2 routers are inter-area routers that can connect different areas. Level 1-2 routers are both intra-area and inter-area routers that can form adjacencies with any other router.

In the exhibit, R1 and R2 are in different areas (49.0001 and 49.0002), so they cannot form a Level 1 adjacency. However, they can form a Level 2 adjacency if they are both configured as Level 1-2 routers. R1 is already configured as a Level 1-2 router, but R2 is configured as a Level 1 router only, because of the level 1 disable command under the lo0.0 interface. This command disables Level 2 routing on the loopback interface, which is used as the router ID for IS-IS.

Therefore, to establish a Level 1 adjacency between R1 and R2, the level 1 disable command under the R2 protocols isis interface lo0.0 hierarchy must be removed. This will enable Level 2 routing on R2 and allow it to form a Level 2 adjacency with R1.

In IS-IS, a designated intermediate system (DIS) is a router that is elected on a multi-access network segment (such as Ethernet) to perform some functions on behalf of other routers on the same segment. A DIS is responsible for sending network link-state advertisements (LSPs), which describe all the routers attached to the network. These LSPs are flooded throughout a single area. A DIS also generates pseudonode LSPs, which represent the multi-access network as a single node in the link-state database. A DIS election is based on the priority value configured on each router’s interface connected to the multi-access network. The priority value ranges from 0 to 127, with higher values indicating higher priority. The router with the highest priority becomes the DIS for the area (Level 1, Level 2, or both). If routers have the same priority, then the router with the highest MAC address is elected as the DIS. By default, routers have a priority value of 64. On a multi-access network, each router only forms an adjacency to the DIS, not to every other router on the segment. This reduces the amount of hello packets and LSP