Last Updated on August 6, 2021 by Admin 3
You administer the IS-IS network shown in the exhibit above.
A DIS has been elected on the multiaccess segment.
Which of the following routers will be the DIS after you connect RouterE to the multiaccess segment? (Select the best answer.)
- RouterA
- RouterB
- RouterC
- RouterD
- RouterE
Explanation:
RouterE will be the designated intermediate system (DIS) after you connect it to the multiaccess segment. The Intermediate SystemtoIntermediate System (IS-IS) DIS is analogous to the Open Shortest Path First (OSPF) designated router (DR). All ISIS routers on the network segment establish adjacencies with the DIS. The DIS serves as a focal point for the distribution of ISIS routing information. If the DIS is no longer detected on the network, a new DIS is elected based on the priority of the remaining routers on the network segment.
The DIS for the multiaccess segment is the router with the highest interface priority. To configure the priority of an interface, you should issue the isis priority command from interface configuration mode. The syntax of the isis priority command is isis priorityvalue [level1 | level2], where value is an integer from 0 through 127. A router with an interface priority of 0 can still become the DIS. If you do not issue the isis prioritycommand on an interface, the default interface priority is 64.
If interface priority values are equal, the router with the highest Media Access Control (MAC) address becomes the DIS if the multiaccess segment is a LAN. If the multiaccess segment is a Frame Relay link, the router with the highest datalink connection identifier (DLCI) becomes the DIS. If the DLCI is the same at both ends, the router with the higher system ID becomes the DIS. Every ISIS router is required to have a unique system ID. If two ISIS routers have the same system ID, an ISIS neighbor relationship will not form.
Unlike the DR in OSPF, the DIS in ISIS can be preempted if a router with a higher priority or a higher MAC address is connected to the network. In this scenario, all of the routers have the same interface priority. Therefore, the router with the highest MAC address becomes the DIS. Before RouterE is connected, RouterD is the DIS because it has the highest MAC address. However, after RouterE is connected, RouterE becomes the DIS because RouterE has a higher MAC address than RouterD.
Neither RouterA, RouterB, nor RouterC will become the DIS unless you increase the interface priority for that router’s interface. Loopback addresses and interface IP addresses are not considered in the election of the DIS.
RouterE will be the designated intermediate system (DIS) after you connect it to the multiaccess segment. The Intermediate SystemtoIntermediate System (IS-IS) DIS is analogous to the Open Shortest Path First (OSPF) designated router (DR). All ISIS routers on the network segment establish adjacencies with the DIS. The DIS serves as a focal point for the distribution of ISIS routing information. If the DIS is no longer detected on the network, a new DIS is elected based on the priority of the remaining routers on the network segment.
The DIS for the multiaccess segment is the router with the highest interface priority. To configure the priority of an interface, you should issue the isis priority command from interface configuration mode. The syntax of the isis priority command is isis priorityvalue [level1 | level2], where value is an integer from 0 through 127. A router with an interface priority of 0 can still become the DIS. If you do not issue the isis prioritycommand on an interface, the default interface priority is 64.
If interface priority values are equal, the router with the highest Media Access Control (MAC) address becomes the DIS if the multiaccess segment is a LAN. If the multiaccess segment is a Frame Relay link, the router with the highest datalink connection identifier (DLCI) becomes the DIS. If the DLCI is the same at both ends, the router with the higher system ID becomes the DIS. Every ISIS router is required to have a unique system ID. If two ISIS routers have the same system ID, an ISIS neighbor relationship will not form.
Unlike the DR in OSPF, the DIS in ISIS can be preempted if a router with a higher priority or a higher MAC address is connected to the network. In this scenario, all of the routers have the same interface priority. Therefore, the router with the highest MAC address becomes the DIS. Before RouterE is connected, RouterD is the DIS because it has the highest MAC address. However, after RouterE is connected, RouterE becomes the DIS because RouterE has a higher MAC address than RouterD.
Neither RouterA, RouterB, nor RouterC will become the DIS unless you increase the interface priority for that router’s interface. Loopback addresses and interface IP addresses are not considered in the election of the DIS.
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