IP Link-State Routing Protocols - CCIE Routing and Switching Exam Certification Guide - page 400

as 00. The NET is configured under the router isis command. In this example, the AFI is 49,
the area is 0001, the System ID is 00aa.0101.0001, and the SEL is 00:
router isis
net 49.0001.00aa.0101.0001.00
Designated IS (DIS)
As with OSPF, IS-IS selects DISs on multiaccess networks. There is no backup DIS as in OSPF.
If the DIS fails, a new DIS is elected. L1 and L2 DISs are selected separately and might or might
not be the same IS. One difference between OSPF and IS-IS in multiaccess subnetworks is that
all IS-IS ISs establish adjacencies with all others in the subnetwork, not just with the DIS.
The IS with the highest priority becomes the DIS. By default, the priority value is 64. The
priority value can be changed to a value from 0 to 127. If the priority is set to 0, the IS is not
eligible to become a DIS for that network. The highest System ID selects the DIS if there is a
tie with the priorities. If a new IS is added to a network with an existing DIS, and the new IS
has a higher priority or an equal priority and a higher System ID, the new IS becomes the DIS.
On p2p networks, the priority is set to 0 because no DIS is elected. The interface isis priority
command changes the priority.
IS-IS Areas
IS-IS uses a two-level hierarchy that is similar to the OSPF areas. ISs are configured to route
L1, L2, or both L1 and L2 routes (L1/L2). L1 ISs are similar to OSPF internal routers. An L2
IS is similar to an OSPF backbone router. An IS that has both L1 and L2 routes is similar to an
OSPF ABR.
Each L1 IS in an area has an identical link-state database. The L1/L2 ISs maintain a separate
link-state database for the L1 routes and L2 routes. The L1/L2 ISs do not advertise L2 routes to
the L1 area. Instead, the L1/L2 IS sets a bit, the attached bit (ATT bit), in the L1 advertisements,
to indicate that it has knowledge of other areas. L1 ISs do not have specific routing information
about destinations outside the area, but they do know which IS has the knowledge. L1 ISs
forward traffic to outside destinations through the nearest L1/L2 IS. L1 ISs are similar to OSPF
internal routers in a totally stubby area.
IS-IS areas are not bounded by the L1/L2 ISs but by the links between L1/L2 ISs and L2
backbone ISs, as shown in Figure 8-9.