IS-IS Network Types

The IS-IS protocol offers support only for the broadcast and point-to-point network types. You will not be able to configure the IS-IS protocol on a WAN interface configured with the multipoint keyword, unless multicast support is also included:

[edit interfaces so-0/1/0] lab@r4# show dce; hold-time up 30 down 30; encapsulation frame-relay; lmi {      n392dce 2;      n393dce 3;      t392dce 25;      lmi-type itu; } unit 100 {      multipoint;      family inet {         address 10.0.2.6/30 {             multipoint-destination 10.0.2.5 dlci 100;         }     }      family iso; }

The lack of a multicast DLCI assignment makes this an illegal configuration:

[edit interfaces so-0/1/0] lab@r4# commit check [edit interfaces so-0/1/0 unit 100 family iso]  'family iso;'  Family ISO not allowed on non-multicast-capable multipoint interfaces DCD_PARSE_ERROR: config file parse failed error: configuration check-out failed 

At the time of this writing, carrier support for multicast Frame Relay and ATM circuits is virtually nonexistent. The grounding of the JNCIP examination in “real world” service provider–like environments means that you are unlikely to see multicast Frame Relay or ATM-related configuration requirements in the lab. With the multicast-dlci keyword present, the candidate configuration will commit but IS-IS will still treat the WAN link as a point-to-point interface.

The Designated Intermediate System

IS-IS makes use of the Designated Router (DR) concept for the same reasons as the OSPF protocol. Since IS-IS does not support NBMA network types, it elects only a DIS on the broadcast network type associated with LAN interfaces. Unlike OSPF, the DIS function in IS-IS is preemptive, which means that a router with a preferred priority setting will overthrow the existing DIS to claim its rightful position on the network. Should the DIS priority be the same, the tie is broken based on the MAC address (also called a Subnetwork Point of Attachment [SNPA]), with the router possessing the highest MAC address being declared the winner. Additional differences between IS-IS and OSPF include the fact that IS-IS does not support the concept of a BDR, and that LAN-attached stations will form a full mesh of IS-IS adjacencies, while in OSPF, such stations will form adjacencies only to the DR and BDR but not among DROther routers.