Arris C4C - Designated Intermediate System (DIS) and Reliable Flooding of Lsps

Chapter 16: Dynamic Routing Protocols
STANDARD Revision 1.0 C4® CMTS Release 8.3 User Guide
© 2016 ARRIS Enterprises LLC. All Rights Reserved. 474
 Link State Packets (LSPs).
 Sequence number packets.
Hello Packets — As is the case with OSPF, Hello packets are used to establish and maintain adjacencies between directly
connected IS-IS neighbors.
Link State Packets — Link state packets are used to distribute the actual IP routing information.
Sequence Number Packets — Sequence number packets control the distribution of LSPs to allow for correct
synchronization of the Link State database.
Designated Intermediate System (DIS) and Reliable Flooding of LSPs
The DIS is sometime referred to as the Pseudonode, which is an abstraction for representing broadcast links as network
nodes. This reduces the amount of router-to-router communications on a broadcast network and as a consequence,
reduces the amount of information (IS-IS PDUs) that is exchanged when multiple nodes interconnect on a LAN.
The election of the DIS is based on interface priority and, as a tie breaker, the MAC address used to encapsulate the Hello
packet. As is the case with OSPF, the DIS plays the critical role of LSP flooding; however it should be noted that unlike
OSPF, there does not exist the concept of a backup DIS (known in OSPF nomenclature as a BDR).
If the DIS becomes unavailable, then DIS election must be restarted.
To help mitigate a DIS outage, the hello interval for DIS routers is set at three times the rate of non-DIS routers. This
scheme allows for quick detection of DIS failures and replacement.
In addition to flooding responsibilities, the DIS will advertise a pseudonode LSP, which represents the broadcast link itself.
This LSP has a zero cost and allows for communication on the broadcast link between individual non-DIS routers.
The DIS router is not guaranteed to remain the DIS if a new router with a higher priority shows up on the LAN; likewise,
there is no mechanism for making a router ineligible for DIS operation.
IS-IS peers are said to be adjacent after Hello packets are exchanged, but before the LSP database synchronization is
complete. This differs from OSPF, and may cause transient routing problems when adjacent routers do not have a
complete forwarding table representing routes within the IS-IS domain.
Use of the LSP overload bit can help solve this issue by informing adjacent routers that traffic should not be sent to a
router whose LSP overload bit is set.