Pseudonodes on Broadcast-Networks

At first a short recapitulation of the last chapter for a better understanding of the task of pseudonodes. IS-IS routers which are connected over common LAN, multicast hello packets to well known ports. With the use of hello packets IS-IS routers form adjacencies with other routers. After that link-state packets are exchanged between IS-IS routers. These packets contain link-state information. Without this information dynamic routing would not be possible.

The problem is that all these packets generate a lot of administration traffic. And for network developers it is very important to reduce this traffic as far as possible. One solution, but only in a so called multi-access media such as LAN, is the use of pseudonodes. As the name implies, this is a virtual node, whose role is played by an elected Designated Intermediate System (DIS) for the LAN. In a selection process, only routers with adjacencies in an up state are considered. Further more, every level has a separate DIS. More precise there is a level 1 DIS and a level 2 DIS. But how does the selection process work? The selection of the DIS is based on the highest interface priority and if the priority is equal it is based on the highest SNPA address. A Cisco router has a default interface priority of 64.

In spite of the fact that IS-IS DISs plays an important role in the network, IS-IS considers no backup DIS. This is an essential difference between the routing-protocols IS-IS and OSPF, because OSPF uses a DIS. But this is not a big problem in IS-IS, because this protocol has a very high periodical database synchronisation update rate. More precisely at an IS-IS LAN the default hello interval for the DIS is 3.3 seconds. For the sake of comparison other nodes have a 10 seconds default hello interval. These values demonstrate the quick detection of DIS failures and immediate replacement. In IS-IS an elected router is not guaranteed the DIS if a new router with a higher priority shows up on a broadcast LAN. In this case, if the router goes in up state, the new router with the higher priority takes over the DIS role. Further more there is no mechanism specified for making a router without ability to be a DIS. But configuring a router’s LAN interface with the lowest priority value relative to the priorities of other nodes on the LAN is one way to reduce the availability to be chosen as DIS.

To sum up, the IS-IS specification (ISO 10589) defines three types of designated intermediate systems, as follows:

  • LAN level 1 DIS
  • LAN level 2 DIS
  • Partition-designated level 2 IS

An IS-IS level 1 or level 2 DIS has two important responsibilities. First of all a DIS is responsible for generating pseudonode link-state packets to report links to all systems on the broadcast subnetwork. Secondly a DIS is also responsible for carrying out flooding over the broadcast LAN for the corresponding routing level. The third IS-IS type is Partition-designated level 2 IS. This type plays only a subordinate role and therefore it is not further treated in this term work.

pseudonode.txt · Last modified: 2009/09/13 14:37 (external edit)
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