Domain Partitions

1) This chapter gives an overview of how the IS-IS routing protocol handles broken links and deals with the domain partitions that can happen due to broken links.

The first part explains level 2 domain partitions and the second part will discuss the level 1 domain partition problems.

Level 2 Domain Partitions

Because of routing overhead the level 1 topology information is hidden from level 2 domains. So all the intermediate systems of a level 2 domain have to be interconnected. Otherwise there will be some unreachable domain parts.


In figure 3-7 there are two level 1 partitions and a broken level 2 partition. Before the link between the intermediate system C and D is broken the network is in a convergent state. That means that all nodes in the network are reachable.
If there is a failure on the link between node C and D there is no way to handle the segmented partition. If the level 1 node F choses the L1/L2 intermediate system B as its closest exit point all nodes which were reachable through the node C wont be reachable any more. Also if the level 1 node F choses the level 1/2 node A as the exit point the complete connection to node D and also the reachability from the area 49.3333 fails!
The obscured thing on analyzing such a broken network is that every node in a level 1 area can choose a different exit point, so some nodes can reach the 49.3333 area via the exit point B and some can not reach this area because they use node A as their closest exit point. Furthermore the level 2 node C can't reach node D.

To prevent such a broken domain it is possible to define all the level 2 intermediate systems as level 1/2 nodes. That means that the level 2 nodes are both, level 2 and also level 1/2 nodes. In many current network designs all nodes are defined as level 2 nodes only in order to build only one big domain. This prevents from such segmentation but also the routing overhead would be enormous. With this technique the real advantage, the domain structure would be lost and so the networks are not able to grow as much as a real domain separated IS-IS network could.

Level 1 Domain Partitions


In figure 3-8 the level 1 area 49.1111 is seperated into 2 level 1 areas with the same domain number. In this case the node A and the node B send advertisements that the area 49.1111 is reachable through them. Now there are 2 different entry points into the level 1 domain. Every level 2 router will select one of this two nodes so again some level 1 nodes are reachable and some are not.

Repairsystem of Level 1 Domain Partitions

The ISO 10589 standard specifies a technique which provides a working routing system although there is a broken level 1 domain.
Due to the broken link the node B can't see the node A of the same domain any more, so it must choose a different intermediate system as the partition repair system. In our case B chooses itself as the partition repair system.
IS B starts with anouncing itself as the partition repair system for domain 49.1111. Node A receives this repair advertisements through the node C and detects that there are two partiton repair systems for domain 49.1111. Now IS A and IS B know that the partition is seperated and needs to be repaired through the level 2 domain. Now node A and B encapsulate all the received LSPs and transmitt them through the level 2 domain to the seperated part of domain 49.1111.
For example every LSP that A receives gets a new CLNS header with the address of node B. A forwards this new LSP through node C to the seperated level 1 domain (to intermediate system B). IS B receives this repair LSP and removes the second CLNS header. Now node B has the needed information of the splitted domain.
In other words there is a virtual link created between node A and B.

Zitat: IS-IS Network Design Solutions page 160
Many details in the partitioning repair mechanism are not clearly explained in ISO 10589. In addition, the process depends on CLNS routing (between A and B in our example), 
which is not deployed in most networks. These reasons, and perhaps most importantly the fact that no real problems arise when a partition takes place within an IP network, 
are why the partition repair mechanism is not implemented by most vendors or used in deployed networks.


1) IS-IS Network Design Solutions page 156 - 162
2) figure 3-7 A partitioned level 2 domain page 157
3) figure 3-8 Level 1 routing domain partitioning page 158
4) IS-IS Network Design Solutions page 160
routenausfall.txt · Last modified: 2009/09/13 14:37 (external edit)
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