Brocade Multi-Service IronWare Routing Configuration Guide (Supporting R05.6.00) - Manual (Page 280)

252 Multi-Service IronWare Routing Configuration Guide
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Relationship to IP route table
Figure 18 shows an example of the results of Designated IS elections. For simplicity, this example
shows four of the five routers in Figure 17 on page 250, with the same domain and areas.
FIGURE 18 Each broadcast network has a Level-1 Designated IS and a Level-2 Designated IS
Designated IS election has the following results in this network topology:
• Router B is the Level-1 Designated IS for broadcast network 1
• Router C is the Level-1 Designated IS for broadcast network 2
• Router D is the Level-2 Designated IS for broadcast network 3
In this example, the IS-IS priorities for the IS-IS interfaces in broadcast network 1 have been
changed by an administrator. The priorities for the interfaces in the other broadcast networks are
still set to the default (64). When there is a tie, IS-IS selects the interface with the highest MAC
address.
Broadcast pseudonode
In a broadcast network, the Designated IS maintains and distributes link state information to other
ISs by maintaining a pseudonode. A pseudonode is a logical host representing all the Level-1 or
Level-2 links among the ISs in a broadcast network. Level-1 and Level-2 have separate
pseudonodes, although the same device can be the pseudonode for Level-1 and Level-2.
Route calculation and selection
The Designated IS uses a Shortest Path First (SPF) algorithm to calculate paths to destination ISs
and ESs. The SPF algorithm uses Link State PDUs (LSPDUs) received from other ISs as input, and
creates the paths as output.
After calculating the paths, the Designated IS then selects the best paths and places them in the
IS-IS route table. The Designated IS uses the following process to select the best paths.
1. Prefer the Level-1 path over the Level-2 path.
2. If there is no Level-1 path, prefer the internal Level-2 path over the external Level-2 path.
3. If there is still more than one path, prefer the path with the lowest metric.
4. If there is more than one path with the lowest metric, load share among the paths.
After selecting the best path to a destination, the software places the path in the IS-IS route table.