Dell PowerConnect B-RX Configuration Manual page 955

Figure 118
shows four of the five routers in
FIGURE 118
Designated IS election has the following results in this network topology:
•
•
•
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.
BigIron RX Series Configuration Guide
53-1001810-01
shows an example of the results of Designated IS elections. For simplicity, this example
Each broadcast network has a Level-1 designated IS and a Level-2 designated IS
Broadcast
Network 1
Router A
10
20
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
Figure 117 on page 881, with the same domain and areas.
Broadcast
Network 2
Router B Router C
64
aaaa.bbbb.1111
64
aaaa.bbbb.1122
In this chapter
Broadcast
Network 3
Router D
64
aaaa.bbbb.1121
64
aaaa.bbbb.1131
29
883