Dell PowerConnect B-RX Configuration Manual page 932

28
Configuring IS-IS (IPv4)
The Designated IS is elected based on the priority of each IS in the broadcast network. When an IS
becomes operational, it sends a Level-1 or Level-2 Hello PDU to advertise itself to other ISs. If the
IS is configured to be both a Level-1 and a Level-2 IS, the IS sends a separate advertisement for
each level.
•
•
If the Designated IS becomes unavailable (for example, is rebooted), the IS with the next highest
priority becomes the new IS. If two or more ISs have the highest priority, the IS with the highest MAC
address becomes the Designated IS.
The priority is an interface parameter. Each interface that is enabled for IS-IS can have a different
priority.
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.
860
The Level-1 IS that has the highest priority becomes the Level-1 Designated IS for the
broadcast network.
The Level-2 IS that has the highest priority becomes the Level-2 Designated IS for the
broadcast network.
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 858, with the same domain and areas.
Broadcast
Network 2
Router B Router C
64
aaaa.bbbb.1111
64
aaaa.bbbb.1122
Broadcast
Network 3
Router D
64
aaaa.bbbb.1121
64
aaaa.bbbb.1131
BigIron RX Series Configuration Guide
53-1001986-01