Neighbors and adjacencies, Designated is – Brocade BigIron RX Series Configuration Guide User Manual
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BigIron RX Series Configuration Guide
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IS-IS overview
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Neighbors and adjacencies
A device configured for IS-IS forms an adjacency with each of the IS-IS devices to which it is directly
connected. An adjacency is a two-way direct link (a link without router hops) over which the two
devices can exchange IS-IS routes and other protocol-related information. The link is sometimes
called a “circuit”. The devices with which the device forms adjacencies are its neighbors, which are
other ISs.
A BigIron RX IS-IS interfaces are configured by default for broadcast circuits.
In
on page 947, Router A has an IS-IS adjacency with Router B. Likewise, Router B has
an IS-IS adjacency with Router A and Router C.
Designated IS
A Designated IS is an IS-IS router that is responsible for gathering and distributing link state
information to other Level-1 or Level-2 ISs within the same broadcast network (LAN). The Level-1
and Level-2 Designated ISs within a broadcast network are independent, although the same device
can be a Level-1 Designated IS and a Level-2 Designated IS at the same time.
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.
•
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.
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.
shows an example of the results of Designated IS elections. For simplicity, this example
shows four of the five routers in
on page 947, with the same domain and areas.