Relationship to ip route table, Intermediate systems and end systems – 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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Portions of the Internet Draft “IS-IS extensions for Traffic Engineering”
draft-ieff-isis-traffic-02.txt (dated 2000) that describe the Extended IP reachability TLV (TLV
type 135) and the extended Intermediate System (IS) reachability TLV (TLV type 22). These
portions provide support for the wide metric version of IS-IS. No other portion is supported on
Brocade’s implementation of IS-IS.
NOTE
The BigIron RX does not support routing of Connectionless-Mode Network Protocol (CLNP) packets.
The device uses IS-IS for TCP/IP only.
Relationship to IP route table
The IS-IS protocol has the same relationship to the device’s IP route table that OSPF has to the IP
route table. The IS-IS routes are calculated and first placed in the IS-IS route table. The routes are
then transferred to the IP route table.
The protocol sends the best IS-IS path for a given destination to the IP route table for comparison to
the best paths from other protocols to the same destination. The CPU selects the path with the
lowest administrative distance and places that path in the IP route table.
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If the path provided by IS-IS has the lowest administrative distance, then the CPU places that
IS-IS path in the IP route table.
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If a path to the same destination supplied by another protocol has a lower administrative
distance, the CPU installs the other protocol’s path in the IP route table instead.
The administrative distance is a protocol-independent value from 1 – 255. Each path sent to the
CPU, regardless of the source of the path (IS-IS, OSPF, static IP route, and so on) has an
administrative distance.
Each route source has a default administrative distance. The default administrative distance for
IS-IS is 115.
You can change the administrative distance for IS-IS and other routes sources.
Intermediate systems and end systems
IS-IS uses the following categories to describe devices within an IS-IS routing domain (similar to an
OSPF Autonomous System):
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Intermediate System (IS) – A device capable of forwarding packets from one device to another
within the domain. In Internet Protocol (IP) terminology, an IS is a router.
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End System (ES) – A device capable of generating or receiving packets within the domain. In IP
terminology, an ES is an end node or IP host.
When you configure IS-IS on a device, the device is an IS.