TAILIEUCHUNG - The Complete IS-IS Routing Protocol- Part 5

The Complete IS-IS Routing Protocol- P5: IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, its well-structured data formats, its flexibility and easy extensibility are all appealing – IS-IS epitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choice in some of the world’s largest networks. | Router Technology Examples 27 Figure . The first generation Interface Processor IP Cards did not embed route-lookup functionality. All the traffic has been passed via the Route Switch Processor RSP . Cisco 7500 Series The Cisco 7500 series of router was the most successful router ever built for Internet core applications. Figure shows the overall structure of the box. Basically it is a redundant shared bus system with one element dual-homed to both buses. The shared buses have different speeds depending on the revision level. Bus speeds range from the CxBus 533 Mbit s half-duplex to the CyBus Gbit s half-duplex and finally the CzBus Gbit s half-duplex . The Route Switch Processor RSP has to run both the routing software and also needs to switch packets. The first-generation interface cards are called Interface Processors and are from Network-Layer viewpoint purely passive devices. The IPs perform Layer-1 Physical Layer and Layer-2 MAC Layer related tasks like verifying CRC checksums SONET messaging or ATM SAR functions. If a packet enters the box an interrupt is signalled to the RSP. The RSP fetches the packet and does a route-lookup to find the corresponding outbound interface. All relevant modifications to the IP header such as TTL decrementing and recalculating the IP header s checksum are done by the RSP. Then the packet is copied to the outgoing interface where it ultimately leaves the chassis. The RSP forwarding module needs to have efficient route-lookup structures in order to spend minimum lookup times before making forwarding decisions. The forwarding information base FIB is known to Cisco Systems as the Cisco Express Forwarding CEF Table. In Figure there are two examples of how the lookup for IP address traverses the CEF Table. The basic structure is a 256-way 4-level structure called an M-tree. The four levels are located at the 8 16 24 and 32 prefix boundaries. Each 28 2. Router Architecture node contains 256 pointers

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