Cisco Systems 12000-Series Routers

Cisco Systems has the largest range of routers in the market today. The Cisco 12000 product line is the one of interest for this discussion, since it is the closest competitor to the Juniper Networks M-series routers. The 12000 series starts from the 12008 router, with a throughput of 40 Gbps, to the 12416 router, with a throughput of 320 Gbps. Considering the 12410 router, which has a throughput of 200 Gbps, we note that it has redundant route processors and switch fabrics, as well as redundant power supplies.

The architecture is also based on the Single Control Card + Multiple Line Card architecture. The line card–to–switch fabric communication is in the form of fixed-size cells. The control card–to–line card interface is via a 1 Mbps serial bus called the Maintenance Bus.

The control card (called the Gigabit Route Processor) runs the Cisco IOS™ (Internetwork Operating System) software. The CPU used is a MIPS family processor. The switch fabric is also known as the forwarding engine This software is used across multiple Cisco routing products but has different profiles based on the product line.

Cisco IOS™ Software

IOS is both the operating system and the software required for the various protocols. In that sense, it is a monolithic piece of code without a separation between the application and the RTOS. IOS provides the capability for both CLI and SNMP management. The CLI can be accessed via a serial port and via telnet.

IOS uses the term “process” to indicate threads and associated data (similar to tasks specified in our description). Processes perform various functions like switching packets, system maintenance, and implementing routing protocols. For use in partitioning data according to memory (SRAM, DRAM, and so on), IOS uses the concept of regions. Free memory is managed via a series of memory pools

Packet buffers are managed via a buffer pool manager. Packet buffer pools are allocated out of memory from memory pools.

Unlike most RTOSes, IOS uses a run-to-completion model of execution. Processes cannot preempt each other, so a process has to relinquish control for another process to run.

IOS device drivers provide a hardware abstraction layer between the hardware and the OS. The interface is via a data structure called the Interface Descriptor Block (IDB), one for each interface.

For more details, the reader is referred to “Inside IOS Software Architecture” by Vijay Bollapragada, et al., Cisco Press, 2000.