Cisco 12000 Series Routers

Cisco 12000 Series Internet Router Architecture: Route Processor

Document ID: 47241

Updated: Jul 07, 2005



This document describes the architecture of the Cisco 12000 Series Internet Router route processor.



There are no specific requirements for this document.

Components Used

The information in this document is based on the following hardware:

  • Cisco 12000 Series Internet Router

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.


For more information on document conventions, see the Cisco Technical Tips Conventions.

Card Description

The Gigabit Route Processor, more commonly called the GRP, is the brain of the system. The GRP:

  • Runs internal routing protocols such as Enhanced Interior Gateway Routing Protocol (EIGRP), Interior Gateway Routing Protocol (IGRP), Intermediate System-to-Intermediate System (IS-IS), Open Shortest Path First (OSPF)

  • Runs external gateway protocols such as Border Gateway Protocol (BGP)

  • Computes the forwarding table

  • Builds the Cisco Express Forwarding tables and Adjacency tables, and distributes them to all the line cards (LCs) in the system over the Switch Fabric.

Additionally, the GRP is also responsible for system control and administrative functions, performing general maintenance functions, such as diagnostics, console port, and line card monitoring.

Note: Once the GRP has sent the routing information base (RIB), basically the route table, and the adjacency database to each LC over the switch fabric, each LC then computes its copy of the Forwarding Information Base (FIB) which should be identical to the one on the Route Processor (RP). Sometimes there are inconsistencies between the FIB on the RP and the LC. This is why you should always check the CEF entry on the RP and the LC when you are troubleshooting accessibility. All LCs make their switching decisions based on the FIB table and then directly send the packet to the appropriate output interface over the fabric.

The GRP is mainly composed of:

  • CPU - The CPU on the GRP is the same R5000 processor used on the Cisco 7500 RSP4. The CPU is responsible primarily for running routing protocols and for maintaining a master copy of the CEF table which is downloaded to the line cards for packet switching.

  • Main Memory (Dynamic RAM - DRAM) - Up to 512 MB used to store Cisco IOS software code and all data structures.

  • Cisco Cell Segmentation and Reassembly (CSAR) Static RAM (SRAM) - 512 KB; this memory is used for reassembling cells arriving from the switching fabric into packets.

  • Ethernet Controller - Designed for out-of-band management: traffic which should not be switched between this port and ports on the LCs.

For more information about the types of memory on a GRP, see Memory Present on the Gigabit Route Processor (GRP).

Below is an overview of the GRP:


The GRP communicates with the line cards, either through the switching fabric or through a redundant 1 Mbps Maintenance Bus. The fabric connection is the main data path for route table distribution and for the movement of packets between the line cards and the GRP (for example, Address Resolution Protocol (ARP), Simple Network Management Protocol (SNMP), and Telnet). The maintenance bus connection enables the GRP to download a bootstrap image, collect or load diagnostic information, and perform general maintenance operations.

GRP Boot Process Overview

The following sequence describes a typical GRP boot process:

  1. System power is turned on.

  2. The GRP decompresses the Bootstrap image (rommon).

  3. The GRP loads the appropriate Cisco IOS software image from the Flash card.

  4. The GRP decompresses the Cisco IOS software image.

  5. Meanwhile, the Maintenance Bus (MBus) is initialized (it receives +5 VDC) and the MBus module in each component in the chassis also powers on.

  6. Redundant GRPs in the chassis arbitrate for mastership over the MBus period.

  7. The Primary RP uses the MBus to instruct the MBus modules on the line cards and switch cards to power on their cards.

  8. The Bootstrap image is downloaded to the line cards across the MBus.

  9. The GRP decompresses the configuration, while the line cards are waiting for the loading of the fabric downloader over the switch fabric.

  10. The line card gets the fabric downloader and loads it into line card memory.

  11. The line card launches and runs the fabric downloader.

  12. The GRP downloads the Cisco IOS software onto line card memory.

  13. The line card launches and runs the Cisco IOS software image.

  14. "IOS RUN" appears on the line card LED.

  15. When the links come UP/UP, BGP peers are established and routes are advertised.

  16. Route advertisements are sent to the RP.

  17. RP updates the routing information table and builds a CEF entry for that prefix.

  18. For each line card that is UP/UP and in sync, the RP sends the update through Inter Processor Communication (IPC).

  19. BGP convergence finishes. All routes are successfully exchanged and integrated into Cisco Express Forwarding.

Redundancy Modes

Support for redundant GRPs was introduced in Cisco IOS Software Releases 12.0(5)S and 11.2(15)GS2.

As from Cisco IOS Software Release 12.0(22)S, the following redundancy modes are supported on the Cisco 12000 Series Internet Router:

  • Route Processor Redundancy (RPR)

  • Route Processor Redundancy Plus (RPR+)

  • Stateful Switchover (SSO)

See How does 12000 Series Internet Router GRP Redundancy Work? for more details about these different redundancy modes.

Note: The failover process can be initiated by the redundancy force-failover command.

Configuring the Ethernet Interface

The Institute of Electrical and Electronic Engineers (IEEE) 802.3 Ethernet interface, located on the GRP, allows connections to external Ethernet networks and is capable of data transmission rates of 10 Mbps and 100 Mbps. At the auto-sensed data transmission rate of 100 Mbps, the Ethernet port provides maximum usable bandwidth that is less than 100 Mbps; a maximum usable bandwidth of approximately 20 Mbps can be expected if you are using either the MII or RJ-45 connection. Transmission speed which is not user-configurable is determined by the network to which the Ethernet interface is connected.

Further, the Ethernet interface does not provide external routing functions; it is primarily designed as a Telnet port into the GRP, and for booting or accessing Cisco IOS software images over a network to which the Ethernet interface is directly connected.

GRP Ethernet port forwarding behavior was changed in Cisco IOS Software Release 12.0(9)S (CSCdm01200), so packets received on a line card are no longer forwarded out of the Ethernet port. As of Release 12.0(9)S, by default:

  • Ethernet 0 is only used for communication to and from the RP.

  • Packets entering E0 and destined out of a line card are dropped.

  • Packets entering a line card or created on a line card that need to be sent out Ethernet 0 are dropped.

With this bug, Cisco Express Forwarding is disabled on Ethernet 0 by default.

On the Cisco 12000 series routers, the GRP Ethernet 0 port is designed to handle packets to and from the GRP. In some versions of code, the software incorrectly allows the Ethernet 0 port to be used to forward packets to the line cards. This forwarding path is unsupported and should not be used as it exposes the router vulnerabilities, including the potential that a large number of packets will be sent through this path due to misconfiguration of another device. This would result in all of the GRP CPU being used to forward the packets at the expense of other router duties.

DDTS CSCdu27273 changes the command line interface so that it is consistent with the supported configurations for the GRP Ethernet 0 port. Specifically, the port can only be used to receive packets destined for the router. These changes have been committed in Cisco IOS Software Releases 12.0(18)ST and 12.0(18)S.

The following links provide two methods for configuring the Ethernet interface:

Related Information

Updated: Jul 07, 2005
Document ID: 47241