This document describes the architecture of the Cisco 12000 Series
Internet Router route processor.
There are no specific requirements for this document.
The information in this document is based on the following hardware:
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
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
Runs external gateway protocols such as Border Gateway Protocol (BGP)
Computes the forwarding table
Express Forwarding tables and
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
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
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
The following sequence describes a typical GRP boot process:
System power is turned on.
The GRP decompresses the Bootstrap image (rommon).
The GRP loads the appropriate Cisco IOS software image from the Flash
The GRP decompresses the Cisco IOS software image.
Meanwhile, the Maintenance Bus (MBus) is initialized (it receives +5
VDC) and the MBus module in each component in the chassis also powers on.
Redundant GRPs in the chassis arbitrate for mastership over the MBus
The Primary RP uses the MBus to instruct the MBus modules on the line
cards and switch cards to power on their cards.
The Bootstrap image is downloaded to the line cards across the
The GRP decompresses the configuration, while the line cards are
waiting for the loading of the fabric downloader over the switch
The line card gets the fabric downloader and loads it into line card
The line card launches and runs the fabric
The GRP downloads the Cisco IOS software onto line card
The line card launches and runs the Cisco IOS software
"IOS RUN" appears on the line card LED.
When the links come UP/UP, BGP peers are established and routes are
Route advertisements are sent to the RP.
RP updates the routing information table and builds a CEF entry for
For each line card that is UP/UP and in sync, the RP sends the update
through Inter Processor Communication (IPC).
BGP convergence finishes. All routes are successfully exchanged and
integrated into Cisco Express Forwarding.
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
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
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