Unicast Forwarding Distribution Module; Fib; Hardware Forwarding; Software Forwarding - Cisco Nexus 7000 Series Configuration Manual

Cisco NX-OS Fowarding Architecture
S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m .

Unicast Forwarding Distribution Module

The unicast forwarding distribution module exists on the active supervisor and distributes the forwarding
path information from the unicast RIB and other sources. The unicast RIB generates forwarding
information which the unicast FIB programs into the hardware forwarding tables on the standby
supervisor and the modules. The unicast forwarding distribution module also downloads the FIB
information to newly inserted modules.
The unicast forwarding distribution module gathers adjacency information, rewrite information, and
other platform-dependent information when updating routes in the unicast FIB. The adjacency and
rewrite information consists of interface, next-hop, and Layer 3 to Layer 2 mapping information. The
interface and next-hop information is received in route updates from the unicast RIB. The Layer 3 to
Layer 2 mapping is received from the adjacency manager.

FIB

The unicast FIB exists on supervisors and switching modules and builds the information used for the
hardware forwarding engine. The unicast FIB receives route updates from the unicast forwarding
distribution module and sends the information along to be programmed in the hardware forwarding
engine. The unicast FIB controls the addition, deletion, and modification of routes, paths, and
adjacencies.
The unicast FIBs are maintained on a per-VRF and per-address-family basis, that is, one for IPv4 and
one for IPv6 for each configured VRF. Based on route update messages, the unicast FIB maintains a
per-VRF prefix and next-hop adjacency information database. The next-hop adjacency data structure
contains the next-hop IP address and the Layer 2 rewrite information. Multiple prefixes could share a
next-hop adjacency information structure.

Hardware Forwarding

Cisco NX-OS supports distributed packet forwarding. The ingress port takes relevant information from
the packet header and passes the information to the local switching engine. The local switching engine
does the Layer 3 lookup and uses this information to rewrite the packet header. The ingress module
forwards the packet to the egress port. If the egress port is on a different module, the packet is forwarded
using the switch fabric to the egress module. The egress module does not participate in the Layer 3
forwarding decision.
The forwarding tables are identical on the supervisor and all the modules.
You also use the show platform fib or show platform forwarding commands to display details on
hardware forwarding.

Software Forwarding

The software forwarding path in Cisco NX-OS is used mainly to handle features that are not supported
in hardware or to handle errors encountered during hardware processing. Typically, packets with IP
options or packets that need fragmentation are passed to the CPU on the active supervisor. All packets
that should be switched in software or terminated go to the supervisor. The supervisor uses the
information provided by the unicast RIB and the adjacency manager to make the forwarding decisions.
The module is not involved in the software forwarding path.
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
1-12
Chapter 1 Overview
OL-20002-02