Adjacency Manager; Unicast Forwarding Distribution Module; Fib - Cisco Nexus 3000 Series Configuration Manual

Chapter 1 Overview
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 3 k - d o c f e e d b a c k @ c i s c o . c o m
The unicast RIB maintains the routing table with directly connected routes, static routes, and routes
learned from dynamic unicast routing protocols. The unicast RIB also collects adjacency information
from sources such as the Address Resolution Protocol (ARP). The unicast RIB determines the best
next-hop for a given route and populates the unicast forwarding information base (FIB) by using the
services of unicast FIB distribution module (FDM).
Each dynamic routing protocol must update the unicast RIB for any route that has timed out. The unicast
RIB then deletes that route and recalculates the best next-hop for that route (if an alternate path is
available).

Adjacency Manager

The adjacency manager maintains adjacency information for different protocols including ARP, Open
Shortest Path First version 2 (OSPFv2), Neighbor Discovery Protocol (NDP), and static configuration.
The most basic adjacency information is the Layer 3 to Layer 2 address mapping discovered by these
protocols. Outgoing Layer 2 packets use the adjacency information to complete the Layer 2 header.
The adjacency manager can trigger ARP requests to find a particular Layer 3 to Layer 2 mapping. The
new mapping becomes available when the corresponding ARP reply is received and processed.

Unicast Forwarding Distribution Module

The unicast forwarding distribution module 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. 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 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. 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.
The unicast FIB also enables and disables unicast reverse path forwarding (RPF) checks per interface.
The Cisco Nexus 3000 Series supports the following two RPF modes that can be configured on each
ingress interface:
•
RPF Strict Check—Packets that do not have a verifiable source address in the routers forwarding
table or do not arrive on any of the return paths to the source are dropped.
Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, NX-OS Release 5.0(3)U1(1)
Cisco NX-OS Fowarding Architecture
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