Manage the Unicast RIB and FIB

About the unicast RIB and FIB

The unicast Routing Information Base (RIB) and Forwarding Information Base (FIB) are integral parts of the Cisco NX-OS forwarding architecture. The RIB maintains the routing table and determines the best next hop for routes, while the FIB is populated with forwarding information for efficient packet delivery.

  • The unicast RIB exists on the active supervisor and maintains the routing table with directly connected, static, and dynamically learned routes.

  • The unicast RIB collects adjacency information from sources such as the Address Resolution Protocol (ARP).

  • The unicast RIB determines the best next hop for each route and populates the unicast FIBs on the modules using the unicast FIB distribution module (FDM).

Unicast RIB and FIB in Cisco NX-OS forwarding architecture

The unicast RIB and FIB are essential for the operation of the Cisco NX-OS forwarding architecture, enabling efficient routing and forwarding of unicast traffic.

  • Directly connected routes, static routes, and routes learned from dynamic unicast routing protocols are maintained in the unicast RIB.

  • Adjacency information is collected from ARP and other sources.

  • The unicast FIB is populated by the RIB and used for forwarding decisions on the modules.

  1. Each dynamic routing protocol updates the unicast RIB when a route times out.

  2. The unicast RIB deletes the expired route and recalculates the best next hop if an alternate path is available.

Unicast RIB and FIB operation

For example, when a dynamic routing protocol detects that a route has timed out, it updates the unicast RIB. The RIB then removes the route and, if another path exists, recalculates the best next hop and updates the FIB accordingly.

Layer 3 consistency checker

The Layer 3 consistency checker is a feature in Cisco NX-OS that detects inconsistencies between the unicast IPv4 RIB on the supervisor module and the FIB on each interface module.

  • Detects missing prefix, extra prefix, wrong next-hop address, and incorrect Layer 2 rewrite string in the ARP or neighbor discovery (ND) cache.

  • Compares FIB entries to the latest adjacency information from the Adjacency Manager (AM) and logs any inconsistencies.

  • Compares unicast RIB prefixes to the module FIB and logs any inconsistencies.

Layer 3 consistency checker operation and behavior

In rare instances, an inconsistency can occur between the unicast RIB and the FIB on each module. Cisco NX-OS supports the Layer 3 consistency checker to detect and log such inconsistencies.

  • Missing prefix

  • Extra prefix

  • Wrong next-hop address

  • Incorrect Layer 2 rewrite string in the ARP or neighbor discovery (ND) cache

The Layer 3 consistency checker compares the FIB entries to the latest adjacency information from the Adjacency Manager (AM) and logs any inconsistencies. The consistency checker then compares the unicast RIB prefixes to the module FIB and logs any inconsistencies. See the Triggering the Layer 3 Consistency Checker section.

You can then manually clear any inconsistencies. See the Clearing Forwarding Information in the FIB section.

When more routes are learned exceeding the hardware limit, the show consistency-checker forwarding ipv4 command is run, consistency may still show as pass. The same is true when it is transitioning from an inconsistent state to a consistent state. It may show as a failure. Until and unless the test forwarding ipv4 inconsistency route command is run again, it doesn't leave this state. This is an expected behavior.

Guidelines and limitations for the unicast RIB

The following guidelines and limitations apply to the URIB or U6RIB.

In a virtual domain context (VDC), when modifying memory resource limits for the IPv4 or IPv6 unicast route, the modified limits do not take effect immediately. You must issue the copy running-config startup-config command followed by the reload command to activate the modified limits. For example, if you issue either of the following commands, you will need to issue copy running-config startup-config , then reload the switch an extra time to activate the new setting:

  • limit-resource u4route-mem

  • limit-resource u6route-mem


Note

If “feature pim” is configured for limit-resource, ensure that the value of limit-resource u4route-mem plus limit-resource u6route-mem is <= 1024 MB (1GB).

Manage the unicast RIB and FIB

Manage the Unicast RIB and FIB refers to the process of handling the Routing Information Base (RIB) and Forwarding Information Base (FIB) in a network device.

  • The RIB stores routing information learned from different protocols.

  • The FIB contains the actual forwarding table used by the device.

  • Management involves updating, maintaining, and synchronizing both tables for correct packet forwarding.

Differences in Command Syntax

If you are familiar with the Cisco IOS command, be aware that the Cisco NX-OS commands for this feature might differ from the Cisco IOS commands that you would use.

Display the module FIB information

To display the FIB information on a module, use the following commands in any mode:

Procedure

Step 1

show forwarding { ipv4 | ipv6 } adjacency module slot

Example:

switch# show forwarding ipv6 adjacency module 2

Displays the adjacency information for IPv4 or IPv6.

Step 2

show forwarding { ipv4 | ipv6 } route module slot

Example:

switch# show forwarding ipv6 route module 2

Displays the route table for IPv4 or IPv6.

Configure load sharing in the unicast FIB

Dynamic routing protocols such as Open Shortest Path First (OSPF) support load balancing with equal-cost multipath (ECMP). The routing protocol determines its best routes based on the metrics configured for the protocol and installs up to the protocol-configured maximum paths in the unicast RIB. The unicast RIB compares the administrative distances of all routing protocol paths in the RIB and selects a best path set from all of the path sets installed by the routing protocols. The unicast RIB installs this best path set into the FIB for use by the forwarding plane.

The forwarding plane uses a load-sharing algorithm to select one of the installed paths in the FIB to use for a given data packet.


Note

Load sharing uses the same path for all packets in a given flow. A flow is defined by the load-sharing method that you configure. For example, if you configure source-destination load sharing, then all packets with the same source IP address and destination IP address pair follow the same path.

To configure the unicast FIB load-sharing algorithm, use the following command in global configuration mode:

Procedure

Step 1

ip load-sharing address { destination port destination | source-destination [ port source-destination | ]} [ universal-id seed ] [ rotate rotate ] [ concatenation ]

Example:

ip load-sharing address source-destination

Configures the unicast FIB load-sharing algorithm for data traffic.

The following options are available for all IP load sharing configurations:

  • The universal-id option sets the random seed for the hash algorithm and shifts the flow from one link to another.

    You do not need to configure the universal ID. Cisco NX-OS chooses the universal ID if you do not configure it. The universal-id range is from 1 to 4294967295.

  • The rotate option causes the hash algorithm to rotate the link picking selection so that it does not continually choose the same link across all nodes in the network. It does so by influencing the bit pattern for the hash algorithm. This option shifts the flow from one link to another and load balances the already load-balanced (polarized) traffic from the first ECMP level across multiple links.

    If you specify a rotate value, the 64-bit stream is interpreted starting from that bit position in a cyclic rotation. The rotate range is from 1 to 63, and the default is 32.

    Note

     

    With multi-tier Layer 3 topology, polarization is possible. To avoid polarization, use a different rotate bit at each tier of the topology.

    Note

     

    To configure a rotation value for port channels, use the port-channel load-balance src-dst ip-l4port rotate rotate command. For more information on this command, see the Cisco Nexus 9000 Series NX-OS Interfaces Configuration Guide .

  • The concatenation option ties together the hash tag values for ECMP and the hash tag values for port channels in order to use a stronger 64-bit hash. If you do not use this option, you can control ECMP load-balancing and port-channel load-balancing independently. The default is disabled.

Step 2

(Optional) show ip load-sharing

Example:

switch(config)# show ip load-sharing
address source-destination

Displays the unicast FIB load-sharing algorithm for data traffic.

Step 3

(Optional) show routing hash source-addr dest-addr [ source-port dest-port ] [ vrf vrf-name ]

Example:

switch(config)# show routing hash 192.0.2.1
10.0.0.1

Displays the route that the unicast RIB and unicast FIB use for a source and destination address pair. The source address and destination address format is x.x.x.x. The source port and destination port range is from 1 to 65535. The VRF name can be any case-sensitive, alphanumeric string up to 64 characters.

This example shows how to display the route selected for a source/destination pair:


                switch# 
                show routing hash 10.0.0.5 192.0.0.2
Load-share parameters used for software forwarding:
load-share mode: address source-destination port source-destination
Universal-id seed: 0xe05e2e85
Hash for VRF "default"
Hashing to path *172.0.0.2 (hash: 0x0e), for route:

Display the routing and adjacency information

To display routing and adjacency information, use the following commands in any mode:

Command

Purpose

show { ip | ipv6 } route [ route-type | interface interface-type number | next-hop ] 

switch# show ip route

Displays the unicast route table. The route-type argument can be a single route prefix or a direct, static, or dynamic route protocol. Use the ? command to see the supported interfaces.

show { ip | ipv6 } adjacency [ prefix | interface-type number [ summary ] | non-best ] [ detail ] [ vrf vrf-id ]

Example: 

switch# show ip adjacency

Displays the adjacency table. The argument ranges are as follows:

  • prefix —Any IPv4 or IPv6 prefix address.

  • interface-type number —Use the ? command to see the supported interfaces.

  • vrf-id —Any case-sensitive, alphanumeric string up to 64 characters.

show { ip | ipv6 } routing [ route-type | interface interface-type number | next-hop | recursive-next-hop | summary | updated { since | until } time ]

Example: 

switch# show routing summary

Displays the unicast route table. The route-type argument can be a single route prefix or a direct, static, or dynamic route protocol. Use the ? command to see the supported interfaces.

Procedure

Enter the appropriate show command to display routing or adjacency information.

Example:

switch# show ip route

This example shows how to display the unicast route table:

switch# show ip route
IP Route Table for Context "default"
'*' denotes best ucast next-hop '**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]
0.0.0.0/0, 1 ucast next-hops, 0 mcast next-hops
*via 10.1.1.1, mgmt0, [1/0], 5d21h, static
0.0.0.0/32, 1 ucast next-hops, 0 mcast next-hops
*via Null0, [220/0], 1w6d, local, discard
10.1.0.0/22, 1 ucast next-hops, 0 mcast next-hops, attached
*via 10.1.1.55, mgmt0, [0/0], 5d21h, direct
10.1.0.0/32, 1 ucast next-hops, 0 mcast next-hops, attached
*via 10.1.0.0, Null0, [0/0], 5d21h, local
10.1.1.1/32, 1 ucast next-hops, 0 mcast next-hops, attached
*via 10.1.1.1, mgmt0, [2/0], 5d16h, am
10.1.1.55/32, 1 ucast next-hops, 0 mcast next-hops, attached
*via 10.1.1.55, mgmt0, [0/0], 5d21h, local
10.1.1.253/32, 1 ucast next-hops, 0 mcast next-hops, attached
*via 10.1.1.253, mgmt0, [2/0], 5d20h, am
10.1.3.255/32, 1 ucast next-hops, 0 mcast next-hops, attached
*via 10.1.3.255, mgmt0, [0/0], 5d21h, local
255.255.255.255/32, 1 ucast next-hops, 0 mcast next-hops
*via Eth Inband Port, [0/0], 1w6d, local

This example shows how to display the adjacency information:


switch# 
show ip adjacency 
IP Adjacency Table for context default
Total number of entries: 2
Address         Age         MAC Address       Pref  Source   Interface    Best
10.1.1.1        02:20:54    00e0.b06a.71eb    50    arp      mgmt0        Yes
10.1.1.253      00:06:27    0014.5e0b.81d1    50    arp      mgmt0        Yes

Trigger the layer 3 consistency checker

You can manually trigger the Layer 3 consistency checker.

To manually trigger the Layer 3 consistency checker, use the following commands in global configuration mode:

Procedure

Step 1

test forwarding [ ipv4 | ipv6 ] [ unicast ] inconsistency [ vrf vrf-name ] [ module { slot | all }]

Example:

switch(config)# test forwarding inconsistency

Starts a Layer 3 consistency check. The vrf-name can be any case-sensitive, alphanumeric string up to 64 characters. The slot range is from 1 to 26.

Step 2

test forwarding [ ipv4 | ipv6 ] [ unicast ] inconsistency [ vrf vrf-name ] [ module { slot | all }] stop

Example:

switch(config)# test forwarding inconsistency stop

Stops a Layer 3 consistency check. The vrf-name can be any case sensitive, alphanumeric string up to 64 characters. The slot range is from 1 to 26.

Step 3

show forwarding [ ipv4 | ipv6 ] [ unicast ] inconsistency [ vrf vrf-name ] [ module { slot | all }]

Example:

switch(config)# show forwarding inconsistency

Displays the results of a Layer 3 consistency check. The vrf-name can be any case-sensitive, alphanumeric string up to 64 characters. The slot range is from 1 to 26.

Step 4

show consistency-checker forwarding unicast

Example:

switch(config)# show consistency-checker forwarding unicast

Displays the results of a Layer 3 consistency check for unicast routes.

Clear the FIB forwarding information

You can clear one or more entries in the FIB. Clearing a FIB entry does not affect the unicast RIB.


Caution
The clear forwarding command disrupts forwarding on the device.

To clear an entry in the FIB, including a Layer 3 inconsistency, use the following command in any configuration mode:

Procedure

clear forwarding { ipv4 | ipv6 } route { * | prefix } [ vrf vrf-name ] module { slot | all }

Example:

switch# clear forwarding ipv4 route * module 1

Clears one or more entries from the FIB. The route options are as follows:

  • * —All routes.

  • prefix —All routes.

The vrf-name can be any case-sensitive, alphanumeric string up to 64 characters. The slot range is from 1 to 26.

Configure maximum routes for the unicast RIB

You can configure the maximum number of routes allowed in the routing table.

Procedure

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

vrf context vrf-name

Example:


switch(config)# vrf context management2
switch(config-vrf)#

Creates a VRF and enters VRF configuration mode.

Step 3

address-family {ipv4 | ipv6} unicast

Example:


switch(config-vrf)# address-family ipv4 unicast
switch(config-vrf-af-ipv4)

Enters the address-family configuration mode.

Step 4

maximum routes max-routes [ threshold [ reinstall threshold ] | warning -only ]

Example:


switch(config-vrf-af-ipv4)# maximum routes 300000

Configures the maximum number of routes allowed in the routing table. The range is from 1 to 4294967295.

You can optionally specify the following:

  • threshold —Percentage of maximum routes that triggers a warning message. The range is from 1 to 100.

  • warning-only —Logs a warning message when the maximum number of routes is exceeded.

  • reinstall threshold —Reinstalls routes that previously exceeded the maximum route limit and were rejected and specifies the threshold value at which to reinstall them. The threshold range is from 1 to 100.

Step 5

(Optional) copy running-config startup-config

Example:

switch(config-vrf-af-ipv4)# copy running-config
startup-config

Saves this configuration change.

Estimate memory requirements for routes

To estimate the memory requirements for routes, use the following command in any mode:

Command Purpose

show routing { ipv6 } memory estimate routes num-routes next-hops num-nexthops

Example: 

switch# show routing memory estimate routes 5000 next-hops 2

Displays the memory requirements for routes. The num-routes range is from 1000 to 1000000. The num-nexthops range is from 1 to 16.

Procedure

Enter the show routing { ipv6 } memory estimate routes num-routes next-hops num-nexthops command.

Example:

switch# show routing memory estimate routes 5000 next-hops 2

Displays the memory requirements for routes. The num-routes range is from 1000 to 1000000. The num-nexthops range is from 1 to 16.

Clear routes in the unicast RIB

You can clear one or more routes from the unicast RIB.


Caution
The * keyword is severely disruptive to routing.

To clear one or more entries in the unicast RIB, use the following commands in any configuration mode:

Procedure

Step 1

clear { ip | ip4 | ipv6 } route { * | { route | prefix/length } [ next-hop interface ]} [ vrf vrf-name ]

Example:

switch(config)# clear ip route 10.2.2.2

Clears one or more routes from both the unicast RIB and all the module FIBs. The route options are as follows:

  • * —All routes.

  • route —An individual IP or IPv6 route.

  • prefix / length —Any IP or IPv6 prefix.

  • next-hop —The next-hop address.

  • interface —The interface to reach the next-hop address.

The vrf-name can be a case-sensitive, alphanumeric string up to 64 characters.

Step 2

clear routing [ multicast | unicast ] [ ip | ip4 | ipv6 ] { * | { route | prefix/length } [ next-hop interface ]} [ vrf vrf-name ]

Example:

switch(config)# clear routing ip 10.2.2.2

Clears one or more routes from the unicast RIB. The route options are as follows:

  • * —All routes.

  • route —An individual IP or IPv6 route.

  • prefix / length —Any IP or IPv6 prefix.

  • next-hop —The next-hop address.

  • interface —The interface to reach the next-hop address.

The vrf-name can be a case-sensitive, alphanumeric string up to 64 characters.

Verify the unicast RIB and FIB configuration

To display the unicast RIB and FIB configuration information, perform one the following tasks:

Command

Purpose

show forwarding adjacency

Displays the adjacency table on a module.

show forwarding distribution {clients | fib-state}

Displays the FIB distribution information.

show forwarding interfaces module slot

Displays the FIB information for a module.

show forwarding {ip | ipv4 | ipv6} route

Displays routes in the FIB.

show {ip | ipv6} adjacency

Displays the adjacency table.

show {ip | ipv6} route

Displays the IPv4 or IPv6 routes from the unicast RIB.

show routing

Displays routes from the unicast RIB.

show system internal access-list dest-miss stats

Displays statistics for packets dropped due to missing the FIB routes for the destinations, also called as DEST MISS. The output displays increment in the DEST MISS counters.

Note

 

Beginning with Cisco NX-OS Release 10.1(1), this feature is supported on Cisco Nexus 9300-FX3 platform switches.