Managing the Unicast RIB and FIB

This chapter describes how to manage routes in the unicast Routing Information Base (RIB) and the Forwarding Information Base (FIB) on the Cisco NX-OS switch.

This chapter includes the following sections:

•Information About the Unicast RIB and FIB

•Licensing Requirements for the Unicast RIB and FIB

•Managing the Unicast RIB and FIB

•Verifying the Unicast RIB and FIB Configuration

•Additional References

•Feature History for Unicast RIB and FIB

Information About the Unicast RIB and FIB

The unicast RIB (IPv4 RIB) and FIB are part of the Cisco NX-OS forwarding architecture, as shown in Figure 10-1.

Figure 10-1 Cisco NX-OS Forwarding Architecture

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 (FIBs) by using the services of the 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).

This section includes the following topics:

•Layer 3 Consistency Checker

•FIB Tables

•Virtualization Support

Layer 3 Consistency Checker

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. This feature detects inconsistencies between the unicast IPv4 RIB and the FIB on each interface module. Inconsistencies include the following:

•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.

FIB Tables

The hardware provides two tables, a TCAM table and a Hash table. The TCAM table is shared between the longest prefix match (LPM) route and the /32 unicast route. The Hash table is shared between the /32 unicast entries and the multicast entries. Each table has approximately 8000 routes.

Virtualization Support

The Unicast RIB and FIB support Virtual Routing and Forwarding instances (VRFs). By default, Cisco NX-OS places you in the default VRF unless you specifically configure another VRF. For more information, see Chapter 9 "Configuring Layer 3 Virtualization."

Licensing Requirements for the Unicast RIB and FIB

The following table shows the licensing requirements for this feature:

Managing the Unicast RIB and FIB

This section includes the following topics:

•Displaying Module FIB Information

•Configuring Load Sharing in the Unicast FIB

•Configuring Per-Packet Load Sharing

•Displaying Routing and Adjacency Information

•Triggering the Layer 3 Consistency Checker

•Clearing Forwarding Information in the FIB

•Estimating Memory Requirements for Routes

•Clearing Routes in the Unicast RIB

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

Displaying Module FIB Information

You can display the FIB information on a switch.

DETAILED STEPS

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

show ip fib adjacency 

Example:

switch# show ip fib adjacency 

show forwarding ipv4 adjacency 

Example:

switch# show forwarding ipv4 
adjacency 

show ip fib interfaces 

Example:

switch# show ip fib interfaces 

show ip fib route 

Example:

switch# show ip fib route 

show forwarding ipv4 route 

Example:

switch# show forwarding ipv4 route 

This example shows the FIB contents on a switch:

switch# show ip fib route 

IPv4 routes for table default/base

------------------+------------------+---------------------

Prefix            | Next-hop         | Interface

------------------+------------------+---------------------

0.0.0.0/32          Drop               Null0

255.255.255.255/32  Receive            sup-eth1

Configuring 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.

You can globally configure the following load-sharing settings:

•load-share mode—Selects the best path based on the destination address and port or the source and the destination address and port.

•Universal ID—Sets the random seed for the hash algorithm. You do not need to configure the Universal ID. Cisco NX-OS chooses the Universal ID if you do not configure it.

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:

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

Example:

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

To display the unicast FIB load-sharing algorithm, use the following command in any mode:

show ip load-sharing 

Example:

switch(config)# show ip load-sharing

To display the route that the unicast RIB and FIB use for a particular source address and destination address, use the following command in any mode:

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

Example:

switch# show routing hash 192.0.2.1 
10.0.0.1

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

switch# show routing hash 10.0.0.5 30.0.0.2

  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 *20.0.0.2 (hash: 0x0e), for route:

Configuring Per-Packet Load Sharing

You can use per-packet load sharing to evenly distribute data traffic in an IP network over multiple equal-cost connections. Per-packet load sharing allows the router to send successive data packets over paths on a packet-by-packet basis rather than on a per-flow basis.

Note Using per-packet load sharing can result in out-of-order packets. Packets for a given pair of source-destination hosts might take different paths and arrive at the destination out of order. Make sure you understand the implications of out-of-order packets to your network and applications. Per-packet load sharing is not appropriate for all networks. Per-flow load sharing ensures packets always arrive in the order that they were sent.

Per-packet load sharing uses the round-robin method to determine which path each packet takes to the destination. With per-packet load sharing enabled on interfaces, the router sends one packet for destination1 over the first path, the second packet for (the same) destination1 over the second path, and so on. Per-packet load sharing ensures balancing over multiple links.

Use per-packet load sharing to ensure that a path for a single source-destination pair does not get overloaded. If most of the traffic passing through parallel links is for a single pair, per-destination load sharing will overload a single link while other links will have very little traffic. Enabling per-packet load sharing allows you to use alternate paths to the same busy destination.

Note Per-packet load sharing on an interface overrides the global load-sharing configuration.

You configure per-packet load sharing on the input interface. This configuration determines the output interface that Cisco NX-OS chooses for the packet.

For example, if you have ECMP paths on two output interfaces, Cisco NX-OS uses the following load-sharing methods for input packets on Ethernet 1/1:

•Per-packet load sharing if you configure per-packet load sharing on Ethernet 1/1.

•Per-flow load sharing.

The configuration for the other interfaces have no effect on the load-sharing method used for Ethernet 1/1 in this example.

To configure per-packet load sharing, use the following command in interface configuration mode:

ip load-sharing per-packet

Example:

switch(config-if)# ip load-sharing 
per-packet

Displaying Routing and Adjacency Information

You can display the routing and adjacency information.

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

show ip route [route-type | interface 
int-type number | next-hop]

Example:

switch# show ip route

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

Example:

switch# show ip adjacency

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

Example:

switch# show routing summary

This example displays the unicast route table:

switch# show ip route

IP Route Table for VRF "default"

'*' denotes best ucast next-hop

'**' denotes best mcast next-hop

'[x/y]' denotes [preference/metric]

192.168.0.2/24, ubest/mbest: 1/0, attached

    *via 192.168.0.32, Eth1/5, [0/0], 22:34:09, direct

192.168.0.32/32, ubest/mbest: 1/0, attached

    *via 192.168.0.32, Eth1/5, [0/0], 22:34:09, local

This example shows the adjacency information:

switch# show ip adjacency

IP Adjacency Table for VRF 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

Triggering 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:

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

Example:

switch(config)# test forwarding 
inconsistency 

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

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

Example:

switch(config)# test forwarding 
inconsistency stop

To display the Layer 3 inconsistencies, use the following commands in any mode:

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

Example:

switch(config)# show forwarding 
inconsistency 

Clearing Forwarding Information in the FIB

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 switch.

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

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

Example:

switch(config)# clear forwarding ipv4 
route *

Estimating Memory Requirements for Routes

You can estimate the memory that a number of routes and next-hop addresses will use.

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

show routing memory estimate routes 
num-routes next-hops num-nexthops

Example:

switch# show routing memory estimate 
routes 1000 next-hops 1

Clearing 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 mode:

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

Example:

switch(config)# clear ip route 
10.2.2.2 

clear routing [multicast | unicast] 
[ip | ipv4] {* | {route | 
prefix/length}[next-hop interface]} 
[vrf vrf-name] 

Example:

switch(config)# clear routing ip 
10.2.2.2 

Verifying the Unicast RIB and FIB Configuration

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

Additional References

For additional information related to managing unicast RIB and FIB, see the following sections:

•Related Documents

•Feature History for Unicast RIB and FIB

Related Documents

Feature History for Unicast RIB and FIB

Table 10-1 lists the release history for this feature.