Cisco NX-OS/IOS OSPF Comparison

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Objective

This tech note outlines the main differences in Open Shortest Path First Version 2 (OSPFv2) support between Cisco® NX-OS Software and Cisco IOS® Software. Sample configurations are included for Cisco NX-OS and Cisco IOS Software for some common features to demonstrate the similarities and differences. Please refer to the NX-OS documentation on Cisco.com for a complete list of supported features.


OSPF Overview

OSPFv2 is an IETF (RFC 2328) standards-based dynamic link-state routing protocol used to exchange network reachability within an autonomous system.


Important Cisco NX-OS and Cisco IOS Software Differences

In Cisco NX-OS:

  • OSPF command-line interface (CLI) configuration and verification commands are not available until you enable the OSPF feature with the feature ospf command.
  • The OSPF protocol requires the Enterprise Services license.
  • The OSPF instance can consists of 20 characters, whereas the IOS supports numbers 1 – 65536.
  • Eight equal-cost paths are supported by default. You can configure up to sixteen.
  • The reference bandwidth used in the OSPF cost calculation is 40 Gbps
  • Networks and interfaces are added to an OSPF instance under the interface configuration mode.
  • An OSPF area can be configured using decimal or decimal dotted notation, but it is always displayed in decimal dotted notation in the configuration and in the show command output.
  • Passive interfaces are applied to the interface as opposed to under the OSPF instance.
  • If a router ID is not manually configured, the loopback 0 IP address is always preferred. If loopback 0 does not exist, Cisco NX-OS selects the IP address for the first loopback interface in the configuration. If no loopback interfaces exist, Cisco NX-OS selects the IP address for the first physical interface in the configuration.
  • Neighbor adjacency changes are not logged by default. The log-adjacency-changes CLI command is required under the OSPF instance.
  • When interface authentication is configured, the OSPF key is encrypted with Data Encryption Standard 3 (3DES) in the configuration. Cisco IOS Software requires the service password command.
  • When you rollover an OSPF authentication key in a combined Cisco NX-OS/Cisco IOS network, you should configure both keys on the Cisco NX-OS router to ensure that there is sufficient overlap between the old key and the new key for a smooth transition to the new key. You should configure the new key as a valid accept key on all the NX-OS and IOS routers before the new key becomes a valid generation key in the keychain. During the overlap period, Cisco NX-OS transmits the new OSPF key and accepts OSPF authenticated packets from both the old key and the new key.


Things You Should Know

The following list provides some additional facts about Cisco NX-OS that should be helpful when designing, configuring, and maintaining an OSPF network.


  • Four OSPF instances can be configured per virtual device context (VDC).
  • Numerous Virtual Route Forwarding (VRF) instances can be associated with an OSPF instance.
  • If you remove the feature ospf command, all relevant OSPF configuration information is also removed.
  • The shutdown command under the OSPF process can be used to disable OSPF while retaining the configuration. Similar functionality can also be applied per interface with the ip ospf shutdown command.
  • The show running-config ospf command displays the current OSPF configuration.
  • An OSPF instance can be restarted with the restart ospf <instance #> command.
  • Graceful Restart (RFC 3623) is enabled by default.
  • OSPF is the only layer 3 routing protocol that supports stateful process restart.
  • You cannot configure multiple OSPF instances on the same interface.
  • An interface can support multi-area adjacencies using the multi-area option with the router ospf interface command.
  • Secondary IP addresses are advertised by default, but can be suppressed per interface.


Configuration Comparison


The following sample code shows configuration similarities and differences between the Cisco NX-OS and Cisco IOS Software CLIs. There are two significant differences: Cisco NX-OS allows OSPF to be enabled and disabled globally, and it has a more interface-centric configuration that makes it easier to read.


Cisco IOS CLI Cisco NX-OS CLI
Enabling the OSPF Feature
Cisco IOS Software does not have the ability to enable or disable OSPF. feature ospf
Configuring an OSPF Instance and Router ID
router ospf 10

router-id 192.168.1.1

router ospf 10

router-id 192.168.1.1

Associating a Network with an OSPF Instance and Area
router ospf 10

network 192.168.1.0 0.0.0.255 area 1

interface Ethernet2/1

ip address 192.168.10.1/24

ip router ospf 10 area 1

Configuring a Passive Interface
router ospf 10

passive-interface GigabitEthernet2/1

network 192.168.1.0 0.0.0.255 area 1

interface Ethernet2/1

ip address 192.168.11.1/24

ip ospf passive-interface

ip router ospf 10 area 0

Configuring Interface Authentication (MD5)
interface GigabitEthernet2/1

ip address 192.168.10.1 255.255.255.0

ip ospf authentication message-digest

ip ospf message-digest-key 1 md5 cisco123

interface Ethernet2/1

ip address 192.168.10.1/24

ip ospf authentication message-digest

ip ospf message-digest-key 1 md5 3 a667d47acc18ea6b

ip router ospf 10 area 1

Configuring a Stub Area with the no summary Option
router ospf 10

area 2 stub no-summary

router ospf 10

area 2 stub no-summary

Creating a Not-So-Stubby Area (NSSA) and Generating a Default Route
router ospf 10

area 3 nssa default-information-originate

router ospf 10

area 3 nssa default-information-originate

Configuring Inter-Area and External Summarization
router ospf 10

area 0 range 159.142.0.0 255.255.0.0 summary-address 172.16.0.0 255.255.0.0

router ospf 10

area 0 range 159.142.0.0/16 summary-address 172.16.0.0/16

Generating a Default Route (Conditional)
router ospf 10

default-information originate

router ospf 10

default-information originate

Generating a Maximum Metric (Max-Metric) Value
router ospf 10

max-metric router-lsa

router ospf 10

max-metric router-lsa



Verification Command Comparison

The following table compares some useful show commands for verifying and troubleshooting an OSPFv2 network configuration.


Cisco NX-OS OSPFv2 Cisco IOS Software OSPFv2 Command Description
show ip ospf show ip ospf Displays the running configuration
show ip ospf border-routers show ip ospf border-routers Displays a list of border routers
show ip ospf database show ip ospf database Displays OSPF database information
show ip ospf interface show ip ospf interface <int type> Displays OSPF database information
show ip ospf interface detail - Displays additional packet statistics for each interface
show ip ospf memory - Displays the memory allocated for OSPF
show ip ospf neighbor show ip ospf neighbors Displays neighbor-specific information
show ip ospf neighbor detail show ip ospf neighbor detail Displays details for each OSPF neighbor
show ip ospf policy statistics - Displays redistribution statistics for a specified protocol
show ip ospf request list show ip ospf request list Displays a list of link-state advertisements (LSAs) that have been requested
show ip ospf retransmission list show module Displays installed modules and their status
show ip ospf route - Displays all routes learned through OSPF
show ip ospf statistics show ip ospf statistics Displays OSPF LSA statistics
show ip ospf summary-address show ip ospf summary-address Displays OSPF-summarized networks
show ip ospf traffic show ip ospf traffic Displays OSPF-related packet counters
show ip ospf vrf - Displays information for a specified OSPF VRF instance

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