Contents

Introduction

Explanation

Introduction

This document extends the concepts described in another document   Understand Selection of Forwarding Address in OSPF  and explains the difference when you use NX-OS, the network operating system for Cisco Nexus-series Ethernet switches.

Explanation

According to above document, the forwarding address is selected on Autonomous System Border Router (ASBR) using these  rules:

1. If there is a loopback interface configured in the area, then IP address of the loopback interface is selected as Forwarding Address (FA).

2. If the first condition is not met, then IP address of the first interface on the Open Shortest Path First (OSPF) interface list is selected as a forwarding address. You can see OSPF interface list by using show ip ospf interface brief command. The interface on top will be the last interface which was attached to OSPF.

However, the second rule is not applicable to NX-OS.

In NX-OS, when there is no loopback interface, OSPF interface ID determines the forwarding address.

IP address of the interface with the highest ID is selected as FA, regardless of whether the interface is passive or not.

Please note that interface ID is dynamic and may change after the system is reloaded.

If an interface is removed from OSPF and is added back, its ID in OSPF interface list may change if another new interface is added into OSPF between the delete and add operations.

The reason is that OSPF interface ID resource is used cyclically.

When an interface is removed from OSPF interface list, the assigned interface ID will be recycled, i.e., when another interface is added to OSPF interface, the ID wil be assigned to the interface.

Only when all recycled IDs are depleted, a new interface ID will be allocated for the next OSPF interface.

An example is provided:

show ip ospf interface brief vrf A

OSPF Process ID 10 VRF A

Total number of interface: 4

Interface               ID     Area            Cost   State    Neighbors Status

Vlan101                452    0.0.0.7         5      BDR      2         up

Vlan201                678    0.0.0.7         100    DR       0         up <—————— highest ID = FA

Vlan301                160    0.0.0.7         100    DR       0         up

Po11                247    0.0.0.7         1000   P2P      1         up

- When vlan201 is removed from OSPF, ID 678 is freed.

- Then, when van 202 is added to OSPF, ID 678 is assigned to vlan 202.

- Then vlan 201 is added back, a new larger ID is assigned to vlan 201.

show ip ospf interface brief vrf B

OSPF Process ID 10 VRF B

Total number of interface: 4

Interface               ID     Area            Cost   State    Neighbors Status

Vlan102                490    0.0.0.7         5      BDR      2         up    <——————  highest ID = FA

Vlan202                51     0.0.0.7         100    DR       0         up

Vlan302                102    0.0.0.7         100    DR       0         up

Po12                275    0.0.0.7         1000   P2P      1         up

interface Vlan101

vrf member A

ip address 192.168.1.1/24 <-----FA

interface Vlan102

vrf member B

ip address 10.1.1.1/24 <---------FA

N7K#show ospf data nssa-external172.16.1.140 det vrf A

OSPF Router with ID () (Process ID 21)

Type-7 AS External Link States (Area 7)

LS age: 162

Options: (No TOS-capability, Type 7/5 translation, No DC)

LS Type: AS External Link

Link State ID: 172.16.1.140 (External Network Number )

Advertising Router: 100.1.1.1

LS Seq Number:80000012

Checksum: 0x504c

Length: 36

Network Mask:255.255.255.255

Metric Type: 1 (Comparable directly to link state metric)

TOS: 0

Metric: 20

Forward Address: 192.168.1.1 ←--- Vlan101

External Route Tag: 0

N7K# show ip ospf data extroute172.16.1.150 det vrf B

OSPF Router with ID () (Process ID 10 VRF B)

Type-5 AS External Link States

LS age: 323

Options: 0x20 (No TOS-capability, DC)

LS Type: Type-5 AS-External

Link State ID:route172.16.1.150 (Network address)

Advertising Router:100.1.1.2

LS Seq Number: 0x8000008f

Checksum: 0x9691

Length: 36

Network Mask: /32

Metric Type: 1 (Same units as link state path)

TOS: 0

Metric: 20

Forward Address: 10.1.1.1 ←--- Vlan 102

External Route Tag: 0