Managing Configuration Comments on IOS XR

Jan 4, 2026 · BGP Routing ·

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Introduction

This post has been written considering Cisco Catalyst 8000v running IOS XE 17.18.01a and Cisco IOS XRv 9000 running IOS XR 24.3.1.

Recently, I had to configure an ASR9K from scratch, starting from a set of configuration lines taken from another device. During the configuration process, I realized that, unlike IOS XE, IOS XR allows comments to be included directly alongside configuration commands. In IOS XE, it is well known that comments cannot be saved within the running configuration. When comments are inserted using the ! character, they are not stored in NVRAM or in the active copy of the configuration file. As a result, comments do not appear when displaying the active configuration using the show running-config command. The situation is different with IOS XR. On this platform, it is possible to include comments directly within the configuration. These comments are stored together with the rest of the configuration, making them visible in the output of commands such as show running-config.

How to Insert a Comment in IOS XR

Therefore, the special character used to insert a comment within the configuration, and thus before executing the usual commit operation to save it, is the ! character. The comment is then associated with the configuration command that follows. A simple example is shown below, based on the following topology.

A simple OSPF adjacency in Area 0 has been configured between the two PEs. PE-TEST-1 advertises the network 1.1.1.1/32 (which corresponds to the IP address assigned to the local Loopback 0 interface) to PE-TEST-2. Below is the configuration of PE-TEST-1.

RP/0/RP0/CPU0:PE-TEST-1#show running-config router ospf 1
Sun Jan  4 07:33:57.059 UTC
router ospf 1
 area 0
  interface Loopback0
   passive enable
  !
  interface GigabitEthernet0/0/0/1
   bfd minimum-interval 50
   bfd fast-detect
   bfd multiplier 3
   network point-to-point
  !
 !
!

Below is the configuration of PE-TEST-2.

RP/0/RP0/CPU0:PE-TEST-2#show running-config router ospf 1
Sun Jan  4 07:34:17.149 UTC
router ospf 1
 area 0
  interface GigabitEthernet0/0/0/0
   bfd minimum-interval 50
   bfd fast-detect
   bfd multiplier 3
   network point-to-point
  !
 !
!

The node PE-TEST-2 correctly receives the advertisement for the network 1.1.1.1/32, as can be verified from PE-TEST-2. The network is advertised by PE-TEST-1 with an OSPF cost of 1.

RP/0/RP0/CPU0:PE-TEST-2#show ip ospf 1 database router adv-router 10.0.0.1
Sun Jan  4 07:35:01.712 UTC


            OSPF Router with ID (10.0.0.2) (Process ID 1)

                Router Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 29
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 10.0.0.1
  Advertising Router: 10.0.0.1
  LS Seq Number: 80000011
  Checksum: 0xab03
  Length: 60
   Number of Links: 3

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 1.1.1.1
     (Link Data) Network Mask: 255.255.255.255
      Number of TOS metrics: 0
       TOS 0 Metrics: 1
    
    [OUTPUT OMITTED]

Now, the OSPF cost of the Loopback 0 interface on PE-TEST-1 is modified, and a comment is added to explain the reason for this choice. The following configuration is then applied.

RP/0/RP0/CPU0:PE-TEST-1(config-ospf)#area 0
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar)#int loopback 0            
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#! BACKUP PATH - COST 450
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#cost 450
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#commi
Sun Jan  4 07:36:13.761 UTC

It is worth noting that a space between the special character ! and the comment text is not mandatory. The comment could therefore also be written as !BACKUP PATH - COST 450.

When displaying the OSPF configuration, the comment is now shown. It is important to emphasize that the comment is associated with the configuration command that follows. In other words, the configuration command passive enable is entirely independent of the comment that was just added.

RP/0/RP0/CPU0:PE-TEST-1#show running-config router ospf 1
Sun Jan  4 07:37:16.152 UTC
router ospf 1
 area 0
  interface Loopback0
   ! BACKUP PATH - COST 450
   cost 450
   passive enable
  !
  interface GigabitEthernet0/0/0/1
   bfd minimum-interval 50
   bfd fast-detect
   bfd multiplier 3
   cost 13
   network point-to-point
  !
 !
!

The OSPF cost for the network 1.1.1.1/32 was subsequently updated on PE-TEST-2 as well.

RP/0/RP0/CPU0:PE-TEST-2#show ip ospf 1 database router adv-router 10.0.0.1   
Sun Jan  4 07:37:16.152 UTC


            OSPF Router with ID (10.0.0.2) (Process ID 1)

                Router Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 26
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 10.0.0.1
  Advertising Router: 10.0.0.1
  LS Seq Number: 80000012
  Checksum: 0x5a90
  Length: 60
   Number of Links: 3

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 1.1.1.1
     (Link Data) Network Mask: 255.255.255.255
      Number of TOS metrics: 0
       TOS 0 Metrics: 450

    [OUTPUT OMITTED]

Modifying the OSPF cost value can be performed normally; the comment remains preserved within the configuration.

How to Modify a Comment in IOS XR

Once a comment has been associated with a command, it can be updated. Two approaches are available for doing so:

Enter the updated comment and then reapply the same command with the same configuration parameters.
Enter the updated comment and then apply the same command with different configuration parameters.

For example, consider the first scenario where we want to update the comment associated with the OSPF cost command on the Loopback 0 interface of PE-TEST-1, while keeping the same cost value. The sequence of commands to achieve this is shown below.

RP/0/RP0/CPU0:PE-TEST-1(config-ospf)#area 0
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar)#int loopback 0            
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#! NEW COMMENT - BACKUP PATH - 450
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#cost 450
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#commi
Sun Jan  4 07:40:11.132 UTC

By verifying the configuration using the standard command, it can be confirmed that the comment has been successfully updated. Additionally, packet capture analysis confirmed that PE-TEST-1 did not send any OSPF LSU messages to PE-TEST-2.

RP/0/RP0/CPU0:PE-TEST-1#show running-config | i COMMENT
Sun Jan  4 07:41:01.261 UTC
   ! NEW COMMENT - BACKUP PATH - 450

In the second scenario, we examine what happens when, after updating the comment, the OSPF cost of the interface is modified from 450 to 150.

RP/0/RP0/CPU0:PE-TEST-1(config-ospf)#area 0
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar)#int loopback 0            
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#! NEW VALUE - BACKUP PATH - 150
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#cost 150
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#commi
Sun Jan  4 07:42:43.721 UTC

By checking the configuration again using the standard command, it can be confirmed that the comment has been successfully updated.

RP/0/RP0/CPU0:PE-TEST-1#show running-config | i NEW
Sun Jan  4 07:43:23.411 UTC
   ! NEW VALUE - BACKUP PATH - 150

How to Remove a Comment in IOS XR

After modifying the command, if it needs to be removed from PE-TEST-1, the specific clear comment command must be used. Once executed, the configuration command associated with that comment should be re-entered. An example is provided below, assuming the objective is to remove the comment ! NEW VALUE - BACKUP PATH - 150.

RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#clear comment
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#cost 150
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#commit
Sun Jan  4 07:45:13.816 UTC

The comment was therefore removed as expected. Additionally, packet capture analysis confirmed that PE-TEST-1 did not send any OSPF LSU messages to PE-TEST-2.

RP/0/RP0/CPU0:PE-TEST-1#show running-config router ospf 1 area 0 interface loopback 0
Sun Jan  4 07:48:42.113 UTC
router ospf 1
 area 0
  interface Loopback0
   cost 150
   passive enable
  !
 !
!

Being linked to the command, the associated comment is removed when the command itself is deleted.

How to Insert a Comment within a Commit in IOS XR

So far, we have seen how it is possible to add, modify and remove comments from the configuration. In IOS XR, however, it is also possible to include comments within a commit (up to a maximum of 60 characters). This functionality, which is reminiscent of version control systems (VCS), can be useful when trying to understand which changes are included in a particular commit. Let us now consider reconfiguring the interface cost while including a descriptive comment with the commit. The commands below illustrate how to perform this operation.

RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#cost 450
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#commit comment ?
  LINE  Comment for this commit (Max 60 characters)
RP/0/RP0/CPU0:PE-TEST-1(config-ospf-ar-if)#commit comment Updated Link Cost
Sun Jan  4 07:51:21.533 UTC

The list of all commits (up to a maximum of 36) can be displayed using the show configuration commit list command.

RP/0/RP0/CPU0:PE-TEST-1#show configuration commit list 
Sun Jan  4 07:52:16.546 UTC
SNo. Label/ID              User      Line                Client      Time Stamp
~~~~ ~~~~~~~~              ~~~~      ~~~~                ~~~~~~      ~~~~~~~~~~
1    1000000036            admin     con0_RP0_CPU0       CLI         Sun Jan  4 07:51:21 2026
2    1000000035            admin     con0_RP0_CPU0       CLI         Sun Jan  4 07:45:13 2026
[OUTPUT OMITTED]

To view the comment, the show configuration commit list detail command that shows the details of each commit must be used.

RP/0/RP0/CPU0:PE-TEST-1#show configuration commit list detail 
Sun Jan  4 07:53:45.823 UTC

   1) CommitId: 1000000036                 Label: NONE
      UserId:   admin                      Line:  con0_RP0_CPU0
      Client:   CLI                        Time:  Sun Jan  4 07:51:21 2026
      Comment:   Updated Link Cost 

   2) CommitId: 1000000035                 Label: NONE
      UserId:   admin                      Line:  con0_RP0_CPU0
      Client:   CLI                        Time:  Sun Jan  4 07:45:13 2026
      Comment:  NONE
[OUTPUT OMITTED]

In the most recent commit, the Comment field displays the value that was previously entered.

I hope this post has been helpful. If you have any additional information to share, feel free to reach out via social media. See you in the next one! 🙂

Refernce

System Management Configuration Guide, Cisco IOS XE 17.x