System Security Configuration Guide for Cisco 8000 Series Routers, IOS XR Releases

SSH clients and servers in FIPS mode

Updated: September 18, 2026

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Explains how SSH clients select documented FIPS-approved ciphers and how SSH version 2 servers use supported key-exchange, cipher, and HMAC algorithms.

SSH clients and servers in FIPS mode are secure connection endpoints that

use documented algorithms during session establishment
negotiate compatible cipher and integrity protection, and
operate after FIPS mode and the required SSH settings are enabled.

The SSH client selects an AES-CTR cipher when it starts a connection. The SSH version 2 server negotiates supported key-exchange, cipher, and HMAC algorithms with the client.

Connect to an SSH server with FIPS-approved ciphers

Connect the router SSH client to a server with a documented FIPS-approved cipher.

The SSH client selects the cipher and key length when it starts the connection.

Before you begin

Enable FIPS mode.

Procedure

Start the SSH session with the destination address, AES-CTR key length, and username.

Example:

Router# ssh 192.0.2.1 cipher aes 128-CTR username sample-ssh-user

AES in Counter mode with a 128-bit, 192-bit, or 256-bit key is the FIPS-compliant cipher for the SSH client.

The router starts an SSH client session with the selected AES-CTR cipher.

Configure a FIPS-compliant SSH server

Enable inbound SSH version 2 connections while the router operates in FIPS mode.

The server negotiates connection algorithms from the set supported in FIPS mode.

Before you begin

Enable FIPS mode.
Review the supported SSH server algorithms for FIPS mode.

Procedure

Enable the SSH version 2 server in global configuration mode, and commit the configuration.

Example:

Router# configure
Router(config)# ssh server v2
Router(config)# commit

Use these values when evaluating an SSH server configuration for FIPS mode.

Table 1. Supported SSH server algorithms
Algorithm category	Supported algorithms
Key exchange	diffie-hellman-group14-sha1 ecdh-sha2-nistp256 ecdh-sha2-nistp384 ecdh-sha2-nistp521
Cipher	aes128-ctr aes192-ctr aes256-ctr aes128-gcm aes256-gcm
HMAC	hmac-sha2-512 hmac-sha2-256 hmac-sha1

The SSH version 2 server negotiates a mutually supported key-exchange algorithm, cipher, and HMAC algorithm with each connecting client.

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