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Cisco ASR 1000 Series Aggregation Services Routers Software Configuration Guide
OL-16506-17
Chapter 3
Telnet and SSH Overview for the Cisco ASR 1000 Series Routers
Telnet and SSH Overview for the Cisco ASR 1000 Series Routers
Telnet and Secure Shell (SSH) on the Cisco ASR 1000 Series Routers can be configured and handled
like Telnet and SSH on other Cisco platforms. For information on traditional Telnet, see the line
command in the Cisco IOS Terminal Services Command Reference guide located at:
http://www.cisco.com/en/US/docs/ios/12_2/termserv/command/reference/trflosho.html#wp1029818.
For information on configuring traditional SSH, see the “Configuring Secure Shell” chapter of the Cisco
IOS Security Configuration Guide located at:
http://www.cisco.com/en/US/docs/ios/12_2/security/configuration/guide/scfssh.html.
The Cisco ASR 1000 Series Routers also introduces persistent Telnet and persistent SSH. Persistent
Telnet and persistent SSH allow network administrators to more clearly define the treatment of incoming
traffic when users access the router through the Management Ethernet port using Telnet or SSH. Notably,
persistent Telnet and persistent SSH provide more robust network access by allowing the router to be
configured to be accessible through the Ethernet Management port using Telnet or SSH even when the
IOS process has failed.
Persistent Telnet and Persistent SSH Overview
In traditional Cisco routers, accessing the router using Telnet or SSH is not possible in the event of an
IOS failure. When Cisco IOS fails on a traditional Cisco router, the only method of accessing the router
is through the console port. Similarly, if all active IOS processes have failed on a Cisco ASR 1000 Series
Router that is not using persistent Telnet or persistent SSH, the only method of accessing the router is
through the console port.
With persistent Telnet and persistent SSH, however, users can configure a transport map that defines the
treatment of incoming Telnet or SSH traffic on the Management Ethernet interface. Among the many
configuration options, a transport map can be configured to direct all traffic to the IOS command-line
interface, diagnostic mode, or to wait for an IOS vty line to become available and then direct users into
diagnostic mode when the user sends a break signal while waiting for the IOS vty line to become
available. If a user uses Telnet or SSH to access diagnostic mode, that Telnet or SSH connection will be
usable even in scenarios when no IOS process is active. Therefore, persistent Telnet and persistent SSH
introduce the ability to access the router via diagnostic mode when the IOS process is not active. For
information on diagnostic mode, see the “Understanding the Diagnostic Mode” section on page 2-7.
See the “Configuring Persistent Telnet” section on page 3-5 and the “Configuring Persistent SSH”
section on page 3-8 for information on the various other options that are configurable using persistent
Telnet or persistent SSH transport maps.