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Tunnel Route Selection

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Tunnel Route Selection

Table Of Contents

Tunnel Route Selection

Contents

Restrictions for Tunnel Route Selection

Information About Tunnel Route Selection

Tunnel Transport Behavior

How to Configure Tunnel Route Selection

Configuring Tunnel Route Selection

Configuration Examples for Tunnel Route Selection

Configuring Tunnel Route Selection: Example

Verifying and Troubleshooting Tunnel Route Selection: Examples

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference

debug tunnel route-via

show interfaces tunnel

tunnel route-via

Feature Information for Tunnel Route Selection


Tunnel Route Selection


First Published: November 17, 2006
Last Updated: November 17, 2006

The Tunnel Route Selection feature allows the tunnel transport to be routed using a subset of the routing table. When there are equal-cost routes to a tunnel destination, normal tunnel transport behavior is to use one of the available routes chosen at random. The Tunnel Route Selection feature allows the explicit configuration of the outgoing interface for the tunnel transport.

Finding Feature Information in This Module

Your Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for Tunnel Route Selection" section.

Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

Restrictions for Tunnel Route Selection

Information About Tunnel Route Selection

How to Configure Tunnel Route Selection

Configuration Examples for Tunnel Route Selection

Additional References

Command Reference

Feature Information for Tunnel Route Selection

Restrictions for Tunnel Route Selection

This feature is supported in the following tunnel modes only:

Generic Routing Encapsulation (GRE) IP

GRE Multipoint

IP in IP

Mobile User Datagram Protocol (UDP)

This feature is not supported on a tunnel when the tunnel transport is a GRE Multipoint tunnel.

Supported Configuration

interface tunnel 0
  tunnel mode gre multipoint
  tunnel route-via tunnel 1
interface tunnel 1
   tunnel mode gre ip

Unsupported Configuration

interface tunnel 0
  tunnel mode gre multipoint
  tunnel route-via tunnel 1
interface tunnel 1
  tunnel mode gre multipoint

Information About Tunnel Route Selection

To configure the Tunnel Route Selection feature, you should understand the following concept:

Tunnel Transport Behavior

Tunnel Transport Behavior

The Tunnel Route Selection feature allows the tunnel transport to be routed using a subset of the routing table by specifying the outgoing interface of the tunnel transport.

The Tunnel Route Selection feature is not the same as an implementation of policy-based routing for the tunnel transport. The Tunnel Route Selection feature will forward traffic using only a subset of the route table, and it cannot introduce routing loops into the network.

Figure 1 compares default tunnel behavior with the Tunnel Route Selection behavior.

Figure 1 Tunnel Route Selection Traffic

How to Configure Tunnel Route Selection

This section describes the following task required to configure the Tunnel Route Selection feature.

Configuring Tunnel Route Selection (required)

Configuring Tunnel Route Selection

Perform the following steps to specify the outgoing interface of the tunnel transport to route the tunnel transport using a subset of the routing table.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface tunnel interface-number

4. tunnel route-via interface-type interface-number {mandatory | preferred}

5. end

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

interface tunnel interface-number

Example:

Router(config)# interface tunnel 0

Configures a tunnel interface and enters interface configuration mode.

Step 4 

tunnel route-via interface-type interface-number {mandatory | preferred}

Example:

Router(config-if)# tunnel route-via ethernet0 mandatory

Specifies the outgoing interface to be used by the tunnel transport.

Step 5 

end

Example:

Router(config-if)# end

Returns to privileged EXEC mode.

Configuration Examples for Tunnel Route Selection

This section provides the following examples required to configure the Tunnel Route Selection feature.

Configuring Tunnel Route Selection: Example

Verifying and Troubleshooting Tunnel Route Selection: Examples

Configuring Tunnel Route Selection: Example

The following example shows Tunnel 0 configured to use Ethernet interface 0 as its preferred outgoing transport interface. Traffic that exits the router using the tunnel 0 interface will be sent out of Ethernet interface 0 if there is a route to the tunnel destination out of Ethernet interface 0. If there is no route out of Ethernet interface 0, the traffic will be forwarded as if the Tunnel Route Selection feature were not configured.

If the tunnel route-via interface-type interface-number mandatory command is configured, and there is no route to the tunnel destination using that interface, a point-to-point tunnel interface will go into a down state.

Router> enable
Router# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)# interface tunnel 0
Router(config-if)# tunnel route-via ethernet0 preferred
Router(config-if)# end
Router# show running-config interface tunnel 0
Building configuration...

Current configuration : 147 bytes
!
interface Tunnel0
 ip unnumbered Loopback0
 tunnel source Loopback0
 tunnel destination 10.73.0.102
 tunnel route-via Ethernet0 preferred
end

Verifying and Troubleshooting Tunnel Route Selection: Examples

To verify your configuration, use the show interfaces tunnel command in privileged EXEC mode. The following example shows that the tunnel transport is routed using a subset of the routing table by specifying the outgoing interface of the tunnel transport.

Router# show running-config interface tunnel 0

Building configuration...

Current configuration : 147 bytes
!
interface Tunnel0
 ip unnumbered Loopback0
 tunnel source Loopback0
 tunnel destination 10.73.0.102
 tunnel route-via Ethernet0 preferred
end

Router# show interfaces tunnel 0 | include route-via

  Tunnel route-via feature is on [Ethernet0, preferred]

To troubleshoot your configuration, use the debug tunnel route-via command in privileged EXEC mode. The following is sample output from the debug tunnel route-via command after the tunnel route-via command was used to route the tunnel transport explicitly using a subset of the routing table.

Router# debug tunnel route-via

Tunnel route-via debugging is on
Router#
*May 23 08:40:53.707: TUN-VIA: Tunnel0 candidate route-via Ethernet0/0, next hop 10.73.2.1
*May 23 08:40:53.707: TUN-VIA: Tunnel0 route-via action is forward
*May 23 08:41:03.719: TUN-VIA: Tunnel0 candidate route-via Ethernet0/0, next hop 10.73.2.1
*May 23 08:41:03.719: TUN-VIA: Tunnel0 route-via action is forward

Router# undebug tunnel route-via

Tunnel route-via debugging is off

Additional References

The following sections provide references related to the Tunnel Route Selection feature.

Related Documents

Related Topic
Document Title

Implementing tunnels

"Implementing Tunnels" chapter in the Cisco IOS Interface and Hardware Component Configuration Guide, Release 12.4T

Interface and hardware component commands: t1 through yellow

Cisco IOS Interface and Hardware Component Command Reference


Standards

Standard
Title

None


MIBs

MIB
MIBs Link

None

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs

RFC
Title

None


Technical Assistance

Description
Link

The Cisco Technical Support & Documentation website contains thousands of pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.

http://www.cisco.com/techsupport


Command Reference

This section documents new and modified commands only.

New Commands

debug tunnel route-via

tunnel route-via

Modified Command

show interfaces tunnel

debug tunnel route-via

To display debugging information about the tunnel transport using a subset of the route table, use the debug tunnel route-via command in privileged EXEC mode. To disable this feature, use the no form of this command.

debug tunnel route-via

no debug tunnel route-via

Syntax Description

This command has no arguments or keywords.

Command Modes

Privileged EXEC

Command History

Release
Modification

12.4(11)T

This command was introduced.


Examples

The following sample output of debug tunnel route-via command displays the outgoing interface for the tunnel transport.

Router# debug tunnel route-via

Tunnel route-via debugging is on

*May 22 11:54:34.803: TUN-VIA: Tunnel0 candidate route-via Ethernet0/0, next hop
 10.73.2.1
*May 22 11:54:34.803: TUN-VIA: Tunnel0 route-via action is forward

Router# no debug tunnel route-via

undebug tunnel route-via 
Tunnel route-via debugging is off

Related Commands

Command
Description

show interface tunnel

Displays information about the physical output tunnel interface.

tunnel route-via

Specifies the outgoing interface of the tunnel transport.


show interfaces tunnel

To display tunnel interface information, use the show interfaces tunnel command in privileged EXEC mode.

show interfaces tunnel number [accounting]

Syntax Description

number

Port line number.

accounting

(Optional) Displays the number of packets of each protocol type that have been sent through the interface.


Command Modes

Privileged EXEC

Command History

Release
Modification

10.0

This command was introduced.

12.2(14)S

This command was integrated into Cisco IOS Release 12.2(14)S.

12.2(28)SB

This command was integrated into Cisco IOS Release 12.2(28)SB.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

12.4(11)T

Support was added to display traffic information when the tunnel route-via command is present in the configuration file.


Examples

The following is sample output from the show interfaces tunnel command.

Router# show interfaces tunnel 4

Tunnel4 is up, line protocol is down
  Hardware is Routing Tunnel
  MTU 1500 bytes, BW 9 Kbit, DLY 500000 usec, rely 255/255, load 1/255
  Encapsulation TUNNEL, loopback not set, keepalive set (10 sec)
  Tunnel source 0.0.0.0, destination 0.0.0.0
  Tunnel protocol/transport GRE/IP, key disabled, sequencing disabled
  Last input never, output never, output hang never
  Last clearing of "show interface" counters never
  Output queue 0/0, 0 drops; input queue 0/75, 0 drops
  Five minute input rate 0 bits/sec, 0 packets/sec
  Five minute output rate 0 bits/sec, 0 packets/sec
     0 packets input, 0 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     0 packets output, 0 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets, 0 restarts

Router# show interfaces tunnel 0 | include route-via

Tunnel route-via feature is on [Ethernet0, preferred]

Router# show interfaces tunnel 0 | include route-via

Tunnel route-via feature is on [Ethernet0, mandatory]

Table 1 describes significant fields shown in the display.

Table 1 show interfaces tunnel Field Descriptions 

Field
Description

Tunnel is {up | down}

Interface is currently active and inserted into ring (up) or inactive and not inserted (down).

On the Cisco 7500 series routers, gives the interface processor type, slot number, and port number.

line protocol is {up | down | administratively down}

Shows line protocol up if a valid route is available to the tunnel destination. Shows line protocol down if no route is available or if the route would be recursive.

Hardware

Specifies the hardware type.

MTU

Maximum transmission unit of the interface.

BW

Bandwidth of the interface, in kilobits per second.

DLY

Delay of the interface, in microseconds.

rely

Reliability of the interface as a fraction of 255 (255/255 is 100 percent reliability), calculated as an exponential average over 5 minutes.

load

Load on the interface as a fraction of 255 (255/255 is completely saturated), calculated as an exponential average over 5 minutes.

Encapsulation

Encapsulation method is always TUNNEL for tunnels.

loopback

Indicates whether loopback is set or not.

keepalive

Indicates whether keepalives are set or not.

Tunnel source

IP address used as the source address for packets in the tunnel.

destination

IP address of the host destination.

Tunnel protocol

Tunnel transport protocol (the protocol that the tunnel is using). This is based on the tunnel mode command, which defaults to GRE.

key

ID key for the tunnel interface, unless disabled.

sequencing

Indicates whether the tunnel interface drops datagrams that arrive out of order. Can be disabled.

Last input

Number of hours, minutes, and seconds since the last packet was successfully received by an interface and processed locally on the router. Useful for knowing when a dead interface failed. This counter is updated only when packets are process-switched, not when packets are fast-switched.

output

Number of hours, minutes, and seconds since the last packet was successfully transmitted by an interface. This counter is updated only when packets are process-switched, not when packets are fast-switched.

output hang

Number of hours, minutes, and seconds (or never) since the interface was last reset because of a transmission that took too long. When the number of hours in any of the "last" fields exceeds 24 hours, the number of days and hours is printed. If that field overflows, asterisks are printed.

Last clearing

Time at which the counters that measure cumulative statistics (such as number of bytes transmitted and received) shown in this report were last reset to zero. Note that variables that might affect routing (for example, load and reliability) are not cleared when the counters are cleared.

*** indicates the elapsed time is too large to be displayed.

0:00:00 indicates that the counters were cleared more than 231 ms (and less than 232 ms) ago.

Output queue, drops
Input queue, drops

Number of packets in output and input queues. Each number is followed by a slash, the maximum size of the queue, and the number of packets dropped because of a full queue.

Five minute input rate,
Five minute output rate

Average number of bits and packets transmitted per second in the last 5 minutes.

The 5-minute input and output rates should be used only as an approximation of traffic per second during a given 5-minute period. These rates are exponentially weighted averages with a time constant of 5 minutes. A period of four time constants must pass before the average will be within two percent of the instantaneous rate of a uniform stream of traffic over that period.

packets input

Total number of error-free packets received by the system.

bytes

Total number of bytes, including data and MAC encapsulation, in the error-free packets received by the system.

no buffer

Number of received packets discarded because there was no buffer space in the main system. Compare with ignored count. Broadcast storms on Ethernet networks and bursts of noise on serial lines are often responsible for no input buffer events.

broadcasts

Total number of broadcast or multicast packets received by the interface.

runts

Number of packets that are discarded because they are smaller than the minimum packet size of them medium.

giants

Number of packets that are discarded because they exceed the maximum packet size of the medium.

CRC

Number of cyclic redundancy checksums generated by the originating LAN station or far-end device that do not match the checksum calculated from the data received. On a LAN, this usually indicates noise or transmission problems on the LAN interface or the LAN bus itself. A high number of CRCs is usually the result of a station transmitting bad data.

frame

Number of packets received incorrectly having a CRC error and a noninteger number of octets.

overrun

Number of times the serial receiver hardware was unable to hand received data to a hardware buffer because the input rate exceeded the receiver's ability to handle the data.

ignored

Number of received packets ignored by the interface because the interface hardware ran low on internal buffers. These buffers are different than the system buffers mentioned previously in the buffer description. Broadcast storms and bursts of noise can cause the ignored count to be increased.

abort

Illegal sequence of one bits on a serial interface. This usually indicates a clocking problem between the serial interface and the data link equipment.

packets output

Total number of messages transmitted by the system.

bytes

Total number of bytes, including data and MAC encapsulation, transmitted by the system.

underruns

Number of times that the far-end transmitter has been running faster than the near-end router's receiver can handle. This may never be reported on some interfaces.

output errors

Sum of all errors that prevented the final transmission of datagrams out of the interface being examined. Note that this may not balance with the sum of the enumerated output errors, because some datagrams may have more than one error, and others may have errors that do not fall into any of the specifically tabulated categories.

collisions

Number of messages retransmitted because of an Ethernet collision. Some collisions are normal. However, if your collision rate climbs to around 4 or 5 percent, you should consider verifying that there is no faulty equipment on the segment and/or moving some existing stations to a new segment. A packet that collides is counted only once in output packets.

interface resets

Number of times an interface has been reset. The interface may be reset by the administrator or automatically when an internal error occurs.

restarts

Number of times that the controller was restarted because of errors.

preferred

If the route is not available, forwards the traffic using any available route.

mandatory

Drops the traffic if the route is not available.


Related Commands

Command
Description

show interfaces

Displays statistics for all interfaces configured on the router or access server.

show ip route

Displays the current state of the routing table.


tunnel route-via

To specify the outgoing interface of the tunnel transport, use the tunnel route-via command in interface configuration mode. To disable the source address selection, use the no form of this command.

tunnel route-via interface-type interface-number {mandatory | preferred}

no tunnel route-via

Syntax Description

interface-type

Indicates the type of interface.

interface-number

Indicates the interface number of the interface configured as the tunnel transport.

mandatory

Drops the traffic if the route is not available.

preferred

If the route is not available, forwards the traffic using any available route.


Command Default

This command is disabled by default. The tunnel transport cannot be routed using a subset of the routing table.

Command Modes

Interface configuration

Command History

Release
Modification

12.4(11)T

This command was introduced.


Usage Guidelines

If the tunnel route-via interface-type interface-number mandatory command is configured, and there is no route to the tunnel destination using that interface, a point-to-point tunnel interface will go into a down state.

Examples

The following example shows the options that are available to configure the interfaces of the tunnel transport and route the tunnel transport using a subset of the routing table:

Router> enable
Router# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)# interface tunnel 0
Router(config-if)# tunnel route-via ethernet0 mandatory

Related Commands

Command
Description

debug tunnel route-via

Displays information about the source address selection.

show interfaces tunnel

Displays information about the physical output tunnel interface.


Feature Information for Tunnel Route Selection

Table 2 lists the release history for this feature.

Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.


Note Table 2 lists only the Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release train. Unless noted otherwise, subsequent releases of that Cisco IOS software release train also support that feature.


Table 2 Feature Information for Tunnel Route Selection 

Feature Name
Releases
Feature Information

Tunnel Route Selection

12.4(11)T

The Tunnel Route Selection feature allows the tunnel transport to be routed using a subset of the routing table. When there are equal-cost routes to a tunnel destination, normal tunnel transport behavior is to use one of the available routes chosen at random. The Tunnel Route Selection feature allows the explicit configuration of the outgoing interface for the tunnel transport.