This document provides an overview of how Generic Routing Encapsulation
(GRE) keepalives work.
Readers of this document should have knowledge of these topics:
The information in this document is based on these software and
The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.
For more information on document conventions, refer to the
Technical Tips Conventions.
The GRE keepalive feature enables the
keepalive interface command for tunnels, and allows
you to configure keepalives for point-to-point GRE tunnels. You can configure
keepalives with the keepalive command, and
optionally with its new extension.
GRE tunnels provide a method to encapsulate arbitrary packets inside a
transport protocol. They also offer an architecture designed to provide the
services required to implement any standard point-to-point encapsulation
scheme. Here are some of the advantages of GRE tunnels:
GRE tunnels provide multi-protocol local networks over a
GRE tunnels provide workarounds for networks that contain protocols
with limited hop counts.
GRE tunnels connect discontinuous sub-networks.
GRE tunnels allow VPNs across WANs.
However, in the current implementation of GRE tunnels, a configured
tunnel does not have the ability to bring down the line protocol of either
tunnel endpoint, if the far end is unreachable. Thus, the traffic sent from the
tunnel is black-holed, and it cannot follow alternative paths because the
tunnel always stays up.
This situation is true for tunnels that rely on static routes or on
routing protocols that aggregate routes to find a route to the tunnel
destination. It is also true in situations where the data in the control plane
follows a different path from the data in the data plane.
This section provides a functional description for the tunnel keepalive
mechanism with the help of an example. This section also lists the software
elements that this feature modifies, and discusses the impact on memory and
The tunnel keepalive mechanism enables, extends and implements an
interface-specific command for tunnel interfaces, and delivers the ability to
bring down the line protocol of a tunnel. For more information, see the
Commands and Configuration section.
The tunnel keepalive mechanism also addresses these additional
The tunnel keepalive mechanism functions even if the far tunnel
endpoint does not support keepalives.
The tunnel keepalive mechanism originates
The tunnel keepalive mechanism processes
The tunnel keepalive mechanism replies to keepalive packets of the
far end, even when the line protocol of the tunnel is
Here is an example of how the tunnel keepalive mechanism works (see
Figure 1 – Example for the Tunnel Keepalive
interface tunnel 0 interface tunnel 0
ip address 126.96.36.199 255.255.255.240 ip address 188.8.131.52 255.255.255.240
tunnel source 184.108.40.206 tunnel source 220.127.116.11
tunnel destination 18.104.22.168 tunnel destination 22.214.171.124
keepalive 5 4 keepalive 5 4
interface loopback 0 interface loopback 0
ip address 126.96.36.199 255.255.255.255 ip address 188.8.131.52 255.255.255.255
A keepalive packet that originates from A to B
�---outer IP header---' �---inner IP header---'
|IP | IP src | IP dst | GRE | IP | IP src | IP dst | GRE |
| |184.108.40.206|220.127.116.11|PT=IP| |18.104.22.168|22.214.171.124| PT=0|
GRE header GRE header
When you enable keepalives on the tunnel endpoint of Router A, the
router at every interval constructs the inner IP header. At the end of the
header, the router also appends a GRE header with a Protocol Type (PT) of 0,
and no other payload. The router then sends that packet through the tunnel,
which results in its encapsulation with the outer IP header, and a GRE header
with the PT of IP. The tunnel keepalive counter increments by one. If there is
a way to reach the far end tunnel endpoint, and the tunnel line protocol is not
down due to other reasons, the packet arrives on Router B. It is then matched
against Tunnel 0, is decapsulated, and forwarded to the destination IP, which
is the tunnel source, Router A. Upon arrival on Router A, the packet is again
decapsulated, and the PT is checked. If the result of the PT check is 0, it
signifies that this is a keepalive packet. In such a case, the tunnel keepalive
counter is reset to 0, and the packet is discarded.
In case Router B is unreachable, Router A continues to construct and
send the keepalive packets along with normal traffic. If the line protocol is
down, the keepalives do not come back to Router A. Therefore, the keepalive
counter continues to increase. The tunnel line protocol stays up only as long
as the tunnel keepalive counter remains zero, or less than a configured value.
If that condition is not true, the next time you attempt to send a keepalive to
Router B, the line protocol is brought down, as soon as the keepalive counter
reaches the configured keepalive value. In the up/down state, the tunnel does
not forward or process any traffic apart from the keepalive packets. For this
to work for keepalive packets only, the tunnel must be forward-and-receive
friendly. So the tunnel lookup algorithm must be successful in all cases, and
must discard only the data packets if the line protocol is down. When a
keepalive packet is received, it implies that the tunnel endpoint is again
reachable. The tunnel keepalive counter is then reset to 0, and the line
protocol comes back up.
The feature places almost no additional demand on the router system
memory and performance is expected to remain unaffected by its addition.
Keepalive packets are treated as ordinary packets, and so it is possible that
they can be dropped under high traffic conditions. For now, you can change the
number of retries to deal with this issue. If this proves to be inadequate
eventually, you can put locally generated keepalive packets in a high priority
queue for transmission. You can then set the TOS value in the IP headers to a
more suitable value, other than the default or configured value.
The feature is included in the basic IP tunnel code and in the GRE
subsystem. Therefore, it must be available with a basic IP package that has the
tunnel and the GRE subsystems.
This section addresses the keepalive command
enabled and extended by this feature only under Cisco bug ID CSCuk26449. Other
commands are documented in the respectiveCisco IOS Configuration
Guides and Command References. The [no] keepalive
<retries> command is enabled and
extended with a second parameter, and is available in Cisco IOS Software
Release 12.2(8)T and later. It has also been ported under Cisco bug ID
CSCuk29980 and CSCuk29983 to Cisco IOS Software Releases 12.1E and 12.2S.
As keepalive is an interface configuration
command that enables keepalives on the tunnel interface, only keepalives for
the GRE/IP mode are supported currently. The second parameter of the command
) is visible and
available only for tunnel interfaces. Values of the first parameter can range
from 1 to 32767. When the value is 0, it is equivalent to "no keepalive". This
parameter has a default value of 10. The values for the second parameter can
range from 1 to 255, and it indicates the number of keepalives that are sent
but not returned, after which the tunnel interface pulls the line protocol
down. Keepalives on tunnel interfaces are disabled by default.
This section provides sample outputs.
Enter configuration commands, one per line. End with CNTL/Z.
cisco-7505(config)#interface tunnel 1
access-expression Build a bridge boolean access expression
keepalive Enable keepalive <=====
timeout Define timeout values for this interface
cisco-7505(config-if)#keepalive ? <=====
<0-32767> Keepalive period (default 10 seconds)
cisco-7505(config-if)#keepalive 5 ? <=====
<1-255> Keepalive retries (default 3 times)
cisco-7505(config-if)#keepalive 5 4 <=====
cisco-7505#show interfaces tunnel 1
Tunnel1 is up, line protocol is up
Hardware is Tunnel
Internet address is 10.1.1.1/24
MTU 1514 bytes, BW 9 Kbit, DLY 500000 usec, rely 255/255, load 1/255
Encapsulation TUNNEL, loopback not set
Keepalive set (5 sec), retries 4 <=====
Tunnel source 126.96.36.199, destination 188.8.131.52
Tunnel protocol/transport GRE/IP, key disabled, sequencing disabled
Tunnel TOS 0xF, Tunnel TTL 128
Checksumming of packets disabled, fast tunneling enabled
Last input never, output 00:57:05, output hang never
Last clearing of "show interface" counters never
Queueing strategy: fifo
Output queue 0/0, 1 drops; input queue 0/75, 0 drops
30 second input rate 0 bits/sec, 0 packets/sec
30 second output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
3 packets output, 1860 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 output buffer failures, 0 output buffers swapped out