This document explains what GRE keepalives are and how they work. The
GRE tunnel keepalives are not supported in conjunction with the
tunnel protection ipsec profile command. This
document discusses this issue.
GRE keepalives are not supported together with IPSec tunnel
protection under any circumstances.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
For more information on document conventions, refer to the
Technical Tips Conventions.
GRE tunnels are designed to be completely stateless. This means that
each tunnel end-point does not keep any information about the state or
availability of the remote tunnel end-point. A consequence of this is that the
local tunnel end-point router does not have the ability to bring the line
protocol of the GRE Tunnel interface down if the remote end of the tunnel is
unreachable. The ability to mark an interface as down when the remote end of
the link is not available is used in order to remove any routes (specifically
static routes) in the routing table that use that interface as the outbound
interface. Specifically, if the line protocol for an interface is changed to
down, then any static routes that point out that interface are removed from the
routing table. This allows for the installation of an alternate (floating)
static route or for Policy Based Routing (PBR) to select an alternate next-hop
Normally, a GRE Tunnel interface comes up as soon as it is configured
and it stays up as long as there is a valid tunnel source address or interface
which is up. The tunnel destination IP address must also be routable. This is
true even if the other side of the tunnel has not been configured. This means
that a static route or PBR forwarding of packets via the GRE tunnel interface
remains in effect even though the GRE tunnel packets do not reach the other end
of the tunnel.
Before GRE keepalives were implemented, there were only three reasons
for a GRE tunnel to shut down:
There is no route to the tunnel destination address.
The interface that anchors the tunnel source is down.
The route to the tunnel destination address is through the tunnel
These three rules (missing route, interface down and mis-routed tunnel
destination) are problems local to the router at the tunnel endpoints and do
not cover problems in the intervening network. For example, these rules do not
cover the case in which the GRE tunneled packets are successfully forwarded,
but are lost before they reach the other end of the tunnel. This causes data
packets that go through the GRE tunnel to be "black holed", even though an
alternate route that uses PBR or a floating static route via another interface
is potentially available. Keepalives on the GRE tunnel interface are used in
order to solve this issue in the same way as keepalives are used on physical
With Cisco IOS® Software Release 12.2(8)T, it is possible to configure
keepalives on a point-to-point GRE tunnel interface. With this change, the
tunnel interface dynamically shuts down if the keepalives fail for a certain
period of time. In order to better understand how GRE tunnel keepalives work,
these sections discuss some other common keepalive mechanisms.
Keepalive messages are sent by one network device via a physical or
virtual circuit to inform another network device that the circuit between them
still functions. The keepalive interval is the period of time between each
keepalive message that is sent by a network device. The keepalive retries is
the number of times that the device continues to send keepalive packets without
response before the interface is brought down.
On broadcast media like an Ethernet, keepalives are slightly different.
Since there are many possible neighbors on the Ethernet, the keepalive is not
designed to determine if the path to any one particular neighbor on the wire is
available. It is only designed to check that the local system has read and
write access to the Ethernet wire itself. The router produces an Ethernet
packet with itself as the source and destination MAC-address and a special
Ethernet type code of 0x9000. The Ethernet hardware sends this packet onto the
Ethernet wire and then immediately receives this packet back again. This checks
the sending and receiving hardware on the Ethernet adapter and the basic
integrity of the wire.
Another well known keepalive mechanism is serial keepalives for HDLC.
Serial keepalives are sent back and forth between two routers and the
keepalives are acknowledged. With the use of sequence numbers, each router
keeps track of the keepalive packets sent and acknowledged. In this way, the
remote routers look at each others keepalives and track if the keepalives they
send are received.
As an illustration of how serial keepalives work, Router 1 and Router 2
are directly connected via Serial0/0 and Serial2/0 respectively. In the
Router 1 output, Serial 0/0 is shut down purposely.
This causes Router 2 to miss three keepalives in order to illustrate how this
failure causes Router 2 to shut down Serial2/0 when keepalives are missed.
This is sample output from the debug serial
interface command for an HDLC connection when keepalives are
enabled. When the difference in the values in the myseq and mineseen fields
exceeds 3 on Router 2, the line goes down and the interface is reset.
17:21:09.685: Serial0/0: HDLC myseq 0, mineseen 0*, yourseen 1, line up
17:21:19.725: Serial0/0: HDLC myseq 1, mineseen 1*, yourseen 2, line up
17:21:29.753: Serial0/0: HDLC myseq 2, mineseen 2*, yourseen 3, line up
17:21:39.773: Serial0/0: HDLC myseq 3, mineseen 3*, yourseen 4, line up
17:21:49.805: Serial0/0: HDLC myseq 4, mineseen 4*, yourseen 5, line up
17:21:59.837: Serial0/0: HDLC myseq 5, mineseen 5*, yourseen 6, line up
17:22:09.865: Serial0/0: HDLC myseq 6, mineseen 6*, yourseen 7, line up
17:22:19.905: Serial0/0: HDLC myseq 7, mineseen 7*, yourseen 8, line up
17:22:29.945: Serial0/0: HDLC myseq 8, mineseen 8*, yourseen 9, line up
17:22:39.965: Serial0/0: HDLC myseq 9, mineseen 9*, yourseen 10, line up
17:22:42.225: %LINK-5-CHANGED: Interface Serial0/0, changed state
to administratively down
17:22:43.245: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0,
changed state to down
*Sep 24 17:21:04.929: Serial2/0: HDLC myseq 0, mineseen 0, yourseen 0, line up
*Sep 24 17:21:14.941: Serial2/0: HDLC myseq 1, mineseen 1*, yourseen 1, line up
*Sep 24 17:21:24.961: Serial2/0: HDLC myseq 2, mineseen 2*, yourseen 2, line up
*Sep 24 17:21:34.981: Serial2/0: HDLC myseq 3, mineseen 3*, yourseen 3, line up
*Sep 24 17:21:45.001: Serial2/0: HDLC myseq 4, mineseen 4*, yourseen 4, line up
*Sep 24 17:21:55.021: Serial2/0: HDLC myseq 5, mineseen 5*, yourseen 5, line up
*Sep 24 17:22:05.041: Serial2/0: HDLC myseq 6, mineseen 6*, yourseen 6, line up
*Sep 24 17:22:15.061: Serial2/0: HDLC myseq 7, mineseen 7*, yourseen 7, line up
*Sep 24 17:22:25.081: Serial2/0: HDLC myseq 8, mineseen 8*, yourseen 8, line up
*Sep 24 17:22:35.101: Serial2/0: HDLC myseq 9, mineseen 9*, yourseen 9, line up
*Sep 24 17:22:45.113: Serial2/0: HDLC myseq 10, mineseen 10*, yourseen 10, line up
*Sep 24 17:22:55.133: Serial2/0: HDLC myseq 11, mineseen 10, yourseen 10, line up
*Sep 24 17:23:05.153: HD(0): Reset from 0x203758
*Sep 24 17:23:05.153: HD(0): Asserting DTR
*Sep 24 17:23:05.153: HD(0): Asserting DTR and RTS
*Sep 24 17:23:05.153: Serial2/0: HDLC myseq 12, mineseen 10, yourseen 10, line up
*Sep 24 17:23:15.173: HD(0): Reset from 0x203758
*Sep 24 17:23:15.173: HD(0): Asserting DTR
*Sep 24 17:23:15.173: HD(0): Asserting DTR and RTS
*Sep 24 17:23:15.173: Serial2/0: HDLC myseq 13, mineseen 10, yourseen 10, line down
17:23:16.201: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial2/0,
changed state to down
17:23:25.193: Serial2/0: HDLC myseq 14, mineseen 10, yourseen 10, line down
The GRE tunnel keepalive mechanism is slightly different than for
Ethernet or serial interfaces. It gives the ability for one side to originate
and receive keepalive packets to and from a remote router even if the remote
router does not support GRE keepalives. Since GRE is a packet tunneling
mechanism for tunneling IP inside IP, a GRE IP tunnel packet can be built
inside another GRE IP tunnel packet. For GRE keepalives, the sender pre-builds
the keepalive response packet inside the original keepalive request packet so
that the remote end only needs to do standard GRE decapsulation of the outer
GRE IP header and then forward the inner IP GRE packet. This mechanism causes
the keepalive response to forward out the physical interface rather than the
tunnel interface. This means that the GRE keepalive response packet is not
affected by any output features on the tunnel interface, such as 'tunnel
protection ...', QoS, and so forth. ).
Note: If an inbound ACL on the GRE tunnel interface is configured, then the
GRE tunnel keepalive packet that the opposite device sends must be permitted.
If not, the opposite device's GRE tunnel will be down.
(access-list <number> permit gre host
<tunnel-source> host <tunnel-destination>)
Another attribute of GRE tunnel keepalives is that the keepalive timers
on each side are independent and do not have to match. The problem with the
configuration of keepalives only on one side of the tunnel is that only the
router that has keepalives configured marks its tunnel interface as down if the
keepalive timer expires. The GRE tunnel interface on the other side, where
keepalives are not configured, remains up even if the other side of the tunnel
is down. The tunnel can become a black-hole for packets directed into the
tunnel from the side that did not have keepalives configured. In a large
hub-and-spoke GRE tunnel network, it might be appropriate to only configure GRE
keepalives on the spoke side and not on the hub side. This is because it is
often more important for the spoke to discover that the hub is unreachable and
therefore switch to a backup path (Dial Backup for example).
A GRE tunnel is a logical interface on a Cisco router that provides a
way to encapsulate passenger packets inside a transport protocol. It is an
architecture designed to provide the services to implement a point-to-point
These packets illustrate the IP tunneling concepts where GRE is the
encapsulation protocol and IP is the transport protocol. The passenger protocol
is also IP (although it can be another protocol like Decnet, IPX or Appletalk).
IP is the transport protocol.
GRE is the encapsulation protocol.
IP is the passenger protocol.
In order to better understand how the tunnel keepalive mechanism works,
consider this example tunnel topology and configuration. The physical
interfaces of Router A and Router B are S1 and S2 respectively and the tunnel
interfaces are called Tu0. There is an IP backbone network in between the two
GRE tunnel endpoint routers.
This is an example of a keepalive packet that originates from Router A
and is destined for Router B. The keepalive response that Router B returns to
Router A is already inside the Inner IP Header. Router B simply decapsulates
the keepalive packet and sends it back out the physical interface (S2). It
processes the GRE keepalive packet just like any other GRE IP data packet.
This output shows the commands you use in order to configure keepalives
on GRE tunnels.
Router# configure terminal
Router(config-if)#keepalive 5 4
!--- The syntax of this command is keepalive [seconds [retries]].
!--- Keepalives are sent every 5 seconds and 4 retries.
!--- Keepalives must be missed before the tunnel is shut down.
!--- The default values are 10 seconds for the interval and 3 retries.
Note: GRE tunnel keepalives are only supported on point-to-point GRE
tunnels. Tunnel keepalives are configurable on multipoint GRE (mGRE) tunnels
but have no effect.
This table shows the configuration of Router A and Router B with
tunnel keepalive configured on both routers.
interface loopback 0
ip address 126.96.36.199 255.255.255.255
interface tunnel 0
ip address 188.8.131.52 255.255.255.240
tunnel source loopback0
tunnel destination 184.108.40.206
keepalive 5 4
interface loopback 0
ip address 220.127.116.11 255.255.255.255
interface tunnel 0
ip address 18.104.22.168 255.255.255.240
tunnel source loopback0
tunnel destination 22.214.171.124
keepalive 5 4
When you enable keepalives on the tunnel endpoint of Router A, the
router at every <period> interval constructs the inner IP header and GRE
header with a Protocol Type (PT) of 0. It then sends that packet out its tunnel
interface, which results in the encapsulation of the packet with the outer IP
header and a GRE header with PT = IP. Router A increments the tunnel keepalive
counter by one. With the assumption that 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, becomes
decapsulated, and is forwarded to the destination IP which is the tunnel source
IP address on Router A. Upon arrival on Router A, the packet becomes
decapsulated and the check of the PT results in 0. This signifies that this is
a keepalive packet. The tunnel keepalive counter is then reset to 0 and the
packet is discarded.
In the case when Router B is unreachable, Router A continues to
construct and send keepalive packets as well as normal traffic. If the
keepalives do not come back, the tunnel line protocol stays up as long as the
tunnel keepalive counter is less than the number of <retries>. If that
condition is not true, then the next time Router A attempts to send a keepalive
to Router B, the line protocol is brought down.
In the up/down state, the tunnel does not forward or process any data
traffic. However, it does continue to send keepalive packets. On the reception
of a keepalive response, with the implication that the tunnel endpoint is again
reachable, the tunnel keepalive counter is reset to 0 and the line protocol on
the tunnel comes up.
Note: If due to a keepalive drop the tunnel goes to up/down state, enable
debug tunnel and debug tunnel
This table shows the sample output of debug tunnel
keepalive on Router A:
debug tunnel keepalive
Tunnel keepalive debugging is on
*Mar 1 01:19:16.019: Tunnel0: sending keepalive, 126.96.36.199->188.8.131.52 (len=24 ttl=0), counter=15
*Mar 1 01:19:21.019: Tunnel0: sending keepalive, 184.108.40.206->220.127.116.11 (len=24 ttl=0), counter=16
*Mar 1 01:19:26.019: Tunnel0: sending keepalive, 18.104.22.168->22.214.171.124 (len=24 ttl=0), counter=17
This diagram shows a sample scenario of what happens when Router A
sends a GRE keepalive to Router B:
This diagram shows a sample scenario of what happens when Router B
sends a GRE keepalive to Router A:
GRE tunnels are sometimes combined with IPSec because IPSec does not
support IP multicast packets. This prevents dynamic routing protocols from
running successfully over an IPSec VPN network. Since GRE tunnels do support IP
multicast, a dynamic routing protocol can be run over a GRE tunnel. Then, you
can encrypt the GRE IP unicast packets by IPSec.
There are two different ways that IPSec can encrypt GRE packets. One
way is with the use of a crypto-map and the other is
to use tunnel protection. Both methods specify that
IPSec encryption is performed after the addition of the GRE encapsulation. When
a crypto-map is used, it is applied to the outbound physical interface(s) for
the GRE tunnel packets. When tunnel protection is
used, it is configured on the GRE tunnel interface. The tunnel
protection command became available in Cisco IOS Software Release
Note: GRE keepalives are not compatible with tunnel protection.
This diagram shows encrypted packets that come into a router via a GRE
tunnel interface. The router has a crypto-map applied on the physical
interface. Once the packets are de-crypted and de-encapsulated, they continue
on to their IP destination as clear-text.
This diagram shows what happens when you configure tunnel
protection on the GRE tunnel interface. The packets come into the
router encrypted via the tunnel interface and are de-crypted and de-capsulated
before they continue towards their destination as clear-text.
There are two key differences between when you use a crypto-map and
when you use tunnel protection:
The IPSec crypto-map is tied to the physical interface and is checked
as packets are forwarded out the physical interface.
Note: The GRE tunnel has already GRE encapsulated the packet by this
Tunnel protection ties the encryption functionality to the GRE tunnel
and is checked after the packet is GRE encapsulated but before the packet is
handed to the physical interface.
A problem occurs if a crypto map is used (configured on the physical
interface) on one side of a GRE IPSec Tunnel and Tunnel Protection is
configured on the other side. Tunnel protection is configured on the tunnel
interface only (not the physical). This type of configuration might be done in
a hub and spoke design. Tunnel protection is configured on the hub router in
order to reduce the size of the configuration and a static crypto map is used
on each spoke. When you configure GRE tunnel keepalive on the spoke in this
scenario, the keepalive fails. This is because the return keepalive from the
HUB traverses the physical interface which has no crypto-map configured.
Therefore, the return keepalive does not get encrypted and the receiving router
(which originally sent the keepalive packet) drops the keepalive response
because it was not IPSec protected (encrypted).
This diagram shows an illustration of the problem:
You can avoid this problem and there can be a benefit if GRE keepalive
is configured on the router where tunnel protection is configured. This point
is illustrated in this diagram:
If the hub uses dynamic crypto-maps and the spoke router uses tunnel
protection, then you can configure GRE keepalives on the spoke router so you
can to trigger a backup interface to dial the Hub if the tunnel interface shuts
down on the spoke.
Although the GRE tunnel interface on the hub remains up, the routing
neighbor and the routes through the tunnel are lost and the alternate route can
be established. On the spoke, the fact that the tunnel interface went down can
trigger it to bring up a dialer interface and call back to the hub (or another
router at the hub), then establish a new connection.
If both the routers are configured with crypto-maps, the tunnel
keepalives can get through in both directions and the GRE tunnel interfaces can
shut down in either or both directions, and trigger a backup connection to be
made. This is the most flexible option.
If both routers are configured with tunnel protection then GRE tunnel
keeaplives cannot be used in either direction. In this case you must use the
routing protocol or other mechanism, such as the Service Assurance Agent, to
discover that the tunnel is not functional.