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Webcast-Catalyst9k
New Member

Gre Keepalive packet size

Hi all,

what is the size of a GRE Keepalive Packet?

Thans

Rgds

Gianluca

3 REPLIES
Cisco Employee

Re: Gre Keepalive packet size

The entire IP packet is 48 bytes. (20 bytes IP header + 4 bytes GRE header + 20 bytes IP header + 4 bytes GRE header).

Frame 10 (62 bytes on wire, 62 bytes captured)

Arrival Time: May 15, 2006 08:44:53.966971000

Time delta from previous packet: 0.420042000 seconds

Time since reference or first frame: 13.029461000 seconds

Frame Number: 10

Packet Length: 62 bytes

Capture Length: 62 bytes

Ethernet II, Src: aa:bb:cc:00:02:00, Dst: aa:bb:cc:00:01:00

Destination: aa:bb:cc:00:01:00 (aa:bb:cc:00:01:00)

Source: aa:bb:cc:00:02:00 (aa:bb:cc:00:02:00)

Type: IP (0x0800)

Internet Protocol, Src Addr: 2.2.2.2 (2.2.2.2), Dst Addr: 1.1.1.1 (1.1.1.1)

Version: 4

Header length: 20 bytes

Differentiated Services Field: 0xc0 (DSCP 0x30: Class Selector 6; ECN: 0x00)

1100 00.. = Differentiated Services Codepoint: Class Selector 6 (0x30)

.... ..0. = ECN-Capable Transport (ECT): 0

.... ...0 = ECN-CE: 0

Total Length: 48

Identification: 0x0007 (7)

Flags: 0x00

.0.. = Don't fragment: Not set

..0. = More fragments: Not set

Fragment offset: 0

Time to live: 255

Protocol: GRE (0x2f)

Header checksum: 0xb4d2 (correct)

Source: 2.2.2.2 (2.2.2.2)

Destination: 1.1.1.1 (1.1.1.1)

Generic Routing Encapsulation (IP)

Flags and version: 0000

0... .... .... .... = No checksum

.0.. .... .... .... = No routing

..0. .... .... .... = No key

...0 .... .... .... = No sequence number

.... 0... .... .... = No strict source route

.... .000 .... .... = Recursion control: 0

.... .... 0000 0... = Flags: 0

.... .... .... .000 = Version: 0

Protocol Type: IP (0x0800)

Internet Protocol, Src Addr: 1.1.1.1 (1.1.1.1), Dst Addr: 2.2.2.2 (2.2.2.2)

Version: 4

Header length: 20 bytes

Differentiated Services Field: 0xc0 (DSCP 0x30: Class Selector 6; ECN: 0x00)

1100 00.. = Differentiated Services Codepoint: Class Selector 6 (0x30)

.... ..0. = ECN-Capable Transport (ECT): 0

.... ...0 = ECN-CE: 0

Total Length: 24

Identification: 0x0006 (6)

Flags: 0x00

.0.. = Don't fragment: Not set

..0. = More fragments: Not set

Fragment offset: 0

Time to live: 255

Protocol: GRE (0x2f)

Header checksum: 0xb4eb (correct)

Source: 1.1.1.1 (1.1.1.1)

Destination: 2.2.2.2 (2.2.2.2)

Generic Routing Encapsulation (0x0000 - unknown)

Flags and version: 0000

0... .... .... .... = No checksum

.0.. .... .... .... = No routing

..0. .... .... .... = No key

...0 .... .... .... = No sequence number

.... 0... .... .... = No strict source route

.... .000 .... .... = Recursion control: 0

.... .... 0000 0... = Flags: 0

.... .... .... .000 = Version: 0

Protocol Type: Unknown (0x0000)

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

Re: Gre Keepalive packet size

Thanks a lot for your answer.

So if I need to calcualte bandwidth consumption due to GRE keepalive, I have to calculate:

48B * 2 (answer too) = 96 Bytes,

for each keepalive sent.

Is it correct?

Rgds

Gianluca

Cisco Employee

Re: Gre Keepalive packet size

Actually, if you look at the information I provided, you can see that each hello contains its own response (a GRE message in a GRE message).

If I send a keepalive from 1.1.1.1 to 2.2.2.2 the internal GRE message is from 2.2.2.2 to 1.1.1.1, which is king of a bidirectional link testing. Once the entire packet is reach by the peer, the external IP and GRE headers are removed and the internal IP and GRE are forwarded back to the sender. This allows for the keepalive to be supported from one side only if required.

In this case the keepalive is 48 bytes and the response is 24 bytes.

Bear in mind that keepalives might be sent from both peers.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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