[c-nsp] understanding BFD "echo mode"
Martin T
m4rtntns at gmail.com
Mon Mar 3 10:07:27 EST 2014
Dimitris,
thank you for confirming this. I managed to find two Cisco routers
supporting BFD in "echo mode" and connected those two directly with each
other:
R1#sh bfd neighbors details
OurAddr NeighAddr LD/RD RH/RS Holddown(mult) State Int
10.10.10.1 10.10.10.2 1/12 Up 0 (3 ) Up Fa0/0
Session state is UP and using echo function with 300 ms interval.
Local Diag: 0, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3
Received MinRxInt: 1000000, Received Multiplier: 3
Holddown (hits): 0(2), Hello (hits): 1000(46642)
Rx Count: 26, Rx Interval (ms) min/max/avg: 1/1000/814 last: 848 ms ago
Tx Count: 27, Tx Interval (ms) min/max/avg: 1/984/809 last: 268 ms ago
Elapsed time watermarks: 2 2 (last: 2)
Registered protocols: BGP
Uptime: 13:20:32
Last packet: Version: 1 - Diagnostic: 0
State bit: Up - Demand bit: 0
Poll bit: 0 - Final bit: 0
Multiplier: 3 - Length: 24
My Discr.: 12 - Your Discr.: 1
Min tx interval: 1000000 - Min rx interval: 1000000
Min Echo interval: 300000
R1#
R2#sh bfd neighbors details
OurAddr NeighAddr LD/RD RH/RS Holddown(mult) State Int
10.10.10.2 10.10.10.1 12/1 Up 0 (3 ) Up Fa0
Session state is UP and using echo function with 300 ms interval.
Local Diag: 0, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3
Received MinRxInt: 1000000, Received Multiplier: 3
Holddown (hits): 0(0), Hello (hits): 1000(43759)
Rx Count: 97, Rx Interval (ms) min/max/avg: 1/1000/869 last: 512 ms ago
Tx Count: 97, Tx Interval (ms) min/max/avg: 1/1000/869 last: 556 ms ago
Elapsed time watermarks: -1 0 (last: 0)
Registered protocols: BGP
Uptime: 13:21:35
Last packet: Version: 1 - Diagnostic: 0
State bit: Up - Demand bit: 0
Poll bit: 0 - Final bit: 0
Multiplier: 3 - Length: 24
My Discr.: 1 - Your Discr.: 12
Min tx interval: 1000000 - Min rx interval: 1000000
Min Echo interval: 300000
R2#
As you said, both BFD echo and BFD control messages are sent simultaneously
if BFD "echo mode" is enabled. Echo messages(to UDP port 3785) are sent
with the rate specified under "bfd interval" configuration and control
messages(to UDP port 3784) are sent with 1000ms interval, which can be even
further increased with "bfd slow-timers" under global configuration. Just
for reference, I captured some BFD echo messages:
01:21:57.166684 00:07:0e:11:a4:b4 > 00:16:46:8a:18:08, ethertype IPv4
(0x0800), length 60: 10.10.10.2.49152 > 10.10.10.2.3785: BFD, Echo, length:
12
01:21:57.166694 00:16:46:8a:18:08 > 00:07:0e:11:a4:b4, ethertype IPv4
(0x0800), length 60: 10.10.10.2.49152 > 10.10.10.2.3785: BFD, Echo, length:
12
01:21:57.209661 00:16:46:8a:18:08 > 00:07:0e:11:a4:b4, ethertype IPv4
(0x0800), length 60: 10.10.10.1.49152 > 10.10.10.1.3785: BFD, Echo, length:
12
01:21:57.209673 00:07:0e:11:a4:b4 > 00:16:46:8a:18:08, ethertype IPv4
(0x0800), length 60: 10.10.10.1.49152 > 10.10.10.1.3785: BFD, Echo, length:
12
Rest can be seen here: http://www.cloudshark.org/captures/ada0ef51d5f5
I have never seen a protocol which sends packets to remote machine(based on
dst MAC address), but uses its own IP address as a destination IP.
Waris,
I did not have specific platform in mind. I just read the Cisco
documentation regarding BFD and didn't fully understand the "echo mode".
regards,
Martin
On Sat, Feb 15, 2014 at 6:13 AM, Waris Sagheer (waris) <waris at cisco.com>wrote:
> Martin,
> Do you have specific platform in mind?
> Hardware BFD offload implementation may be different on platforms.
>
> Best Regards,
>
> *Waris Sagheer*
> Technical Marketing Manager
> Service Provider Access Group (SPAG)
> waris at cisco.com
> Phone: *+1 408 853 6682 <%2B1%20408%20853%206682>*
> Mobile: *+1 408 835 1389 <%2B1%20408%20835%201389>*
>
> CCIE - 19901
>
>
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> From: Dimitris Befas <dimitris.befas at gmail.com>
> Date: Friday, February 7, 2014 at 7:03 AM
> To: 'Martin T' <m4rtntns at gmail.com>, "cisco-nsp at puck.nether.net" <
> cisco-nsp at puck.nether.net>
> Subject: Re: [c-nsp] understanding BFD "echo mode"
>
> Hi Martin,
>
> Exactly. Both modes (control & echo) are used when using the echo mode.
> The
> hardware handled echo packets are not eligible to remote cpu fluctuations
> because of the fact that we use the actual remote forwarding router
> mechanism. So you have more reliable and fast failure detection with echo
> mode.
> Farther, because with echo mode you actually use control (or asycnhronous)
> mode also, you may configure the slow timer to slow down the probable
> reaction of the control bfd packets and the cpu load that these packets
> intoduce.
>
>
> http://www.cisco.com/en/US/docs/switches/datacenter/sw/6_x/nx-os/interfaces/
> configuration/guide/if_bfd.pdf
> page 3
>
> Dimitris
>
> -----Original Message-----
> From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net<cisco-nsp-bounces at puck.nether.net>]
> On Behalf Of
> Martin T
> Sent: Thursday, February 6, 2014 8:46 PM
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] understanding BFD "echo mode"
>
> Hi,
>
> some Cisco routers support BFD in "echo mode". Am I correct that BFD
> "echo
> packets" are send besides BFD "control messages" once "echo mode" is
> enabled
> and Cisco routers are able to handle former in hardware while BFD "control
> messages" are punted?
>
>
>
> regards,
> Martin
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