Question

BFD protecting OSPF between Extreme and Cisco

  • 9 January 2019
  • 2 replies
  • 409 views

Hello, I just wanted to post this sample configuration to see if this is how I would get BFD to protect an OSPF route on a particular interface between EXOS and Cisco IOS.

What I am after is a 300ms timer... not too short to cause false positives but not too long to cause delay in detection.

EXOS side on Extreme X690 running core liscence - VLAN specified below is the transport vlan on an interface to an ethrenet handoff device provided by the ISP:
configure ospf vlan WAN-WS400 bfd on
configure bfd vlan WAN-WS400 detection-multiplier 3 receive-interval 300 transmit-interval 300
enable bfd vlan WAN-WS400


Cisco IOS side on a 2901 router specifying BFD directly on the external WAN interface that faces the ISP where the X690 is connected to in another city:

interface GigabitEthernet0/2.400
ip ospf bfd
bfd interval 300 min_rx 300 multiplier 3

My expectation here is that BFD hellos would have to not be exchanged for 900 (300 times 3) and if so it would inform the OSPF process and tear down that adjacency. I have a backup link at this site on with OSPF on it which is a tunnel interface via AT&T LTE. At the HQ it terminates into another Cisco 2901 which participates OSPF to the X690.

In testing today if I disconnect the WAN link, it takes about 40 seconds for the path to re-converge over the LTE backup. I'd like to shorten this down to only a few seconds if possible. Its imperative that there are no false positives, as the latency over the LTE link is terrible (around 350ms). Because of this I only think I need to use BFD protection on the primary link. A ping right now to a server at that location is showing 1ms.

Please let me know if I am missing anything or misunderstanding this feature. Is 300ms ok? I found a gtac article recommending 1000ms, but then another article stating 300 and up work (for example setting to 200 caused bfd down events).

Thanks in advance!

2 replies

Userlevel 2
I am in a similar situation. I'm wondering how things went for you? I was thinking about 700ms for the timers. Kind of the middle between the 300 and 1000.
Userlevel 7
Hi Keith,

did you verify if the BFD session is actually active?

  1. show bfd
  2. show bfd counters
  3. show bfd session client ospf
There are more verification commands starting with show bfd if you need to dig deeper.

BFD is a bit strange in that a session that is never established does not count as a failure for failover purposes. This allow preconfiguring it on one side, it will only become active if both sides use it.

The 40s failover time looks more like default adjacency loss after exceeding the dead interval.

It might help in some situations, i.e., if the interface goes down and there is only one other OSPF router connected to the link, if you use point-to-point interfaces. On EXOS this is done with configure ospf add vlan NAME area AREA link-type point-to-point. On Cisco IOS you use ip ospf network point-to-point in interface configuration mode. But BFD is a more general mechanism.

Thanks,
Erik

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