OTV Advance Functions

OTV Advance Functions

Posted on Jan 24, 2020 (0)

OTV Advance Functions

In this section we will discuss about other OTV function like STP isolation, Unknown Unicast traffic suppression, ARP optimization etc. We will discuss it one by one here.

STP Isolation:

OTV helps in STP isolation by not transmitting the BPDU across the Overlay. By this every site has its own STP domain and can use independent configuration.

But while doing, as soon as two OTV edge device is used chances of STP loop occurs. And also we will learn in details how STP loop is avoided in MultiHoming topology.

Unknown Unicast Handling:

OTV does not transport unknown unicast packet via Overlay. Unknown unicast packet can be flooded out to internal interface but later it can be controlled by STP.

When OTV edge device receives the unknown unicast frame, let suppose for destination MAC C, and while Layer 2 lookup it finds that there is no entry for MAC C on MAC table, Layer -2 traffic is flooded out to internal interface but on Overlay interface

ARP Optimization:

OTV performs the ARP optimization due to which the ARP flooded traffic can be reduced over Overlay.

  • Let’s suppose the Host A on Site -1 send ARP request for MAC of Host B having IP-AB.
  • As ARP is layer -2 broadcast, and is flooded across Internal interface and Overlay interface to all remote sites. Now Host B receives the ARP request and reply its MAC address MAC B and sent back this reply to Site-1 DC.
  • OTV edge device on Site-1 snoops that ARP reply and put its entry in its ARP cache with (IP-AB and MAC B) called ARP Neighbour discovery (ND) cache.
  • Now if any subsequent ARP request is sent for Host B, OTV edge device on Site-1 will reply on behalf of Host B and request is not forwarded.

Due to this feature, ARP table timer and CAM table timer must be correct else it can cause the black-holing traffic. Let’s suppose that ARP aging timer is larger than CAM table aging timer, and if this happens traffic drop occurs. Let’s understand by this below example:

Host A on Site-1 sends the ARP request for Host B on Site-2 to get its MAC address for layer 2 traffic.

  • ARP request is sent across all internal interface and Overlay interface and reaches to Site-2 and finally Host B replies it with ARP reply to Host A on site-1 which further snooped by OTV edge device on Site-1 and its information is stored in ARP cache.
  • Now Let’s suppose MAC AB stops communicating due to CAM table timer expires the entry of Host A “ MAC A” also expires and due to which OTV update is triggered and is sent to OTV edge device of Site-1 so that it can remove the entry of MAC AB on its MAC table . Now process does not do any changes in ARP table.
  • Now Host A on Site-1 sends the unicast frame to Host B for MAC AB on site-2.
  • Once Unicast packet is reached to OTV edge device on Site-1 which has valid ARP entry for the destination HOST B, but lookup for MAC AB in the CAM table will not hit and hence traffic will be dropped.

In order to prevent this situation, ARP aging timer should be set lower than CAM aging table. Here are default aging timer mentioned.

  • OTV ARP aging timer: 480 Seconds
  • OTV CAM table aging timer: 1800 Sec


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