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OTV Data Traffic

OTV Data Traffic

Posted on Jan 24, 2020 (0)

OTV Data Traffic

In this section we will learn how Data traffic (Unicast traffic and Multicast traffic) is delivered over OTV overlay tunnel.

Unicast Traffic:

Let’s suppose that Host A in Site-A wants to talk to Host C in Site 2, and Control Plane MAC reachability information is already communicated and adjacency is from between OTV Sites.

  • Host A sends the ethernet Layer-2 packet for destination Host C , This layer 2 frame is local switched and reaches to OTV edge device where it lookup for MAC table and finds that HOST C is reachable on IP-B.
  • The Layer-2 Packet is sent to overlay interface where it is encapsulated to IP packet with Source IP is IP address of its Join Interface and Destination IP is IP address of Join interface of Site-2 OTV edge Device which is IP-B.
  • Now Layer 3 packet is forwarded to join interface and which is further routed to layer-3 transport infrastructure.
  • Site-2 receives the IP packet and Layer -3 packet is decapsulated and then layer-2 Lookup is done for destination MAC address MAC C.
  • OTV edge device find the egress interface where MAC C is connected and forwards it to internal interface and finally HOST C receives the data traffic.

Now before going to understand how multicast traffic is sent over Transport lets understand the frame format of OTV.

OTV Frame Format:



When any OTV encapsulation is done, it increase the 42 bytes in overall frame. While adding 42 bytes, it removes the CRC and 802.1q from original Layer-2 frame.

  • It added the OTV SHIM which contains VALN ID and Overlay ID and an External IP header.
  • All OTV frames are DF bit set means MTU needs to be consistence and OTV network support the extra MTU which it needs. For this a sufficient MTU size must be configured all across L3 network which is used for OTV traffic.
  • Fragmentation and Re-assembly of OTV packet is not done in Nexus 7000 Platform.

Data Plane: Multicast Traffic

Here we will learn how Multicast traffic flow from source to destination via Multicast enable transport or Unicast enabled transport (Adjacency Server Mode).

Multicast traffic over Multicast Enabled transport:

In order to send the Multicast traffic over Multicast enabled transport , a set of Source Specific Multicast ( SSM ) groups to carry Layer 2 multicast streams is used. These group is different from ASM group which we used for control plane adjacency.

Let’s suppose Multicasts stream is started on Site-1 for Group G1.

  • Once Local OTV edge device receives the Multicast traffic it creates the mapping between Group G1 and specific SSM group G2 available in transport infrastructure. The range for SSM group can be 232.0.0.0/8 enabled by default once PIM is configured.
  • Now this mapping is communicated to all OTV edge device by OTV control protocol and this mapping also specifies the specific VLAN suppose VLAN X to which multicast source belongs and IP address of the OTV edge device that created Mapping.

  • Now Client on Site-2 sends the IGMP join report to join the Group G1 which indicates that Host on Site-2 is interested to receive Multicast frame/ stream send by Site-1 Source.
  • OTV Edge device at site-2 snoops the IGMP message and note that there is active receiver interested for Group g1 streams belonging to VLAN X.
  • OTV edge device at Site-2 send the OTV control message to all remote site that there is an active receiver for Group G1.
  • Once OTV edge device on Site-1 receives the GM-update, it will the update the OIL list that group G1 needs to be delivered across Overlay.
  • Now OTV edge device on Site-2 fins the mapping information G1 mapped to G2 (SSM Group) from Site-1 with Site-1 IP address IP-A Now OTV edge Device will send the IGMPV3 report to join the ( IPA, G2) SSM Group.
  • By doing this, SSM tree is built across transport Infrastructure that will be further used to transport Multicast streams G1.

Now Once SSM tree is created, let’s see how multicast streams are delivered.

  • OTV edge device will receive the Multicast stream for group G1 and then lookup for OIL list to see that is there any active receiver for this group over overly.
  • Once OIL lookup is done and there is a HIT then edge device encapsulate the multicast frame with source IP is IP A of its join interface and Destination IP is G2 ( SSM Multicast address ) group dedicated to delivery of multicast data .
  • This encapsulated Multicast data traverse across SSM tree and reaches to all remote site who are interested in receiving G1 streams.
  • OTV edge device decapsulate the OTV data and deliver it to interested receiver

Multicast traffic over Unicast Enabled transport (Adjacency Server Mode):

When your transport infrastructure is unicast mode only that means it does not support multicast transport, Layer 2 Multicast frame can be sent across OTV sites by head-end replication. Let see how receivers join the Multicast Group G1.

  • Client on Site-2 sends the IGMP report to join the group G1.
  • OTV edge device on Site-2 snoops the IGMP report and confirms that there is an active receiver for group G1 on VLAN 100.
  • This Information is send across all OTV edge device remote site via GM-Update packet by using the Unicast Neighbor list which the Site-2 has.
  • The Site-1 OTV edge device receives the GM-Update and updates its Data-Group mapping table that there is an interested receiver on VLAN 100 for Group G1 and can be reached over IP IP-B.

Once Data mapping table is updated by SITE-1 OTV edge device and Source for G1 Groups starts sending multicast frame on G1 following actions happens:

  • As soon as Source starts sending Multicast streams on Group G1, OTV edge device consults the Data Group mapping table list and finds that there is active receiver on IP-B or IP C
  • OTV edge device encapsulate the Multicast frame with Source IP IP-A and Destination IP IP-B and IP-C.
  • These Unicasts frames are now transported over Layer-3 Transport and reached to respective destination OTV edge device.
  • Destination OTV edge device will decapsulate the frame and deliver it to respective intended Receiver for G1 over VLAN 100.




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