SPBM Layer 3 VSN

SPBM Layer 3 VSN

In the preceding figure, the BEBs connect over the SPBM cloud running IS-IS. VRF red and green are configured on the BEBs. VRF red on BEB A has to send and receive routes from VRF red on BEB D. Similar operations are required for VRF green on BEB A and BEB D.

IS-IS TLV 184 is used to advertise SPBM Layer 3 VSN route information across the SPBM cloud. To associate advertised routes with the appropriate VRF, each VRF is associated with an I-SID. All VRFs in the network that share the same I-SID participate in the same VSN.

Note

IPv4 Layer 3 VSN and IPv6 Layer 3 VSN coexist and share the same I-SID. You need to configure the I-SID only one time. The advantage of having two separate VPNs, one for IPv4 and one for IPv6, is because it gives the user an option to enable them separately.

In the preceding example, I-SID 101 is associated with VRF green and I-SID 102 is associated with VRF red. The I-SID ties the advertised routes to a particular VRF. This identifier must be the same on all edge nodes for a particular VRF, and must be unique across all the VRFs on the same node

When IS-IS receives an update from an edge node, it looks for the Layer 3 VSN TLV, and if one exists, it looks at the I-SID identifier. If that identifier is mapped to a local VRF, it extracts the IPv4 or IPv6 routes and adds them to the RTM of that VRF.

With SPBM Layer 3 VSN, the packet forwarding works in a similar fashion as the IP Shortcuts on the Global Router, with the difference that the encapsulation includes the I-SID to identify the VRF that the packet belongs to. The following figure shows the packet forwarding for VRF red.

Packet forwarding in SPBM Layer 3 VSN

When BEB A receives traffic from VRF red that must be forwarded to the far-end location, it performs a lookup and determines that VRF red is associated with I-SID 102 and that BEB D is the destination for I-SID 102. BEB A then encapsulates the IP data into a new BMAC header, using destination BMAC: D.

Note

With SPBM Layer 3 VSN, the CMAC header is all null. This header does not have any significance in the backbone. It is included to maintain the same 802.1ah format for ease of implementation.

At BEB D, the node strips off the BMAC encapsulation, and performs a lookup to determine the destination for traffic with I-SID 102. After identifying the destination as VRF red, the node forwards the packet to the destination VRF.

Note

IPv4 Layer 3 VSN and IPv6 Layer 3 VSN coexist and share the same I-SID. The advantage of having two separate VPNs, one for IPv4 and one for IPv6 is because it gives user an option to enable them separately.

IPv6 Layer 3 VSN Limitations and Considerations