Configuration Guide

Table Of Contents
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October 2021
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3. SPB Support Topologies
3.1
SPB L2 VSN
Figure 2: SPB L2 VSN
A SPB L2 VSN topology is simply made up of a number of Backbone Edge Bridges (BEB) used to terminate
Layer 2 VSNs. The control plane uses IS-IS for forwarding at a Layer 2 level. Only the BEB bridges are
aware of any VSN and associated MAC addresses while the backbone bridges simply forward traffic at the
Backbone MAC (B-MAC) level. The backbone switches will know how to reach every B-MACs using the
shortest path determined by IS-IS. Note that the backbone System ID or B-MAC can be manually
provisioned to help ease trouble-shooting when looking at the B-MAC unicast forwarding table. In summary,
all switches in the backbone will only learn B-MAC addresses to make forwarding decisions while the BEB
will learn both the B-MACs and Customer MACs (C-MAC) for each VSN. A Backbone Service Instance
Identifier (ISID) will be assigned on the BEB to each VLAN. All VLANs in the network that share the same
ISID will be able to participate in the same VSN. If SMLT clusters are used, two backbone VLANs (B-VLAN)
are required with a primary B-VLAN and a secondary B-VLAN. . In general two backbone VLANs should
always be used (even if no SMLT cluster is in use) since the use of 2 backbone VLANs allows IS-IS to
compute equal cost trees where if 2 shortest equal cost paths exist, SPB will load balance VSN traffic
across both paths.
In summary:
At minimum, one B-VLAN must be assigned to each SPB switch
o
For SMLT, two B-VLANs are required
TLVs and sub-TLVs are used to identify SPB instance, link metric’s, B-VLAN, B-MAC, and
number of ISID’s