Design Reference

IS-IS
SPBM eliminates the need for multiple overlay protocols in the core of the network by reducing the
core to a single Ethernet-based link-state protocol, Intermediate System to Intermediate System (IS-
IS). IS-IS provides virtualization services, both Layer 2 and Layer 3, using a pure Ethernet
technology base. SPBM also uses IS-IS to discover and advertise the network topology, which
enables it to compute the shortest path to all nodes in the SPBM network.
Spanning Tree is a topology protocol that prevents loops but does not scale very well. Because
SPBM uses IS-IS, which has its own mechanisms to prevent loops, SPBM does not have to use
Spanning Tree to provide a loop-free Layer 2 domain.
SPBM uses the IS-IS shortest-path trees to populate forwarding tables for the individual backbone
MAC (B-MAC) addresses of each participating node. Depending on the topology, SPBM supports as
many Equal Cost Multipath trees as there are backbone VLAN IDs (B-VIDs) provisioned (with a
maximum of 16 B-VIDs allowed by the standard and two allowed in this release) per IS-IS instance.
IS-IS interfaces operate in point-to-point mode only, which means that for any port or MLT interface
where IS-IS is enabled, there can be only one IS-IS adjacency across that interface.
B-MAC
An SPBM backbone includes Backbone Edge Bridges (BEB) and Backbone Core Bridges (BCB). A
BEB performs the same functionality as a BCB, but it also terminates one or more Virtual Service
Networks (VSNs). A BCB does not terminate any VSNs. BCBs forward encapsulated VSN traffic
based on the Backbone MAC Destination Address (B-MAC-DA). A BCB can access information to
send that traffic to any Backbone Edge Bridges (BEBs) in the SPBM backbone.
To forward customer traffic across the service provider backbone, the BEB for the VSN
encapsulates the customer Ethernet packet received at the edge into a Backbone MAC header
using the 802.1ah MAC-in-MAC encapsulation. This encapsulation hides the customer MAC (C-
MAC) address in a backbone MAC (B-MAC) address pair. MAC-in-MAC encapsulation defines a
BMAC-DA and BMAC-SA to identify the backbone source and destination addresses. The
originating node creates a MAC header for delivery from end to end. Intermediate BCB nodes within
the SPBM backbone perform packet forwarding using BMAC-DA alone. When the packet reaches
the intended egress BEB, the B-MAC header is removed and the original customer packet is
forwarded.
I-SID
SPBM introduces a service instance identifier called I-SID. SPBM uses I-SIDs to separate services
from the infrastructure. After you create an SPBM infrastructure, you can add additional services
(such as VLAN extensions) by provisioning the endpoints only. The SPBM endpoints are BEBs,
which mark the boundary between the core MAC-in-MAC SPBM domain and the edge customer
802.1Q domain. I-SIDs are provisioned on the BEBs to be associated with a particular service
instance. In the SPBM core, the bridges are BCBs. BCBs forward encapsulated traffic based on the
BMAC-DA.
The SPBM B-MAC header includes an I-SID. The length of the I-SID is 32 bits with a 24-bit ID. I-
SIDs identify and transmit virtualized traffic in an encapsulated SPBM frame. These I-SIDs are used
in a VSN for VLANs or VRFs across the MAC-in-MAC backbone:
• For a Layer 2 VSN, the I-SID is associated with a customer VLAN, which is then virtualized
across the backbone. Layer 2 VSNs offer an any-any LAN service type. Layer 2 VSNs
associate one VLAN per I-SID.
SPBM design guidelines
74 Network Design Reference for Avaya VSP 4000 Series January 2015
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