H3C Technologies H3C S12500 Series Switches - Manual (Page 20)

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Loop prevention
SPBM nodes collect topology data independently to calculate forwarding paths. Transient loops might
occur during SPT recalculation caused by a network flapping because the latency varies for an LSP to
reach the SPBM nodes. SPBM uses the Agreement Protocol (AP) to prevent transient loops. AP introduces
a delay for ISIS-SPB to issue forwarding entries to the data plane. After a topology recalculation, ISIS-SPB
issues new forwarding entries to the data plane only if topology data is synchronized between the SPBM
node and its adjacent neighbors.
NOTE:
In this document, "AP" refers to agreement protocol. In other documents, "AP" can also refer to access
points.
Multicast B-VLAN
By default, the device uses one B-VLAN to transmit both unicast and multicast traffic. If tandem multicast
replication is used, you must enable the multicast VLAN feature for the device to use separate VLANs for
unicast and multicast traffic.
If the multicast B-VLAN feature is enabled, you must assign an odd B-VLAN ID to an I-SID for SPBM to
transmit unicast traffic. SPBM automatically chooses the next higher even B-VLAN ID (odd B-VLAN ID plus
1) for multicast traffic.
ISIS-SPB only advertises the odd B-VLAN IDs. It uses the odd B-VLAN IDs when populating the unicast
FDB table with routes. When populating the multicast FDB table with routes, it uses the even B-VLAN IDs.
Fast channel for quick spread of LSPs
The fast channel feature provides a quicker LSP propagation method than conventional LSP propagation.
This feature spreads ISIS-SPB LSPs in the SPBM network by forwarding LSPs as data packets along the SPT
in a dedicated SPB VSI. An SPBM node does not need to complete updating its LSDB before it
propagates the topology change to the next node.
The I-SID for the fast-channel VSI is 255. To use the fast-channel feature, you must create a VSI and assign
I-SID 255 to the VSI.
You cannot associate any Ethernet service instance with the VSI that uses I-SID 255 for data transmission.
Protocols and standards
• IEEE 802.1aq, Shortest Path Bridging
• IEEE 802.1ah, Virtual Bridged Local Area Networks Amendment 7: Provider Backbone Bridges
• ISO 10589, ISO IS-IS Routing Protocol
• RFC 6329, IS-IS Extensions Supporting IEEE 802.1aq Shortest Path Bridging