Cisco 6500 - Manual (Page 19)

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Cisco IOS Software Configuration Guide, Release 12.2SX
OL-13013-06
Chapter 4 Configuring Virtual Switching Systems
Understanding Virtual Switching Systems
Hardware forwarding is distributed across all DFCs on the VSS. The supervisor engine on the VSS active
chassis sends FIB updates to all local DFCs, remote DFCs, and the VSS standby supervisor engine PFC.
All hardware routing uses the router MAC address assigned by the VSS active supervisor engine. After
a switchover, the original MAC address is still used.
The supervisor engine on the VSS active chassis performs all software forwarding (for protocols such
as IPX) and feature processing (such as fragmentation and TTL exceed). If a switchover occurs, software
forwarding is disrupted until the new VSS active supervisor engine obtains the latest CEF and other
forwarding information.
In virtual switch mode, the requirements to support non-stop forwarding (NSF) are the same as in
standalone mode. For additional information about NSF requirements, refer to the Catalyst 6500 Series
Switch Cisco IOS Configuration Guide, Release 12.2SX.
From a routing peer perspective, EtherChannels remain operational during a switchover (only the links
to the failed chassis are down).
The VSS implements path filtering by storing only local paths (paths that do not traverse the VSL) in the
FIB entries. Therefore, IP forwarding performs load sharing among the local paths. If no local paths to
a given destination are available, the VSS updates the FIB entry to include remote paths (reachable by
traversing the VSL).
IPv6 and MPLS
In Cisco IOS Release 12.2(33)SXI2 and later releases, the VSS supports IPv6 unicast and MPLS.
IPv4 Multicast
The IPv4 multicast protocols run on the VSS active supervisor engine. Internet Group Management
Protocol (IGMP) and Protocol Independent Multicast (PIM) protocol packets received on the VSS
standby supervisor engine are transmitted across VSL to the VSS active chassis.
The VSS active supervisor engine sends IGMP and PIM protocol packets to the VSS standby supervisor
engine in order to maintain Layer 2 information for stateful switchover (SSO).
The VSS active supervisor engine distributes multicast FIB and adjacency table updates to the VSS
standby supervisor engine and switching module DFCs.
For Layer 3 multicast in the VSS, learned multicast routes are stored in hardware in the VSS standby
supervisor engine. After a switchover, multicast forwarding continues, using the existing hardware
entries.
Note To avoid multicast route changes as a result of the switchover, we recommend that all links carrying
multicast traffic be configured as MEC rather than Equal Cost Multipath (ECMP).
In virtual switch mode, the VSS active chassis does not program the multicast expansion table (MET)
on the VSS standby chassis. The VSS standby supervisor engine programs the outgoing interface
hardware entries for all local multicast receivers
If all switching modules on the VSS active chassis and VSS standby chassis are egress capable, the
multicast replication mode is set to egress mode; otherwise, the mode is set to ingress mode.
In egress mode, replication is distributed to DFCs that have ports in outgoing VLANs for a particular
flow. In ingress mode, replication for all outgoing VLANs is done on the ingress DFC.
For packets traversing VSL, all Layer 3 multicast replication occurs on the ingress chassis. If there are
multiple receivers on the egress chassis, replicated packets are forwarded over the VSL.