5-19
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 5 Configuring Virtual Switching Systems
Understanding Virtual Switching Systems
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 supervisor engine. The VSS
Active supervisor engine generates IGMP and PIM protocol packets to be sent over ports on either VSS
member.
The VSS Active supervisor engine syncs Multicast Forwarding Information Base (MFIB) state to the
VSS Standby supervisor engine. On both member switches, all multicast routes are loaded in hardware
with replica expansion table (RET) entries programmed for only local outgoing interfaces. Both member
switches are capable of performing hardware forwarding.
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).
For packets traversing VSL, all Layer 3 multicast replication occurs on the egress switch. If there are
multiple receivers on the egress switch, only one packet is replicated and forwarded over the VSL, and
then replicated to all local egress ports.
Software Features
Software features run only on the VSS Active supervisor engine. Incoming packets to the VSS Standby
switch that require software processing are sent across the VSL to the VSS Active supervisor engine.
System Monitoring
The following sections describe system monitoring and system management for a VSS:
• Environmental Monitoring, page 5-19
• File System Access, page 5-19
• Diagnostics, page 5-20
• Network Management, page 5-21
Environmental Monitoring
Environmental monitoring runs on both supervisor engines. The VSS Standby switch reports
notifications to the VSS Active supervisor engine. The VSS Active switch gathers log messages for both
switches. The VSS Active switch synchronizes the calendar and system clock to the VSS Standby
switch.
File System Access
File system access on VSS is the same as it is on dual supervisor standalone system. All files on a
standby switch are accessible with slave prefix as following:
Switch# dir ?
/all List all files
/recursive List files recursively
all-filesystems List files on all filesystems
bootflash: Directory or file name
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