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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 4 Administering the Switch
Managing the MAC Address Table
Layer 2 Firewall or Cache
In this topology, a rewritten Layer 3 packet is routed back to a Layer 2 firewall (or cache) before exiting.
When the packet reenters the switch from the firewall, it possesses the switch’s MAC address because
the packet was previously routed. If the ingress port is a switch port, the switch learns the router’s MAC
address. For a routed port or SVI, however, the switch does not learn the address. Source misses are
generated continuously for all arriving data packets and the switch shows a very high CPU utilization.
By disabling learning on the VLAN that the firewall or cache egress is connected to, you will routinely
suppress the source miss and do not observe high CPU utilization (Figure 4-4).
Figure 4-4 Disabling MAC Address Learning: Layer 2 Firewall/Cache
Feature Compatibility
The following features are compatible with disabling MAC address learning on a VLAN:
• EtherChannel—The learning disable feature has no impact on EtherChannel provided that the MAC
learning state is either disabled or enabled for a VLAN on EtherChannel ports.
• Switch Virtual Interface (SVI, Layer 3 on a VLAN)— The learning disable feature has no impact
on SVI. Although disabling MAC address learning on a SVI VLAN causes flooding, it does not
impact any Layer 3 feature.
• REP—The learning disable feature has no impact on REP provided that the MAC learning state is
either disabled or enabled for an active VLAN on a port where REP is running.
• Unicast, Multicast, and Broadcast—When you enable learning on a VLAN, learning is disabled on
all types of traffic.
• DAI, ESMP, and IGMP snooping— These features do not interact with the learning disable feature.
• Control packets— Control packets arrive at the CPU even if learning is disabled.
• RSPAN— Learning on a VLAN and on an RSPAN are compatible.
• VLAN translation—To disable learning on a VLAN that is being translated, you must disable
learning on the translated VLAN.
Gi 3/2
VLAN 10
Gi 3/1
VLAN 10
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balancer 1
Load
balancer 2
Web server 1
Web server 1