Multilayer Switching

Multilayer Switching (MLS) is a switch feature that allows the switch to route traffic between VLANs and routed interfaces in a highly optimized and efficient manner. Cisco Express Forwarding (CEF) is an example technology used to facilitate MLS (see Figure 4-1). Cisco Express Forwarding (CEF) does the following:


Separates control plane hardware from data plane hardware.

• Controls plane runs in software and builds FIB and adjacency table.
  

• The data plane uses hardware to forward most IP unicast traffic.
  

• Handles traffic that must be forwarded in software (much slower) and includes:

—Packets originating from device.
—Packets with IP header options.
—Tunneled traffic.
—802.3 (IPX) frames.
—Load sharing traffic.
—FIB is an optimized routing table, stored in TCAM.
—Builds adjacencies from ARP data.
—Eliminates recursive loops.


ARP Throttling

ARP throttling is a tool to limit ARPs into a VLAN. ARPs, you may recall, are sent as broadcast. Once an ARP is sent for a given IP, the switch prevents repetitive ARPs for a short period of time:

• First packet to destination forwarded to Route Processor.

• Subsequent traffic dropped until MAC is resolved.

• It prevents overwhelming the Route Processor (RP) with redundant ARP requests.

• It helps during Denial of Service attacks.

• It is removed when MAC is resolved or in two seconds.

Configuring and Troubleshooting CEF

By default, CEF is on and supports per destination load sharing. To disable CEF:

• 4500—Use (config)#no ip cef.
  

• 3500/3700—On each interface, use (config)#no ip routecache cef.
  

• 6550 with policy feature card, distributed FC, and multilayer switch FC—cannot be disabled.

View CEF information with the following:

#show interface fastethernet 2/2 | begin L3


View switching statistics with the following:

#show interface fastethernet 2/2 | include switched


View FIB with the following:

#show ip cef


View detailed CEF FIB entry with the following:

#show ip cef fastethernet 2/2 10.0.0.1 detail


Troubleshoot CEF drops with the following:

#debug ip cef drops


Troubleshoot packets not forwarded by CEF with the following:

#debug ip cef receive


Troubleshoot CEF events with the following:

#debug ip cef events