Release 15.1SY Supervisor Engine 720 Software Configuration Guide
IP Unicast Layer 3 Switching
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IP Unicast Layer 3 Switching

Table Of Contents

IP Unicast Layer 3 Switching

Prerequisites for Hardware Layer 3 Switching

Restrictions for Hardware Layer 3 Switching

Information About Layer 3 Switching

Hardware Layer 3 Switching

Layer 3-Switched Packet Rewrite

Default Settings for Hardware Layer 3 Switching

How to Configure Hardware Layer 3 Switching

Displaying Hardware Layer 3 Switching Statistics


IP Unicast Layer 3 Switching


Prerequisites for Hardware Layer 3 Switching

Restrictions for Hardware Layer 3 Switching

Information About Layer 3 Switching

Default Settings for Hardware Layer 3 Switching

How to Configure Hardware Layer 3 Switching

Displaying Hardware Layer 3 Switching Statistics


NoteFor complete syntax and usage information for the commands used in this chapter, see these publications:

http://www.cisco.com/en/US/products/ps11846/prod_command_reference_list.html

Cisco IOS Release 15.1SY supports only Ethernet interfaces. Cisco IOS Release 15.1SY does not support any WAN features or commands.

For information about IP multicast Layer 3 switching, see Chapter 39 "IPv4 Multicast Layer 3 Features."



Tip For additional information about Cisco Catalyst 6500 Series Switches (including configuration examples and troubleshooting information), see the documents listed on this page:

http://www.cisco.com/en/US/products/hw/switches/ps708/tsd_products_support_series_home.html

Participate in the Technical Documentation Ideas forum


Prerequisites for Hardware Layer 3 Switching

None.

Restrictions for Hardware Layer 3 Switching

IPX traffic is fast switched on the route proccessor (RP).

Hardware Layer 3 switching supports the following ingress and egress encapsulations:

Ethernet V2.0 (ARPA)

802.3 with 802.2 with 1 byte control (SAP1)

802.3 with 802.2 and SNAP

Information About Layer 3 Switching

Hardware Layer 3 Switching

Layer 3-Switched Packet Rewrite

Hardware Layer 3 Switching

Hardware Layer 3 switching allows the PFC and DFCs, instead of the RP, to forward IP unicast traffic between subnets. Hardware Layer 3 switching provides wire-speed forwarding on the PFC and DFCs, instead of in software on the RP. Hardware Layer 3 switching requires minimal support from the RP. The RP routes any traffic that cannot be hardware Layer 3 switched.

Hardware Layer 3 switching supports the routing protocols configured on the RP. Hardware Layer 3 switching does not replace the routing protocols configured on the RP.

Hardware Layer 3 switching runs equally on the PF3 and DFCs to provide IP unicast Layer 3 switching locally on each module. Hardware Layer 3 switching provides the following functions:

Hardware access control list (ACL) switching for policy-based routing (PBR)

Hardware flow-based switching for TCP intercept and reflexive ACL forwarding decisions

Hardware Cisco Express Forwarding (CEF) switching for all other IP unicast traffic

Hardware Layer 3 switching on the PFC supports modules that do not have a DFC. The RP forwards traffic that cannot be Layer 3 switched.

Traffic is hardware Layer 3 switched after being processed by access lists and quality of service (QoS).

Hardware Layer 3 switching makes a forwarding decision locally on the ingress-port module for each packet and sends the rewrite information for each packet to the egress port, where the rewrite occurs when the packet is transmitted from the switch.

Hardware Layer 3 switching generates flow statistics for Layer 3-switched traffic. Hardware Layer 3 flow statistics can be used for NetFlow Data Export (NDE). (See Chapter 49 "Configuring NetFlow Data Export (NDE)".)

Layer 3-Switched Packet Rewrite

When a packet is Layer 3 switched from a source in one subnet to a destination in another subnet, the switch performs a packet rewrite at the egress port based on information learned from the RP so that the packets appear to have been routed by the RP.

Packet rewrite alters five fields:

Layer 2 (MAC) destination address

Layer 2 (MAC) source address

Layer 3 IP Time to Live (TTL)

Layer 3 checksum

Layer 2 (MAC) checksum (also called the frame checksum or FCS)


Note Packets are rewritten with the encapsulation appropriate for the next-hop subnet.


If Source A and Destination B are in different subnets and Source A sends a packet to the RP to be routed to Destination B, the switch recognizes that the packet was sent to the Layer 2 (MAC) address of the RP.

To perform Layer 3 switching, the switch rewrites the Layer 2 frame header, changing the Layer 2 destination address to the Layer 2 address of Destination B and the Layer 2 source address to the Layer 2 address of the RP. The Layer 3 addresses remain the same.

In IP unicast and IP multicast traffic, the switch decrements the Layer 3 TTL value by 1 and recomputes the Layer 3 packet checksum. The switch recomputes the Layer 2 frame checksum and forwards (or, for multicast packets, replicates as necessary) the rewritten packet to Destination B's subnet.

A received IP unicast packet is formatted (conceptually) as follows:

Layer 2 Frame Header
Layer 3 IP Header
Data
FCS

Destination

Source

Destination

Source

TTL

Checksum

   

RP MAC

Source A MAC

Destination B IP

Source A IP

n

calculation1


After the switch rewrites an IP unicast packet, it is formatted (conceptually) as follows:

Layer 2 Frame Header
Layer 3 IP Header
Data
FCS

Destination

Source

Destination

Source

TTL

Checksum

   

Destination B MAC

RP MAC

Destination B IP

Source A IP

n-1

calculation2


Hardware Layer 3 Switching Examples

Figure 32-1 shows a simple network topology. In this example, Host A is on the Sales VLAN (IP subnet 171.59.1.0), Host B is on the Marketing VLAN (IP subnet 171.59.3.0), and Host C is on the Engineering VLAN (IP subnet 171.59.2.0).

When Host A initiates an HTTP file transfer to Host C, Hardware Layer 3 switching uses the information in the local forwarding information base (FIB) and adjacency table to forward packets from Host A to Host C.

Figure 32-1 Hardware Layer 3 Switching Example Topology

Default Settings for Hardware Layer 3 Switching

Feature
Default Value

Hardware Layer 3 switching enable state

Enabled (cannot be disabled)

Cisco IOS CEF enable state on RP

Enabled (cannot be disabled)

Cisco IOS dCEF enable state on RP

Enabled (cannot be disabled)


How to Configure Hardware Layer 3 Switching


Note For information on configuring unicast routing on the RP, see Chapter 34 "Layer 3 Interfaces."


Hardware Layer 3 switching is permanently enabled. No configuration is required.

To display information about Layer 3-switched traffic, perform this task:

Command
Purpose

Router# show interface {{type slot/port} | {port-channel number}} | begin L3

Displays a summary of Layer 3-switched traffic.


This example shows how to display information about hardware Layer 3-switched traffic on Gigabit Ethernet port 3/3:

Router# show interface gigabitethernet 3/3 | begin L3 
  L3 in Switched: ucast: 0 pkt, 0 bytes - mcast: 12 pkt, 778 bytes mcast
  L3 out Switched: ucast: 0 pkt, 0 bytes - mcast: 0 pkt, 0 bytes 
     4046399 packets input, 349370039 bytes, 0 no buffer
     Received 3795255 broadcasts, 2 runts, 0 giants, 0 throttles
<...output truncated...>
Router#
 
   

Note The Layer 3 switching packet count is updated approximately every five seconds.


Cisco IOS CEF and dCEF are permanently enabled. No configuration is required to support hardware Layer 3 switching.

With a PFC (and DFCs, if present), hardware Layer 3 switching uses per-flow load balancing based on IP source and destination addresses. Per-flow load balancing avoids the packet reordering that can be necessary with per-packet load balancing. For any given flow, all PFC- and DFC-equipped switches make exactly the same load-balancing decision, which can result in nonrandom load balancing.

The Cisco IOS CEF ip load-sharing per-packet, ip cef accounting per-prefix, and ip cef accounting non-recursive commands on the RP apply only to traffic that is CEF-switched in software on the RP. The commands do not affect traffic that is hardware Layer 3 switched on the PFC or on DFC-equipped switching modules.

Displaying Hardware Layer 3 Switching Statistics

Hardware Layer 3 switching statistics are obtained on a per-VLAN basis.

To display hardware Layer 3 switching statistics, perform this task:

Command
Purpose

Router# show interfaces {{type slot/port} | {port-channel number}}

Displays hardware Layer 3 switching statistics.


This example shows how to display hardware Layer 3 switching statistics:

Router# show interfaces gigabitethernet 9/5 | include Switched 
L2 Switched: ucast: 8199 pkt, 1362060 bytes - mcast: 6980 pkt, 371952 bytes
L3 in Switched: ucast: 0 pkt, 0 bytes - mcast: 0 pkt, 0 bytes mcast
L3 out Switched: ucast: 0 pkt, 0 bytes - mcast: 0 pkt, 0 bytes 
 
   

To display adjacency table information, perform this task:

Command
Purpose

Router# show adjacency [{{type slot/port} | {port-channel number}} | detail | internal | summary]

Displays adjacency table information. The optional detail keyword displays detailed adjacency information, including Layer 2 information.


This example shows how to display adjacency statistics, which are updated approximately every 60 seconds:

Router# show adjacency gigabitethernet 9/5 detail 
Protocol Interface                 Address
IP       GigabitEthernet9/5        172.20.53.206(11)
                                   504 packets, 6110 bytes
                                   00605C865B82
                                   000164F83FA50800
                                   ARP        03:49:31 

Tip For additional information about Cisco Catalyst 6500 Series Switches (including configuration examples and troubleshooting information), see the documents listed on this page:

http://www.cisco.com/en/US/products/hw/switches/ps708/tsd_products_support_series_home.html

Participate in the Technical Documentation Ideas forum