Cisco 7600 Series Router Software Configuration Guide, Cisco IOS Release 15S

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Updated:: July 31, 2014

Chapter: Configuring IP Unicast Layer 3 Switching

Understanding How Layer3 Switching Works
- Understanding Hardware Layer3 Switching
- Understanding Layer3-Switched Packet Rewrite
  - Hardware Layer3 Switching Examples
Default Hardware Layer3 Switching Configuration
Configuration Guidelines and Restrictions
Configuring Hardware Layer3 Switching
Displaying Hardware Layer3 Switching Statistics

Configuring IP Unicast Layer 3 Switching

This chapter describes how to configure IP unicast Layer 3 switching on Cisco 7600 series routers.

Note For complete syntax and usage information for the commands used in this chapter, refer to these publications:

The Cisco 7600 Series Routers Command References at this URL:

http://www.cisco.com/en/US/products/hw/routers/ps368/prod_command_reference_list.html

The Release 12.2 publications at this URL:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/index.htm

This chapter consists of these sections:

Note ● IPX traffic is fast switched on the MSFC. For more information, refer to this URL:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fatipx_c/index.htm

For information about IP multicast Layer 3 switching, see Chapter31, “Configuring IPv4 Multicast Layer 3 Switching”

Understanding How Layer 3 Switching Works

These sections describe Layer 3 switching:

Understanding Hardware Layer 3 Switching

Hardware Layer 3 switching allows the PFC and DFCs, instead of the MSFC, 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 MSFC. Hardware Layer 3 switching requires minimal support from the MSFC. The MSFC routes any traffic that cannot be hardware Layer 3 switched.

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

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 NetFlow switching for TCP intercept, 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 MSFC 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 Cisco 7600 series router.

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 56, “Configuring NetFlow and NDE”.)

Understanding 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 Cisco 7600 series router performs a packet rewrite at the egress port based on information learned from the MSFC so that the packets appear to have been routed by the MSFC.

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 MSFC to be routed to Destination B, the router recognizes that the packet was sent to the Layer 2 (MAC) address of the MSFC.

To perform Layer 3 switching, the router 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 MSFC. The Layer 3 addresses remain the same.

In IP unicast and IP multicast traffic, the router decrements the Layer 3 TTL value by 1 and recomputes the Layer 3 packet checksum. The router 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
MSFC MAC	Source A MAC	Destination B IP	Source A IP	n	calculation1

After the router 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	MSFC MAC	Destination B IP	Source A IP	n-1	calculation2

Hardware Layer 3 Switching Examples

Figure 29-1 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 29-1 Hardware Layer 3 Switching Example Topology

Default Hardware Layer 3 Switching Configuration

Table 29-1 shows the default hardware Layer 3 switching configuration.

Table 29-1 Default Hardware Layer 3 Switching Configuration
Feature	Default Value
Hardware Layer 3 switching enable state	Enabled (cannot be disabled)
Cisco IOS CEF enable state on MSFC	Enabled (cannot be disabled)
Cisco IOS dCEF¹ enable state on MSFC	Enabled (cannot be disabled)

^1.dCEF = Distributed Cisco Express Forwarding

Configuration Guidelines and Restrictions

Follow these guidelines and restrictions when configuring hardware Layer 3 switching:

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

Configuring Hardware Layer 3 Switching

Note For information on configuring unicast routing on the MSFC, see Chapter22, “Configuring 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 ² slot/port } \| { port-channel number }} \| begin L3	Displays a summary of Layer 3-switched traffic.

^2.type = ethernet, fastethernet, gigabitethernet, or tengigabitethernet

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

Router# show interface fastethernet 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 are not supported on the C7600 series routers.

For information about Cisco IOS CEF and dCEF on the MSFC, refer to these publications:

The “Cisco Express Forwarding” sections at this URL:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fswtch_c/swprt1/index.htm

The Cisco IOS Switching Services Command Reference publication at this URL:

http://www.cisco.com/en/US/docs/ios/12_2/switch/command/reference/fswtch_r.html

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 ³ slot/port } \| { port-channel number }}	Displays hardware Layer 3 switching statistics.

^3.type = ethernet, fastethernet, gigabitethernet, or tengigabitethernet

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 ⁴ slot/port } \| { port-channel number }} \| detail \| internal \| summary ]	Displays adjacency table information. The optional detail keyword displays detailed adjacency information, including Layer 2 information.