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Understanding How Layer 3 Switching Works, Understanding Layer 3-Switched Packet Rewrite

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Understanding How Layer 3 Switching Works Chapter 26 Configuring IP Unicast Layer 3 Switching Understanding How Layer 3 Switching Works These sections describe Layer 3 switching: • Understanding Hardware Layer 3 Switching, page 26-2 • Understanding Layer 3-Switched Packet Rewrite, page 26-2 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 51, "Configuring 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. 26-2 Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX OL-4266-08

Section	Page
Contents	3
Preface	32
Audience	32
Related Documentation	32
Conventions	33
Product Overview	35
Supported Hardware and Software	35
User Interfaces	35
Configuring Embedded CiscoView Support	36
Understanding Embedded CiscoView	36
Installing and Configuring Embedded CiscoView	36
Displaying Embedded CiscoView Information	37
Software Features Supported in Hardware by the PFC and DFC	37
Software Features Supported in Hardware by the PFC3, PFC2, DFC3, and DFC	37
Software Features Supported in Hardware by the PFC3 and DFC3	38
Command-Line Interfaces	41
Accessing the CLI	42
Accessing the CLI through the EIA/TIA-232 Console Interface	42
Accessing the CLI through Telnet	42
Performing Command Line Processing	43
Performing History Substitution	44
Cisco IOS Command Modes	44
Displaying a List of Cisco IOS Commands and Syntax	45
Securing the CLI	46
ROM-Monitor Command-Line Interface	47
Configuring the Router for the First Time	49
Default Configuration	50
Configuring the Router	50
Using the Setup Facility or the setup Command	50
Setup Overview	50
Configuring the Global Parameters	51
Configuring Interfaces	56
Using Configuration Mode	58
Checking the Running Configuration Before Saving	58
Saving the Running Configuration Settings	59
Reviewing the Configuration	59
Configuring a Default Gateway	60
Configuring a Static Route	60
Configuring a BOOTP Server	62
Protecting Access to Privileged EXEC Commands	63
Setting or Changing a Static Enable Password	63
Using the enable password and enable secret Commands	63
Setting or Changing a Line Password	64
Setting TACACS+ Password Protection for Privileged EXEC Mode	64
Encrypting Passwords	65
Configuring Multiple Privilege Levels	65
Setting the Privilege Level for a Command	66
Changing the Default Privilege Level for Lines	66
Logging In to a Privilege Level	66
Exiting a Privilege Level	67
Displaying the Password, Access Level, and Privilege Level Configuration	67
Recovering a Lost Enable Password	67
Modifying the Supervisor Engine Startup Configuration	68
Understanding the Supervisor Engine Boot Configuration	68
Understanding the Supervisor Engine Boot Process	68
Understanding the ROM Monitor	68
Configuring the Software Configuration Register	69
Modifying the Boot Field and Using the boot Command	70
Modifying the Boot Field	71
Verifying the Configuration Register Setting	72
Specifying the Startup System Image	72
Understanding Flash Memory	72
Flash Memory Features	73
Security Features	73
Flash Memory Configuration Process	73
CONFIG_FILE Environment Variable	73
Controlling Environment Variables	74
Configuring a Supervisor Engine 720	75
Using the Bootflash or Bootdisk on a Supervisor Engine 720	76
Using the Slots on a Supervisor Engine 720	76
Configuring Supervisor Engine 720 Ports	76
Configuring and Monitoring the Switch Fabric Functionality	76
Understanding How the Switch Fabric Functionality Works	77
Switch Fabric Functionality Overview	77
Forwarding Decisions for Layer 3-Switched Traffic	77
Switching Modes	77
Configuring the Switch Fabric Functionality	78
Monitoring the Switch Fabric Functionality	78
Displaying the Switch Fabric Redundancy Status	79
Displaying Fabric Channel Switching Modes	79
Displaying the Fabric Status	80
Displaying the Fabric Utilization	80
Displaying Fabric Errors	81
Configuring a Supervisor Engine 32	83
Flash Memory on a Supervisor Engine 32	84
Supervisor Engine 32 Ports	84
Configuring the Supervisor Engine 2 and the Switch Fabric Module	85
Using the Slots on a Supervisor Engine 2	85
Understanding How the Switch Fabric Module Works	86
Switch Fabric Module Overview	86
Switch Fabric Module Slots	86
Switch Fabric Redundancy	86
Forwarding Decisions for Layer 3-Switched Traffic	86
Switching Modes	87
Configuring the Switch Fabric Module	87
Configuring the Switching Mode	88
Configuring Fabric-Required Mode	88
Configuring an LCD Message	89
Monitoring the Switch Fabric Module	89
Displaying the Module Information	91
Displaying the Switch Fabric Module Redundancy Status	91
Displaying Fabric Channel Switching Modes	91
Displaying the Fabric Status	92
Displaying the Fabric Utilization	92
Displaying Fabric Errors	92
Configuring NSF with SSO Supervisor Engine Redundancy	95
Understanding NSF with SSO Supervisor Engine Redundancy	95
NSF with SSO Supervisor Engine Redundancy Overview	96
SSO Operation	96
NSF Operation	97
Cisco Express Forwarding	97
Multicast MLS NSF with SSO	98
Routing Protocols	98
BGP Operation	98
OSPF Operation	99
IS-IS Operation	100
IETF IS-IS Configuration	100
Cisco IS-IS Configuration	101
EIGRP Operation	101
NSF Benefits and Restrictions	102
Supervisor Engine Configuration Synchronization	103
Supervisor Engine Redundancy Guidelines and Restrictions	103
Redundancy Configuration Guidelines and Restrictions	104
Hardware Configuration Guidelines and Restrictions	104
Configuration Mode Restrictions	105
NSF Configuration Tasks	105
Configuring SSO	106
Configuring Multicast MLS NSF with SSO	106
Verifying Multicast NSF with SSO	107
Configuring CEF NSF	107
Verifying CEF NSF	107
Configuring BGP NSF	108
Verifying BGP NSF	108
Configuring OSPF NSF	109
Verifying OSPF NSF	109
Configuring IS-IS NSF	110
Verifying IS-IS NSF	111
Configuring EIGRP NSF	113
Verifying EIGRP NSF	113
Synchronizing the Supervisor Engine Configurations	114
Copying Files to the Redundant Supervisor Engine	114
Configuring RPR and RPR+ Supervisor Engine Redundancy	115
Understanding RPR and RPR+	116
Supervisor Engine Redundancy Overview	116
RPR Operation	116
RPR+ Operation	117
Supervisor Engine Configuration Synchronization	117
RPR Supervisor Engine Configuration Synchronization	118
RPR+ Supervisor Engine Configuration Synchronization	118
Supervisor Engine Redundancy Guidelines and Restrictions	118
Redundancy Guidelines and Restrictions	118
RPR+ Guidelines and Restrictions	119
Hardware Configuration Guidelines and Restrictions	119
Configuration Mode Restrictions	120
Configuring Supervisor Engine Redundancy	120
Configuring Redundancy	120
Synchronizing the Supervisor Engine Configurations	121
Displaying the Redundancy States	121
Performing a Fast Software Upgrade	122
Copying Files to an MSFC	123
Configuring Interfaces	125
Understanding Interface Configuration	126
Using the Interface Command	126
Configuring a Range of Interfaces	128
Defining and Using Interface-Range Macros	130
Configuring Optional Interface Features	130
Configuring Ethernet Interface Speed and Duplex Mode	131
Speed and Duplex Mode Configuration Guidelines	131
Configuring the Ethernet Interface Speed	131
Setting the Interface Duplex Mode	132
Configuring Link Negotiation on Gigabit Ethernet Ports	132
Displaying the Speed and Duplex Mode Configuration	133
Configuring Jumbo Frame Support	134
Understanding Jumbo Frame Support	134
Jumbo Frame Support Overview	134
Bridged and Routed Traffic Size Check at Ingress 10, 10/100, and 100 Mbps Ethernet and 10-Gigabit Ethernet Ports	135
Bridged and Routed Traffic Size Check at Ingress Gigabit Ethernet Ports	135
Routed Traffic Size Check on the PFC	135
Bridged and Routed Traffic Size Check at Egress 10, 10/100, and 100 Mbps Ethernet Ports	135
Bridged and Routed Traffic Size Check at Egress Gigabit Ethernet and 10-Gigabit Ethernet Ports	135
Ethernet Ports	135
Ethernet Port Overview	135
Layer 3 Ethernet Ports	136
Layer 2 Ethernet Ports	136
VLAN Interfaces	136
Configuring MTU Sizes	136
Configuring the MTU Size	136
Configuring the Global Egress LAN Port MTU Size	137
Configuring IEEE 802.3x Flow Control	137
Configuring the Port Debounce Timer	138
Adding a Description for an Interface	140
Understanding Online Insertion and Removal	140
Monitoring and Maintaining Interfaces	141
Monitoring Interface Status	141
Clearing Counters on an Interface	141
Resetting an Interface	142
Shutting Down and Restarting an Interface	142
Checking the Cable Status Using the TDR	143
Configuring LAN Ports for Layer 2 Switching	145
Understanding How Layer 2 Switching Works	145
Understanding Layer 2 Ethernet Switching	146
Layer 2 Ethernet Switching Overview	146
Switching Frames Between Segments	146
Building the Address Table	146
Understanding VLAN Trunks	147
Trunking Overview	147
Encapsulation Types	147
Layer 2 LAN Port Modes	148
Default Layer 2 LAN Interface Configuration	149
Layer 2 LAN Interface Configuration Guidelines and Restrictions	149
Configuring LAN Interfaces for Layer 2 Switching	150
Configuring a LAN Port for Layer 2 Switching	151
Configuring a Layer 2 Switching Port as a Trunk	152
Configuring the Layer 2 Switching Port as an ISL or 802.1Q Trunk	152
Configuring the Layer 2 Trunk to Use DTP	153
Configuring the Layer 2 Trunk Not to Use DTP	153
Configuring the Access VLAN	154
Configuring the 802.1Q Native VLAN	154
Configuring the List of VLANs Allowed on a Trunk	155
Configuring the List of Prune-Eligible VLANs	155
Completing Trunk Configuration	156
Verifying Layer 2 Trunk Configuration	156
Configuration and Verification Examples	157
Configuring a LAN Interface as a Layer 2 Access Port	158
Configuring a Custom IEEE 802.1Q EtherType Field Value	159
Configuring Flex Links	161
Understanding Flex Links	161
Configuring Flex Links	162
Flex Links Default Configuration	162
Flex Links Configuration Guidelines and Restrictions	162
Configuring Flex Links	163
Monitoring Flex Links	164
Configuring EtherChannels	165
Understanding How EtherChannels Work	165
EtherChannel Feature Overview	166
Understanding How EtherChannels Are Configured	166
EtherChannel Configuration Overview	166
Understanding Manual EtherChannel Configuration	167
Understanding PAgP EtherChannel Configuration	167
Understanding IEEE 802.3ad LACP EtherChannel Configuration	168
Understanding Port Channel Interfaces	169
Understanding Load Balancing	169
EtherChannel Feature Configuration Guidelines and Restrictions	169
Configuring EtherChannels	171
Configuring Port Channel Logical Interfaces for Layer 3 EtherChannels	171
Configuring Channel Groups	172
Configuring the LACP System Priority and System ID	174
Configuring EtherChannel Load Balancing	175
Configuring the EtherChannel Min-Links Feature	176
Configuring VTP	177
Understanding How VTP Works	177
Understanding the VTP Domain	178
Understanding VTP Modes	178
Understanding VTP Advertisements	179
Understanding VTP Version 2	179
Understanding VTP Pruning	180
VTP Default Configuration	181
VTP Configuration Guidelines and Restrictions	181
Configuring VTP	182
Configuring VTP Global Parameters	182
Configuring a VTP Password	183
Enabling VTP Pruning	183
Enabling VTP Version 2	184
Configuring the VTP Mode	185
Displaying VTP Statistics	186
Configuring VLANs	189
Understanding How VLANs Work	189
VLAN Overview	190
VLAN Ranges	190
Configurable VLAN Parameters	191
Understanding Token Ring VLANs	191
Token Ring TrBRF VLANs	191
Token Ring TrCRF VLANs	192
VLAN Default Configuration	194
VLAN Configuration Guidelines and Restrictions	196
Configuring VLANs	197
VLAN Configuration Options	197
VLAN Configuration in Global Configuration Mode	197
VLAN Configuration in VLAN Database Mode	198
Creating or Modifying an Ethernet VLAN	198
Assigning a Layer 2 LAN Interface to a VLAN	200
Configuring the Internal VLAN Allocation Policy	200
Configuring VLAN Translation	201
VLAN Translation Guidelines and Restrictions	201
Configuring VLAN Translation on a Trunk Port	203
Enabling VLAN Translation on Other Ports in a Port Group	203
Mapping 802.1Q VLANs to ISL VLANs	204
Saving VLAN Information	205
Configuring Private VLANs	207
Understanding How Private VLANs Work	207
Private VLAN Domains	208
Private VLAN Ports	209
Primary, Isolated, and Community VLANs	209
Private VLAN Port Isolation	210
IP Addressing Scheme with Private VLANs	210
Private VLANs Across Multiple Routers	211
Private VLAN Interaction with Other Features	211
Private VLANs and Unicast, Broadcast, and Multicast Traffic	212
Private VLANs and SVIs	212
Private VLAN Configuration Guidelines and Restrictions	212
Secondary and Primary VLAN Configuration	213
Private VLAN Port Configuration	215
Limitations with Other Features	215
Configuring Private VLANs	217
Configuring a VLAN as a Private VLAN	217
Associating Secondary VLANs with a Primary VLAN	218
Mapping Secondary VLANs to the Layer 3 VLAN Interface of a Primary VLAN	219
Configuring a Layer 2 Interface as a Private VLAN Host Port	220
Configuring a Layer 2 Interface as a Private VLAN Promiscuous Port	221
Monitoring Private VLANs	223
Configuring Cisco IP Phone Support	225
Understanding Cisco IP Phone Support	225
Cisco IP Phone Connections	226
Cisco IP Phone Voice Traffic	226
Cisco IP Phone Data Traffic	227
Cisco IP Phone Power Configurations	227
Locally Powered Cisco IP Phones	227
Inline-Powered Cisco IP Phones	228
Other Cisco IP Phone Features	228
Default Cisco IP Phone Support Configuration	229
Cisco IP Phone Support Configuration Guidelines and Restrictions	229
Configuring Cisco IP Phone Support	230
Configuring Voice Traffic Support	230
Configuring Data Traffic Support	231
Configuring Inline Power Support	232
Configuring IEEE 802.1Q Tunneling	235
Understanding How 802.1Q Tunneling Works	235
802.1Q Tunneling Configuration Guidelines and Restrictions	237
Configuring 802.1Q Tunneling	240
Configuring 802.1Q Tunnel Ports	240
Configuring the Router to Tag Native VLAN Traffic	240
Configuring Layer 2 Protocol Tunneling	243
Understanding How Layer 2 Protocol Tunneling Works	243
Configuring Support for Layer 2 Protocol Tunneling	244
Configuring Standard-Compliant IEEE MST	247
Understanding MST	247
MST Overview	248
MST Regions	248
IST, CIST, and CST	249
IST, CIST, and CST Overview	249
Spanning Tree Operation Within an MST Region	250
Spanning Tree Operations Between MST Regions	250
IEEE 802.1s Terminology	251
Hop Count	252
Boundary Ports	252
Standard-Compliant MST Implementation	253
Changes in Port-Role Naming	253
Spanning Tree Interoperation Between Legacy and Standard-Compliant Routers	254
Detecting Unidirectional Link Failure	254
Interoperability with IEEE 802.1D-1998 STP	255
Understanding RSTP	255
Port Roles and the Active Topology	256
Rapid Convergence	257
Synchronization of Port Roles	258
Bridge Protocol Data Unit Format and Processing	259
BPDU Format and Processing Overview	259
Processing Superior BPDU Information	260
Processing Inferior BPDU Information	260
Topology Changes	261
Configuring MST	261
Default MST Configuration	262
MST Configuration Guidelines and Restrictions	262
Specifying the MST Region Configuration and Enabling MST	263
Configuring the Root Bridge	265
Configuring a Secondary Root Bridge	266
Configuring Port Priority	267
Configuring Path Cost	268
Configuring the Switch Priority	269
Configuring the Hello Time	270
Configuring the Forwarding-Delay Time	271
Configuring the Transmit Hold Count	271
Configuring the Maximum-Aging Time	272
Configuring the Maximum-Hop Count	272
Specifying the Link Type to Ensure Rapid Transitions	272
Designating the Neighbor Type	273
Restarting the Protocol Migration Process	274
Displaying the MST Configuration and Status	274
Configuring STP and Prestandard IEEE 802.1s MST	275
Understanding How STP Works	276
STP Overview	276
Understanding the Bridge ID	276
Bridge Priority Value	277
Extended System ID	277
STP MAC Address Allocation	277
Understanding Bridge Protocol Data Units	278
Election of the Root Bridge	278
STP Protocol Timers	279
Creating the Spanning Tree Topology	279
STP Port States	280
STP Port State Overview	280
Blocking State	282
Listening State	283
Learning State	284
Forwarding State	285
Disabled State	286
STP and IEEE 802.1Q Trunks	286
Understanding How IEEE 802.1w RSTP Works	287
IEEE 802.1w RSTP Overview	287
RSTP Port Roles	287
RSTP Port States	288
Rapid-PVST	288
Understanding How Prestandard IEEE 802.1s MST Works	288
IEEE 802.1s MST Overview	289
MST-to-PVST Interoperability	290
Common Spanning Tree	292
MST Instances	292
MST Configuration Parameters	292
MST Regions	293
MST Region Overview	293
Boundary Ports	293
IST Master	294
Edge Ports	294
Link Type	294
Message Age and Hop Count	294
Default STP Configuration	295
STP and MST Configuration Guidelines and Restrictions	295
Configuring STP	296
Enabling STP	296
Enabling the Extended System ID	298
Configuring the Root Bridge	298
Configuring a Secondary Root Bridge	300
Configuring STP Port Priority	301
Configuring STP Port Cost	302
Configuring the Bridge Priority of a VLAN	304
Configuring the Hello Time	305
Configuring the Forward-Delay Time for a VLAN	306
Configuring the Maximum Aging Time for a VLAN	306
Enabling Rapid-PVST	307
Specifying the Link Type	307
Restarting Protocol Migration	307
Configuring Prestandard IEEE 802.1s MST	307
Enabling MST	308
Displaying MST Configurations	309
Configuring MST Instance Parameters	313
Configuring MST Instance Port Parameters	314
Restarting Protocol Migration	314
Configuring Optional STP Features	317
Understanding How PortFast Works	318
Understanding How BPDU Guard Works	318
Understanding How PortFast BPDU Filtering Works	318
Understanding How UplinkFast Works	319
Understanding How BackboneFast Works	320
Understanding How EtherChannel Guard Works	322
Understanding How Root Guard Works	323
Understanding How Loop Guard Works	323
Enabling PortFast	324
Enabling PortFast BPDU Filtering	326
Enabling BPDU Guard	328
Enabling UplinkFast	328
Enabling BackboneFast	329
Enabling EtherChannel Guard	330
Enabling Root Guard	330
Enabling Loop Guard	331
Configuring Layer 3 Interfaces	333
Layer 3 Interface Configuration Guidelines and Restrictions	334
Configuring Subinterfaces on Layer 3 Interfaces	334
Configuring IPv4 Routing and Addresses	336
Configuring IPX Routing and Network Numbers	340
Configuring AppleTalk Routing, Cable Ranges, and Zones	341
Configuring Other Protocols on Layer 3 Interfaces	342
Configuring UDE and UDLR	343
Understanding UDE and UDLR	343
UDE and UDLR Overview	343
Supported Hardware	344
Understanding UDE	344
UDE Overview	344
Understanding Hardware-Based UDE	344
Understanding Software-Based UDE	345
Understanding UDLR	345
Configuring UDE and UDLR	345
Configuring UDE	345
UDE Configuration Guidelines	346
Configuring Hardware-Based UDE	346
Configuring Software-Based UDE	347
Configuring UDLR	348
UDLR Back-Channel Tunnel Configuration Guidelines	348
Configuring a Receive-Only Tunnel Interface for a UDE Send-Only Port	349
Configuring a Send-Only Tunnel Interface for a UDE Receive-Only Port	349
Router A Configuration	350
Router B Configuration	350
Configuring PFC3BXL and PFC3B Mode Multiprotocol Label Switching	351
PFC3BXL and PFC3B Mode MPLS Label Switching	351
Understanding MPLS	352
Understanding PFC3BXL and PFC3B Mode MPLS Label Switching	352
IP to MPLS	353
MPLS to MPLS	354
MPLS to IP	354
MPLS VPN Forwarding	354
Recirculation	354
Supported Hardware Features	355
Supported Cisco IOS Features	355
MPLS Guidelines and Restrictions	357
PFC3BXL and PFC3B Mode MPLS Supported Commands	357
Configuring MPLS	358
MPLS Per-Label Load Balancing	358
Basic MPLS Load Balancing	358
MPLS Layer 2 VPN Load Balancing	358
MPLS Layer 3 VPN Load Balancing	358
MPLS Configuration Examples	358
PFC3BXL or PFC3B Mode VPN Switching	360

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26-2

Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX

OL-4266-08

Chapter 26

Configuring IP Unicast Layer 3 Switching

Understanding How Layer 3 Switching Works

These sections describe Layer 3 switching:

•

Understanding Hardware Layer 3 Switching, page 26-2

•

Understanding Layer 3-Switched Packet Rewrite, page 26-2

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 51, “Configuring 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.