© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-1 Implementing Inter-VLAN Routing Deploying Multilayer Switching with Cisco Express Forwarding

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-2 Multilayer Switching A multilayer switch combines the ability of a switch, which forwards frames based on a Layer 2 header, and a router, which forwards packets based on a Layer 3 and Layer 4 header. A multilayer switch can therefore do the following:  Switch within a VLAN  Route between VLANs  Filter traffic with Layer 2 or Layer 3 ACLs An advantage of multilayer switches is that they can route at hardware speed.

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-3 IP Unicast Frame and Packet Rewrite Incoming IP Unicast Packet Rewritten IP Unicast Packet

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-4 CAM and TCAM Tables CAM and TCAM tables are used for very high-speed lookup in large tables. CAM works with binary operation:  Matches based on 0 or 1 values; no bits are ignored.  “Hit” returns a result (output port).  Used for MAC address lookup. TCAM works with ternary operation:  Matches based on 0, 1, or X (“don’t care”).  Longest match returns “hit.”  Table structure broken into groups of patterns and associated masks.  Useful for lookups where not all values in key must have exact match (ACL, lookups).

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-5 Distributed Hardware Forwarding In Layer 3 switches, the control path and data path are relatively independent:  The control path code, such as routing protocols, runs on the route processor.  Data packets are forwarded by the switching fabric.

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-6 Layer 3 Switch Processing A Layer 3 switch combines the functions of a switch and a router, and performs three major functions:  Packet switching  Route processing  Intelligent network services

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-7 Cisco Switching Methods Process switching  Slowest method — every packet examined by CPU; all forwarding decisions made in software Fast switching (route caching)  Faster method — first packet in each flow examined by CPU; forwarding decision cached in hardware for subsequent packets in flow Cisco Express Forwarding (topology-based switching)  Fastest method — hardware forwarding table created regardless of traffic flows; all packets switched using hardware  Fast but does have limitations  Switching mode for multilayer switches

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-8 Route Caching  First packet in a stream is routed in software.  Destination MAC address must be for default gateway.  Forwarding decision is programmed in the hardware forwarding table for subsequent packets.

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-9 Topology-Based Switching  Central FIB built by Cisco Express Forwarding regardless of traffic flow  Per-destination load balancing  Currently the predominant method

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-10 Multilayer Switches Based on Cisco Express Forwarding  Cisco Express Forwarding caches routing information in the FIB table, Layer 2 next-hop addresses, and frame header rewrite information in the adjacency table.

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-11 Verifying Cisco Express Forwarding  Displays information about entries in the Cisco Express Forwarding FIB. Switch# show ip cef Prefix Next Hop Interface /32 receive /24 attached GigabitEthernet0/ /32 receive /32 receive / GigabitEthernet0/2 Switch# show ip cef [type mod/port | vlan_interface] [detail] Switch# show ip cef vlan 10 detail IP CEF with switching (Table Version 11), flags=0x0 10 routes, 0 reresolve, 0 unresolved (0 old, 0 new), peak 0 13 leaves, 12 nodes, bytes, 14 inserts, 1 invalidations 0 load sharing elements, 0 bytes, 0 references.../ /24, version 6, epoch 0, attached, connected 0 packets, 0 bytes via Vlan10, 0 dependencies

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-12 Verifying Cisco Express Forwarding (Cont.)  Displays information about entries in the Cisco Express Forwarding adjacency table.  Displays information about packets dropped due to incomplete or nonexistent Cisco Express Forwarding adjacencies. Switch# show adjacency [{type mod/port | port-channel number} | detail | internal | summary] Switch# show cef drop Switch# show adjacency Protocol Interface Address IP GigabitEthernet0/ (5) IP GigabitEthernet0/ (5) Switch#show adjacency gigabitethernet 1/5 detail Protocol Interface Address IP GigabitEthernet1/ (11) 504 packets, 6110 bytes ARP 03:49:31

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-13 Summary  Multilayer switches can forward traffic, based on either Layer 2 or Layer 3 header information.  Multilayer switches rewrite frame and packet headers, using information from tables cached in hardware. Layer 3 (multilayer) switching is high-performance packet switching in hardware.  Multilayer switching can use centralized or distributed switching, and route caching or topology-based switching.  Multilayer switching functionality can be implemented using Cisco Express Forwarding.  Cisco Express Forwarding utilizes two tables in hardware to forward packets: an FIB and an adjacency table.

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—4-14