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  • Table of Contents
  • Layer 2 Technologies
    • Ethernet Switching
      • L2 Switch Operations
      • Spanning Tree
        • 802.1d – STP
        • 802.1w – RSTP
        • 802.1s – MSTP
      • VTP 101
      • Private VLANs
      • VLANs
      • EtherChannel 101
    • Layer 2 WAN Protocols
      • HDLC
        • HDLC 101
      • PPP
        • PPP 101
        • PPP Authentication - PAP
        • PPP Authentication – CHAP
        • PPP Authentication – EAP
        • PPP Multilink
        • PPPoFR – PPP over Frame Relay
        • PPPoE – PPP over Ethernet
      • Frame Relay
        • Frame Relay 101
        • Frame Relay 102
        • Frame Relay Encapsulations – IETF vs Cisco
        • Multilink Frame Relay
        • Frame Relay Switching
        • Routing over Frame Relay
      • Bridging
        • Bridging on a router
        • MTU 101
    • Wireless
      • Wireless Principles
      • Wireless Implementations
      • Wireless Roaming
      • Wireless Authentication
        • WPA2 PSK
        • WPA2 802.1X
  • IPv4
    • IPv4 Addressing
      • Backup Interfaces
      • FHRP 101
      • DHCP 101
      • DNS 101
      • ARP 101
      • IPv4 101
      • Tunnel Interfaces
        • GRE Tunnels
      • BFD – Bidirectional Forwarding Detection
    • IPv4 Routing
      • How the routing table is built
        • How CEF works
        • Routing Order of Operations
        • NSF – Non Stop Forwarding
      • RIP
        • RIP 101
      • EIGRP
        • EIGRP 101
        • EIGRP Metric
        • More EIGRP Features
      • OSPF
        • OSPF 101
        • OSPF Areas
        • OSPF LSAs
        • OSPF Mechanics
      • IS-IS
        • IS-IS 101
        • IS-IS Mechanics – CLNP
      • BGP
        • BGP 101
        • BGP Attributes
        • More BGP
      • Route Redistribution
      • Policy based Routing
      • PfR 101 – Perfromance Routing
      • ODR
  • IPv6
    • IPv6-101
    • IPv6 Routing
    • Interconnecting IPv6 and IPv4
  • MPLS
    • MPLS 101
    • MPLS L3 VPN
  • Multicast
    • Multicast 101
    • PIM 101
    • IGMP 101
    • Inter Domain Multicast
    • IPv6 Multicast
    • Multicast features on switches
  • Security
    • NAT 101
    • NAT for Overlapping Networks
    • ACLs 101
    • ACLs 102
    • Cisco IOS Firewall
    • Zone Based Firewall
    • AAA 101
    • Controlling CLI Access
    • Control Plane
    • Switch Security
      • Switchport Traffic Control
      • Switchport Port Security
      • DHCP Snooping and DAI
      • 802.1x
      • Switch ACLs
    • IPSec VPN 101
      • IKE / ISAKMP 101
      • IPSEC Crypto Maps 101
      • IPSEC VTI 101
      • DMVPN 101
    • EAP 101
  • Network Services
    • NTP 101
    • HTTP 101
    • File Transfer 101 – TFTP & FTP
    • WCCP 101
  • QoS
    • QoS 101
    • Classification and Marking
    • Congestion Management
      • Legacy Congestion Management
      • SPD – Selective Packet Discard
      • CBWFQ
      • IP RTP Priority
    • Congestion Avoidance – WRED
    • Policing and Shaping
      • CAR 101
    • Compression and LFI
      • Header and Payload Compression
      • LFI for MultiLink PPP
    • Frame Relay QoS
      • Per VC Frame Relay QoS
    • RSVP 101
    • Switching QoS
  • Network Optimization
    • NetFlow 101 – TNF – Traditional NetFlow
    • NetFlow 102 – FNF – Flexible NetFlow
    • IP SLA 101
    • IP Accounting 101
    • Logging 101
    • SNMP and RMON 101
    • Cisco CLI Tips and Tricks
    • AutoInstall
    • Enhanced Object Tracking
    • Troubleshooting 101
    • SPAN, RSPAN, ERSPAN
  • Network Architecture
    • Hierarchical Network Architecture
    • SD Access
    • SD WAN
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  • CAM - Content Addressable Memory
  • TCAM - Ternary Content Addressable Memory

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  1. Layer 2 Technologies
  2. Ethernet Switching

L2 Switch Operations

CAM - Content Addressable Memory

A switch operates by forwarding frames based on the L2 MAC Destination Address. For each frame it receives, the switch looks up the destination address in the mac address-table and will find out on which port the destination is expected to be found.

The mac address-table is usually automatically built by the switch but it can also accept configurations to manipulate the mac address-table.

In order to build the mac address-table the switch uses the L2 MAC Source Address of a frame to update the table, recording the MAC address, the switchport the VLAN for the incoming frame and a timestamp of the arrival time. If the information already exists, only the timestamp is updated.

When looking up a Destination MAC address in the table there are 2 options

  • an entry with the MAC address, port and VLAN exists in the table: In this case the frame is forwarded on the port.

  • no entry is found: In this case the frame is forwarded to all ports in the same VLAN as the incoming port. This operation is also known as "Unknown Unicast Flooding"

The mac address-table is also known as CAM (Content Addressable Memory). A CAM works differently than a RAM (Random Access Memory). With RAM you can ask for a the content at a specific address, while with CAM you can ask for the address of a specific content.

To verify the contents of the mac address-table, you can use:

Sw# show mac address-table [interface INTF]

The mac address-table has an aging time. Each entry is kept in the table for until the aging time expires. By default this is set to 300 seconds (5 minutes) but it can be changed in config mode:

Sw# show mac address-table aging-time
Global Aging Time:  300
Vlan    Aging Time
----    ----------
Sw# conf t
Sw(config)# mac address-table aging-time SEC
# Use 0 to disable aging

TCAM - Ternary Content Addressable Memory

Ternary CAM means this memory supports a third state as well, besides 0 and 1. The third state is X="don't care". This is implemented through a VMR format (Value, Mask, Result)

The TCAM is used to hold security ACLs and QoS ACLs and frames would be tested against the TCAM entries to see if the frame should be sent or with what piority. TCAM is also used for L3 forwarding.

The allocation of memory for TCAM tables is limited and statically allocated during the boot process but it can be slightly tweaked to use one of the possible SDM (Switching Database Manager) Templates.

Sw# show sdm prefer
...
Sw(config)# sdm prefer {vlan|advanced}

A reload will be required for the SDM templates to take effect.

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Last updated 2 years ago

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