Layer 2 Switching on the Mobile Router

Table Of Contents

FESMIC Switch Port Functionality

Port-Based VLAN

802.1Q Trunking

Inter-VLAN Routing

VLAN Trunk Protocol (VTP)

802.1P CoS

Spanning Tree Protocol (STP)

Switch Virtual Interface

Creating a SVI

IP Multicast Layer 3 Switching

Enabling IP PIM on Layer 3 Interfaces

Verifying IP Multicast Layer 3 Hardware Switching Summary

Verifying the IP Multicast Routing Table

Storm Control

Storm Control Configuration

IGMP Snooping

IGMP Snooping Configuration

FESMIC Switch Port Functionality

The 10/100 Fast Ethernet ports on the FESMIC default to Layer 2 switch ports. The FESMIC is a "learning bridge," as defined in 802.1D with the Virtual Local Area Network (VLAN) capabilities of 802.1P/Q. The BCM5618 is fully capable of line-rate switching for all four 10/100 Fast Ethernet ports.

VLANs provide the segmentation services traditionally provided by routers in LAN configurations, as shown in Figure 14-1. VLANs make it easy to move an network or to change a network design.

•Broadcast control—Just as switches physically isolate collision domains for attached hosts and only forward traffic out a particular port, VLANs provide logical collision domains that confine broadcast and multicast traffic to the bridging domain.VLANs solve the scalability problems of large flat networks by breaking a single broadcast domain into several smaller broadcast domains.

•Security—VLANs improve security by isolating groups. High-security users can be grouped into a VLAN, possibly on the same physical segment. If you do not include a router in a VLAN, no one outside that VLAN can communicate with the users inside the VLAN and vice versa. This extreme level of security can be highly desirable. Users outside that VLAN cannot penetrate into the VLAN without an appropriate routing through secure Layer 3 routing services.

•Performance—Users that require high-performance networking can be assigned to their own VLAN. You might, for example, assign an engineer who is testing a multicast application and the servers that the engineer is using to a single VLAN. The engineer experiences improved network performance by being on a "dedicated LAN." The rest of the engineering group experiences improved network performance, because the traffic generated by the network-intensive application is isolated to another VLAN. This of course implies some areas of physical isolation of separate VLANs or prioritized service by tagging support and prioritized queuing classes within the switches and bridges of the 802.1Q VLAN.

•Network management—Software on the switch allows you to assign users to VLANs. Changing the cabling to change connectivity is no longer necessary in the switched LAN environment because network management tools allow you to reconfigure the LAN logically in seconds.

Figure 14-1 Traditional LAN Segmentation versus VLAN Segmentation

Port-Based VLAN

By default, the four 10/100 Fast Ethernet interfaces on the FESMIC are defaulted to Layer 2 switch ports and all four interfaces belong to VLAN 1. You can partition the switch ports to belong to different VLAN groups by using the switchport vlan access <vlan-id> command. The following is a brief function description of a FESMIC port-based VLAN:

•Each VLAN has its own MAC address table.

•Packets received are forwarded only to ports that are members of the same VLAN as the receiving port. VLAN partitions provide hard firewalls for all traffic in different VLANs.

•A VLAN comes into existence when a user adds a VLAN to the local VLAN database. A maximum of 32 VLANs are supported. VLAN IDs can range from 1 to 1005.

•By default, a spanning tree instance is created for each VLAN.

802.1Q Trunking

A trunk is a point-to-point link between one or more Ethernet switch ports and another networking device, such as a router or a switch. Trunks carry the traffic of multiple VLANs over a single link, and they allow you to extend VLANs across an entire network, as shown in Figure 14-2. The IEEE 802.1q protocol is an industry-standard trunking encapsulation.

Figure 14-2 802.1Q Trunk Port Application

The 802.1Q trunk port is used for VLAN extension from one switch to another 802.1Q-capable switch, and used for an 802.1Q-capable router for inter-VLAN routing. The FESMIC supports both the VLAN extension and inter-VLAN routing.

The 802.1Q uses an internal tagging mechanism. Internal tagging means that a tag is inserted within the frame. Note that on an 802.1Q trunk, one VLAN is not tagged. This VLAN, named the native VLAN, must be configured the same on each side of the trunk. We can deduce to which VLAN a frame belongs when we receive a frame with no tag. The EtherType field identifying the 802.1Q frame is 0x8100. In addition to the 12-bit VLAN-ID, 3 bits are reserved for 802.1P priority tagging, as shown in Figure 14-3. Also, note that inserting a tag into a frame that already has the maximum Ethernet size creates a 1522 byte frame, that can be considered a "baby giant" by the receiving equipment.

The FESMIC is capable of 802.1Q tagging, only supporting 802.1Q trunking encapsulation. It does not support the Cisco proprietary ISL encapsulation.

Figure 14-3 802.1Q Tag Format in an Ethernet Frame

Inter-VLAN Routing

In a VLAN network, traffic and stations for multiple network layer subnets (VLANs) can coexist on a single physical LAN segment. In practice, a single VLAN corresponds to a network subnet, and a VLAN trunking capable router is required to forward traffic from a first VLAN to a second VLAN for a Layer 2 switch.

The FESMIC enables the Cisco 3200 Series router to become one of first IOS Ethernet switching routers to deliver intelligent Layer 2 switching capability and Layer 3 inter-VLAN routing in a single box solution, as shown in Figure 14-4

Figure 14-4 Switching Router Network Topology

In a typical IOS-managed Layer 2 switch, there would be one Layer 3 Switch Virtual Interface (SVI) that allows you to configure the device over a Layer 3 protocol by using SNMP or a Telnet application. This is referred to as the management VLAN for the switch. The default management VLAN is usually the native VLAN 1. The configurable VLAN device allows you to configure any VLAN to be the management VLAN, but there can be only one virtual Layer 3 interface in one VLAN.

A switch routing module, like the FESMIC, allows you to use the SVI to configure more than one virtual Layer 3 interface to support routing between the different VLANs, and the virtual Layer 3 interface of any other router interface in the system, as shown in Figure 14-5.

You can manage the switching router with any switch virtual Layer 3 interface created in the system. The FESMIC router switch port is an interface capable of handling Layer 3 switching functionality in hardware. The SVI architecture has the framework to support such a functionality.

•A SVI represents a VLAN of switch ports as one interface to the routing function in the system.

•There is at most one SVI associated with a VLAN.

•It is not necessary to configure an SVI for every known VLAN. It is only necessary to configure a SVI when you want to route between VLANs or want to provide IP host connectivity to the rest of the network by using any of the mobile access router routed interfaces.

•One management SVI, interface VLAN 1, is created at system initialization to permit remote administration. Additional SVIs exist only when explicitly configured by a user.

Figure 14-5 Switch Virtual Interface Architecture

VLAN Trunk Protocol (VTP)

VLAN Trunk Protocol (VTP) is a Layer 2 messaging protocol that maintains VLAN configuration consistency by managing the addition, deletion, and renaming of VLANs within a VTP domain. A VTP domain (also called a VLAN management domain) is made up of one or more switches that share the same VTP domain name and that are interconnected with trunks. VTP minimizes configuration errors and configuration inconsistencies that can result in a number of problems, such as duplicate VLAN names, incorrect VLAN-type specifications, and security violations.

The FESMIC supports both VTP version 1 and version 2.

•VTP server mode—You can create, modify, or delete VLANs and specify other configuration parameters (such as VTP version and VTP pruning) for the entire VTP domain. VTP servers advertise their VLAN configuration to other switches in the same VTP domain and synchronize their VLAN configuration with other switches, based on advertisements received over trunk links. VTP server is the default mode.

•VTP clients mode— Behaves the same way as VTP servers, but you cannot create, change, or delete VLANs on a VTP client.

•VTP transparent mode—Switches do not participate in VTP. A VTP-transparent switch does not advertise its VLAN configuration and does not synchronize its VLAN configuration based on received advertisements. However, in VTP version 2, transparent switches do forward VTP advertisements that they receive out their trunk interfaces.

VTP Server Example

The following example shows how to configure the switch as a VTP server:

Router# vlan database
Router(vlan)# vtp server
Setting device to VTP SERVER mode.
Router(vlan)# vtp domain Lab_Network
Setting VTP domain name to Lab_Network
Router(vlan)# vtp password WATER
Setting device VLAN database password to WATER.
Router(vlan)# exit
APPLY completed.
Exiting....
Router#
VTP Client Example

The following example shows how to configure the switch as a VTP client:

Router# vlan database
Router(vlan)# vtp client
Setting device to VTP CLIENT mode.
Router(vlan)# exit

In CLIENT state, no apply attempted.
Exiting....
Router#
Disabling VTP (VTP Transparent Mode) Example

The following example shows how to configure the switch as VTP transparent:

Router# vlan database
Router(vlan)# vtp transparent
Setting device to VTP TRANSPARENT mode.
Router(vlan)# exit
APPLY completed.
Exiting....
Router#
VTP Version 2 Example

The following example shows how to enable VTP version 2:

Router# vlan database
Router(vlan)# vtp v2-mode

V2 mode enabled.
Router(vlan)# exit

APPLY completed.
Exiting....
Router#

802.1P CoS

The IEEE 802.1P specification defines eight levels of priority (0 thru 7), with priority 7 being the highest priority. This information is carried in the 3-bit priority field of the VLAN tag header.

The FESMIC supports up to two class of service (CoS) queues per port. For the tagged packets, the incoming packet priority can be mapped into one of the queues, based on the priority field in the tag header or from the result of filtering mechanism. For untagged packets, the CoS priority is derived either from a programmable field within the ARL (MAC address table) or from the result of filtering mechanism.

After the packets are mapped into a CoS queue, they are forwarded or conditioned using these scheduling algorithms:

•Strict priority-based scheduling—Any packets residing in the higher priority queues are transmitted first. Only when these queues are empty will packets of lower priority be transmitted. The disadvantage of this scheme is the potential starvation of packets in lower priority queues.

•Weighted round-robin scheduling—This scheme alleviates the starvation of packets in lower priority queues by providing a certain minimum bandwidth to all queues for transmission. This bandwidth is programmable as the maximum number of packets of each CoS.

The FESMIC 10/100 Fast Ethernet interfaces default to use the strict priority-based scheduling. After system boots, you can enable weighted round-robin scheduling.

Mapping 802.1P priority to IP precedence bits is not supported.

Spanning Tree Protocol (STP)

Spanning Tree Protocol (STP) is a link management protocol that provides path redundancy while preventing undesirable loops in the network. For an Ethernet network to function properly, only one active path can exist between any two stations. When two ports on a switch are in a loop, the spanning tree port priority and port path cost setting determine which port to put in the forwarding state and which port to put in the blocking state.

The 802.1Q standard defines the method for running multiple VLANs over single or multiple physical LAN segments and defines a unique spanning tree instance to be created on each of the VLAN instances for all the VLANs in a network.

A mono spanning tree (MST) network lacks some flexibility, compared to a per VLAN spanning tree (PVST) network, which runs one instance of STP per VLAN. One spanning tree is created for every new VLAN created on a FESMIC interface. STP is enabled by default on VLAN 1 and on all newly created VLANs.

Cisco developed PVST+ to allow running several STP instances (even over an 802.1Q network) by using a tunneling mechanism. Although beyond the scope of this document, PVST+ can be briefly described as utilizing a Cisco device to connect a MST zone (typically another vendor's 802.1Q-based network) to a PVST zone (typically a Cisco 802.1Q-based network). There is no specific configuration to enter in order to achieve this. PVST+ is a spanning tree that allows the coexistence of both PVST and Shared Spanning Tree Protocol (SSTP) in a mixed vendor environment.

The STP described in IEEE 802.1D standard takes a substantial amount of time to converge to a loop free topology. It fails to take advantage of the point-to-point wiring found in modern networks. PVST is enabled on all switch platforms. Rapid Spanning Tree Protocol (RSTP), specified in IEEE 802.1w[9], improves the operation of STP, while maintaining compatibility with equipment based on the (original) 802.1d Spanning Tree standard.

Note The Cisco Shared Spanning Tree Architecture documents use the terms MST and SST to mean "Mono Spanning Tree" and "Shared Spanning Tree" respectively. The IEEE 802.1s[10] uses the same terms but with exactly opposite meanings, i.e. MST is "Multiple Spanning Trees" and SST is" Single Spanning Tree."

When you connect two Cisco switches through 802.1Q trunks, the switches exchange spanning-tree bridge packet data units (BPDUs) on each VLAN allowed on the trunks. The BPDUs on the native VLAN of the trunk are sent untagged to the reserved IEEE 802.1d spanning-tree multicast MAC address (01-80-C2-00-00-00). The BPDUs on all other VLANs on the trunk are sent tagged to the reserved Shared Spanning Tree Protocol (SSTP).

One spanning tree is created for every new VLAN that is created on the FESMIC. STP is enabled by default on VLAN 1 and on all the newly created VLANs.

PVST and PVST+ are enabled by default on the FESMIC.

For detailed information on how STP works, go to http://www.cisco.com.

Switch Virtual Interface

A Switch Virtual Interface (SVI) represents a VLAN of switch ports as one interface to the routing or bridging function in the system. Only one SVI can be associated with a VLAN, but it is necessary to configure an SVI for a VLAN only when you wish to route between VLANs, fallback-bridge nonroutable protocols between VLANs, or to provide IP host connectivity to the switch. By default, an SVI is created for the default VLAN (VLAN 1) to permit remote switch administration. Additional SVIs must be explicitly configured. In Layer 2 mode, SVIs provide IP host connectivity only to the system; in Layer 3 mode, you can configure routing across SVIs.

SVIs are created the first time that you enter the vlan interface configuration command on a VLAN interface. The VLAN corresponds to the VLAN tag associated with data frames on an ISL or 802.1Q encapsulated trunk or the VLAN ID configured for an access port. Configure a VLAN interface for each VLAN for which you want to route traffic, and assign it an IP address.

SVIs support routing protocol and bridging configurations.

Creating a SVI

To make any of the 2-port FESMIC or the 4-port FESMIC switchports routable, do the following:

Step 1 Create a VLAN ID that will be used for the VLAN.

Step 2 From the enable prompt, (not the global configuration prompt) enter the following commands:

Router#vlan database
! your prompt is now "Router(vlan)#"
Router(vlan)#vlan 7
Router(vlan)#exit

Note If you skip Step 2, your switchport virtual interface line protocol will be down.

Step 3 Go to global configuration mode and enter your switchport.

Router>conf t
Router#interface FastEthernet3/0
Router(config-if)#switchport access vlan 7

Step 4 Configure the IP address for the interface by entering the SVI

Router(config-if)#interface configuration:
Router(config-if)#interface vlan 7
Router(config-if)#ip address 7.7.7.7 255.255.255.0

The 10/100 Fast Ethernet 3/0 switchport can be pinged by through the VLAN interface. You can now attach any Layer 3 features to interface with the VLAN.

IP Multicast Layer 3 Switching

This section describes how to configure IP multicast Layer 3 switching.

You must enable IP multicast routing globally before you can enable IP multicast Layer 3 switching on Layer 3 interfaces.

For complete information and procedures, refer to these publications:

•Cisco IOS IP Configuration Guide, Release 12.2, at this URL:

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

•Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2 at this URL:

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

•Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2 at this URL:

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

•Cisco IOS IP Command Reference, Volume 3 of 3: Routing Protocols, Release 12.2 at this URL:

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

To enable IP multicast routing globally, Use this command in global configuration mode:

Command

Purpose

Router(config)# ip multicast-routing

Enables IP multicast routing globally.

Enabling IP PIM on Layer 3 Interfaces

You must enable PIM on the Layer 3 interfaces before IP multicast Layer 3 switching functions on those interfaces.

To enable IP PIM on a Layer 3 interface, use the following commands beginning in global configuration mode:

Command

Purpose

Step 1

Router(config)# interface vlan vlan_id {slot/port}

Selects the interface to be configured.

Step 2

Router(config-if)# ip pim {dense-mode | sparse-mode | sparse-dense-mode}

Enables IP PIM on a Layer 3 interface.

This example shows how to enable PIM on an interface using the default mode (sparse-dense-mode):

Router(config-if)# ip pim
Router(config-if)#

This example shows how to enable PIM sparse mode on an interface:

Router(config-if)# ip pim sparse-mode
Router(config-if)#
Verifying IP Multicast Layer 3 Hardware Switching Summary

The show ip pim interface count command verifies the IP multicast Layer 3 switching enable state on IP PIM interfaces and the number of packets received and sent on the interface.

Note The show interface statistics command does not verify hardware-switched packets, only packets switched by software.

Use the following show commands to verify IP multicast Layer 3 switching information for an IP PIM Layer 3 interface, as illustrated below:

Step 1 Enter the show ip pim interface count command.

Router# show ip pim interface count

State:* - Fast Switched, D - Distributed Fast Switched
      H - Hardware Switching Enabled
Address Interface FS Mpackets In/Out
10.15.1.20 GigabitEthernet4/8 * H 952/4237130770
10.20.1.7 GigabitEthernet4/9 * H 1385673757/34
10.25.1.7 GigabitEthernet4/10* H 0/34
10.11.1.30 FastEthernet6/26 * H 0/0
10.37.1.1 FastEthernet6/37 * H 0/0
1.22.33.44 FastEthernet6/47 * H 514/68

Step 2 Enter the show ip mroute count command.

Router# show ip mroute count
IP Multicast Statistics
56 routes using 28552 bytes of memory
13 groups, 3.30 average sources per group
Forwarding Counts:Pkt Count/Pkts per second/Avg Pkt Size/Kilobits per second
Other counts:Total/RPF failed/Other drops(OIF-null, rate-limit etc)

Group:224.2.136.89, Source count:1, Group pkt count:29051
Source:132.206.72.28/32, Forwarding:29051/-278/1186/0, Other:85724/8/56665
Router#

Note The -tive counter means that the outgoing interface list of the corresponding entry is NULL, and this indicates that this flow is still active.

Step 3 Enter the show ip interface vlan 10 command.

Router# show ip interface vlan 10
Vlan10 is up, line protocol is up
Internet address is 10.0.0.6/8
Broadcast address is 255.255.255.255
Address determined by non-volatile memory
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Multicast reserved groups joined: 224.0.0.1 224.0.0.2 224.0.0.13 224.0.0.10
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Security level is default
Split horizon is enabled
ICMP redirects are always sent
ICMP unreachables are never sent
ICMP mask replies are never sent
IP fast switching is enabled
IP fast switching on the same interface is disabled
IP Flow switching is disabled
IP CEF switching is enabled
IP Fast switching turbo vector
IP Normal CEF switching turbo vector
IP multicast fast switching is enabled
IP multicast distributed fast switching is disabled
IP route-cache flags are Fast, CEF
Router Discovery is disabled
IP output packet accounting is disabled
IP access violation accounting is disabled
TCP/IP header compression is disabled
RTP/IP header compression is disabled
Probe proxy name replies are disabled
Policy routing is disabled
Network address translation is disabled
WCCP Redirect outbound is disabled
WCCP Redirect exclude is disabled
BGP Policy Mapping is disabled
IP multicast multilayer switching is enabled
IP mls switching is enabled
Router#
Verifying the IP Multicast Routing Table

Use the show ip mroute command to verify the IP multicast routing table.

Step 1 Enter the show ip mroute command.

Router# show ip mroute 230.13.13.1

IP Multicast Routing Table
Flags:D - Dense, S - Sparse, s - SSM Group, C - Connected, L - Local,
       P - Pruned, R - RP-bit set, F - Register flag, T - SPT-bit set,
       J - Join SPT, M - MSDP created entry, X - Proxy Join Timer Running
       A - Advertised via MSDP, U - URD, I - Received Source Specific Host
           Report
Outgoing interface flags:H - Hardware switched
Timers:Uptime/Expires
Interface state:Interface, Next-Hop or VCD, State/Mode

(*, 230.13.13.1), 00:16:41/00:00:00, RP 10.15.1.20, flags:SJC
  Incoming interface:GigabitEthernet4/8, RPF nbr 10.15.1.20
  Outgoing interface list:
GigabitEthernet4/9, Forward/Sparse-Dense, 00:16:41/00:00:00, H

(*, 230.13.13.2), 00:16:41/00:00:00, RP 10.15.1.20, flags:SJC
Incoming interface:GigabitEthernet4/8, RPF nbr 10.15.1.20, RPF-MFD
  Outgoing interface list:
    GigabitEthernet4/9, Forward/Sparse-Dense, 00:16:41/00:00:00, H

(10.20.1.15, 230.13.13.1), 00:14:31/00:01:40, flags:CJT
Incoming interface:GigabitEthernet4/8, RPF nbr 10.15.1.20, RPF-MFD
  Outgoing interface list:
    GigabitEthernet4/9, Forward/Sparse-Dense, 00:14:31/00:00:00, H
(132.206.72.28, 224.2.136.89), 00:14:31/00:01:40, flags:CJT
  Incoming interface:GigabitEthernet4/8, RPF nbr 10.15.1.20, RPF-MFD
Outgoing interface list:Null
Router#

Note The RPF-MFD flag indicates that the flow is completely hardware switched. The H flag indicates that the flow is hardware-switched on the outgoing interface.

Storm Control

A packet storm occurs when a large number of broadcast, unicast, or multicast packets are received on a port. Forwarding these packets can cause the network to slow down or to time out. Storm control is configured for the switch as a whole, although it operates on a per-interface basis. By default, storm control is disabled.

Storm control prevents switch ports on a LAN from being disrupted by a broadcast, multicast, or unicast storm on one of the interfaces. A LAN storm occurs when packets flood the LAN, creating excessive traffic and degrading network performance. Errors in the protocol-stack implementation or in the network configuration can cause a storm.

Storm control monitors incoming traffic statistics over a time period and compares the measurement with a predefined suppression level threshold. The threshold represents the percentage of the total available bandwidth of the port. If the threshold of a traffic type is reached, further traffic of that type is suppressed until the incoming traffic falls below the threshold level.

The graph in Figure 6 shows broadcast traffic patterns on an interface over a given period of time. In this example, the broadcast traffic exceeded the configured threshold between time intervals T1 and T2 and between intervals T4 and T5. When the amount of specified traffic exceeds the threshold, all traffic of that kind is dropped. Therefore, broadcast traffic is blocked during those intervals. At the next time interval, if broadcast traffic does not exceed the threshold, it is again forwarded.

Figure 6 Broadcast Suppression Example

When storm control is enabled, the switch monitors the packets that are passing from an interface to the switching bus and determines whether the packet is unicast, multicast, or broadcast. The switch monitors the number of broadcast, multicast, or unicast packets received within the 1-second time interval, and when a threshold for one type of traffic is reached, that type of traffic is dropped. This threshold is specified as a percentage of the total available bandwidth that can be used by broadcast (multicast or unicast) traffic.

The combination of broadcast suppression threshold numbers and the 1-second time interval control the way the suppression algorithm works. A higher threshold allows more packets to pass through. A threshold value of 100 percent means that no limit is placed on the traffic.

Note Because packets do not arrive at uniform intervals, the 1-second time interval during which traffic activity is measured can affect the behavior of storm control.

The switch continues to monitor traffic on the port. When the utilization level falls back below the threshold level, the type of traffic that was dropped is forwarded again.

Use the storm-control broadcast, storm-control multicast, and storm-control unicast global configuration commands to set up the storm control threshold value.

Storm Control Configuration

This section describes how to configure storm control on your router. It consists of the following configuration information and procedures:

• Enabling Storm Control

• Verifying Storm Control

By default, unicast, broadcast, and multicast suppression is disabled on the switch.

Enabling Storm Control

Enable storm-control globally and enter the percentage of total available bandwidth that you want to be used by a all traffic (multicast, unitcast,); entering 100 percent would allow all traffic.

To enable a particular type of storm-control, use the following commands beginning in privileged EXEC mode:

Command

Purpose

Step 1

Router# configure terminal

Enters global configuration mode.

Step 2

Router(config)# [no] storm-control broadcast threshold <0-100>

Specifies the broadcast suppression level for an interface as a percentage of total bandwidth. A threshold value of 100 percent means that no limit is placed on broadcast traffic.

Use the no keyword to restore the defaults.

Step 3

Router(config)# [no] storm-control multicast threshold <0-100>

Specifies the multicast suppression level for an interface as a percentage of total bandwidth.

Use the no keyword to restore the defaults.

Step 4

Router(config)# [no] storm-control unicast threshold <0-100>

Specifies the unicast suppression level for an interface as a percentage of total bandwidth.

Use the no keyword to restore the defaults.

Step 5

Router(config)# end

Returns to privileged EXEC mode.

Verifying Storm Control

Use the show storm-control command to view switch port characteristics, including the storm control levels set on the interface.

To verify storm-control statistics on an interface, use the following commands, beginning in privileged EXEC mode:

Command

Purpose

show interface [interface-id] counters broadcast

Verifies the broadcast suppression discard counter for all interfaces or a specific interface. Verify the number of packets discarded.

show interface [interface-id] counters multicast

Verifies the multicast suppression discard counter for all interfaces or a specific interface. Verify the number of packets discarded.

show interface [interface-id] counters unicast

Verifies the unicast suppression discard counter for all interfaces or a specific interface. Verify the number of packets discarded.

IGMP Snooping

Internet Group Management Protocol (IGMP) snooping allows the switch to "listen in" on the IGMP conversation between hosts and routers. When a switch "hears" an IGMP report from a host for a given multicast group, the switch adds the host's port number to the Group Destination Address (GDA) list for that group. And, when the switch hears an IGMP leave, it removes the host's port from the content-addressable memory (CAM) table entry.

The purpose of IGMP snooping is to restrain multicast traffic in a switched network. By default, a LAN switch floods multicast traffic within the broadcast domain, and this can consume a lot of bandwidth if many multicast servers are sending streams to the segment.

Multicast traffic is flooded because a switch usually learns MAC addresses by looking into the source address field of all the frames it receives. But, since a multicast MAC address is never used as source address for a packet and since the addresses do not appear in the MAC address table, the switch has no method for learning the addresses.

IGMP Snooping Configuration

IGMP snooping is enabled by default on a VLAN. Multicast routing has to be enabled on the router first and then PIM (Multicast routing protocol) has to be enabled on the VLAN interface so that the switch acknowledges the IGMP join and leave messages which are sent from the hosts connected to the switch. For example:

Router(config)# ip multicast-routing
Router(config-if)# interface VLAN1
ip-address 192.168.10.1 255.255.255.0
ip pim sparse-mode

To verify multicasting support, use the show ip igmp group command:

Router# show ip igmp group

To verify IGMP snooping, use the show mac-address-table multicast igmp-snooping command:

Router# show mac-address-table multicast igmp-snooping

To verify the multicast routing table, use the show ip mroute command:

Router# sh ip mroute

	Command	Purpose
Step 1	`Router(config)# interface vlan vlan_id {slot/port}`	Selects the interface to be configured.
Step 2	`Router(config-if)# ip pim {dense-mode \| sparse-mode \| sparse-dense-mode}`	Enables IP PIM on a Layer 3 interface.

	Command	Purpose
Step 1	Router# configure terminal	Enters global configuration mode.
Step 2	Router(config)# [no] storm-control `broadcast` threshold <0-100`>`	Specifies the broadcast suppression level for an interface as a percentage of total bandwidth. A threshold value of 100 percent means that no limit is placed on broadcast traffic. Use the no keyword to restore the defaults.
Step 3	Router(config)# [no] storm-control `multicast threshold` <0-100`>`	Specifies the multicast suppression level for an interface as a percentage of total bandwidth. Use the no keyword to restore the defaults.
Step 4	Router(config)# [no] storm-control `unicast` threshold <0-100`>`	Specifies the unicast suppression level for an interface as a percentage of total bandwidth. Use the no keyword to restore the defaults.
Step 5	Router(config)# end	Returns to privileged EXEC mode.

Command	Purpose
`show interface` [interface-id] counters broadcast	Verifies the broadcast suppression discard counter for all interfaces or a specific interface. Verify the number of packets discarded.
`show interface` [interface-id] counters multicast	Verifies the multicast suppression discard counter for all interfaces or a specific interface. Verify the number of packets discarded.
`show interface` [interface-id] counters unicast	Verifies the unicast suppression discard counter for all interfaces or a specific interface. Verify the number of packets discarded.