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My apologies if this is not the right forum to ask this, but I am a newbie when it comes to networking. I am currently going through the CISCO Networking Essentials course, and one of the Activities in the second module left me confused. Basically computers with MAC addresses 0E and 0F are both connected to a hub, which connects to a single Layer 2 switch port (Fa9). The question starts with the assumption that a MAC address table entry already exists for 0E (Fa9) but none yet for 0F. If computer with MAC 0E were to address a frame to computer with MAC 0F, how would that ever reach its destination? As I understand it, the switch, not "seeing" 0F in its table would flood the frame out all connected ports except Fa9 (incoming), but that means the frame will never exit via Fa9, so how is computer with MAC 0F ever supposed to receive the frame? Thanks in advance for your help. asked Apr 26 at 3:56
A hub simply repeats electric signals received on a one interface out of all other interfaces. So device 0F, as well as the switch, will receive the frame. In that case the switch will not forward the frame out the same port it received it on, but it doesn't matter since 0F already received it. Regarding learning of 0F - The switch will eventually learn 0F's address once 0F sends a frame. answered Apr 26 at 4:14
manish mamanish ma 1,4968 silver badges15 bronze badges 2 Layer 2 Learning and Forwarding for VLANs Overview
Understanding Layer 2 Forwarding Tables on Switches, Routers and NFX Series DevicesYou can configure Layer 2 MAC address and VLAN learning and forwarding properties in support of Layer 2 bridging. Unicast media access control (MAC) addresses are learned to avoid flooding the packets to all the ports in a VLAN. A source MAC entry is created in its source and destination MAC tables for each MAC address learned from packets received on ports that belong to the VLAN. When you configure a VLAN, Layer 2 address learning is enabled by default. The VLAN learns unicast media access control (MAC) addresses to avoid flooding the packets to all the ports in the VLAN. Each VLAN creates a source MAC entry in its source and destination MAC tables for each source MAC address learned from packets received on the ports that belong to the VLAN. Note: Traffic is not flooded back onto the interface on which it was received. However, because this “split horizon” occurs at a late stage, the packet statistics displayed by commands such as You can optionally disable MAC learning either for the entire device or for a specific VLAN or logical interface. You can also configure the following Layer 2 learning and forwarding properties:
Understanding Layer 2 Forwarding Tables on Security DevicesThe SRX Series device maintains forwarding tables that contain MAC addresses and associated interfaces for each Layer 2 VLAN. When a packet arrives with a new source MAC address in its frame header, the device adds the MAC address to its forwarding table and tracks the interface at which the packet arrived. The table also contains the corresponding interface through which the device can forward traffic for a particular MAC address. If the destination MAC address of a packet is unknown to the device (that is, the destination MAC address in the packet does not have an entry in the forwarding table), the device duplicates the packet and floods it on all interfaces in the VLAN other than the interface on which the packet arrived. This is known as packet flooding and is the default behavior for the device to determine the outgoing interface for an unknown destination MAC address. Packet flooding is performed at two levels: packets are flooded to different zones as permitted by configured Layer 2 security policies, and packets are also flooded to different interfaces with the same VLAN identifier within the same zone. The device learns the forwarding interface for the MAC address when a reply with that MAC address arrives at one of its interfaces. You can specify that the SRX Series device use ARP queries and traceroute requests (which are ICMP echo requests with the time-to-live values set to 1) instead of packet flooding to locate an unknown destination MAC address. This method is considered more secure than packet flooding because the device floods ARP queries and traceroute packets—not the initial packet—on all interfaces. When ARP or traceroute flooding is used, the original packet is dropped. The device broadcasts an ARP or ICMP query to all other devices on the same subnetwork, requesting the device at the specified destination IP address to send back a reply. Only the device with the specified IP address replies, which provides the requestor with the MAC address of the responder. ARP allows the device to discover the destination MAC address for a unicast packet if the destination IP address is in the same subnetwork as the ingress IP address. (The ingress IP address refers to the IP address of the last device to send the packet to the device. The device might be the source that sent the packet or a router forwarding the packet.) Traceroute allows the device to discover the destination MAC address even if the destination IP address belongs to a device in a subnetwork beyond that of the ingress IP address. When you enable ARP queries to locate an unknown destination MAC address, traceroute requests are also enabled. You can also optionally specify that traceroute requests not be used; however, the device can then discover destination MAC addresses for unicast packets only if the destination IP address is in the same subnetwork as the ingress IP address. Whether you enable ARP queries and traceroute requests or ARP-only queries to locate unknown destination MAC addresses, the SRX Series device performs the following series of actions:
Layer 2 Learning and Forwarding for VLANs Acting as a Switch for a Layer 2 Trunk PortLayer 2 learning is enabled by default. A set of VLANs, configured to function as a switch with a Layer 2 trunk port, learns unicast media access control (MAC) addresses to avoid flooding packets to the trunk port. Note: Traffic is not flooded back onto the interface on which it was received. However, because this “split horizon” occurs at a late stage, the packet statistics displayed by commands such as You can optionally disable Layer 2 learning for the entire set of VLANs as well as modify the following Layer 2 learning and forwarding properties:
Understanding the Unified Forwarding Table
Benefits of Unified Forwarding TablesTraditionally, forwarding tables have been statically defined and have supported only a fixed number of entries for each type of address. The unified forwarding table (UFT) provides the following benefits:
Using the Unified Forwarding Table to Optimize Address StorageOn the QFX5100, EX4600, EX4650, QFX5110, QFX5200, and QFX5120 switches, you can control the allocation of forwarding table memory available to store the following:
UFT essentially combines the three distinct forwarding tables to create one table with flexible resource allocation. You can select one of five forwarding table profiles that best meets your network needs. Each profile is configured with different maximum values for each type of address. For example, for a switch that handles a great deal of Layer 2 traffic, such as a virtualized network with many servers and virtualized machines, you would likely choose a profile that allocates a higher percentage of memory to MAC addresses. For a switch that operates in the core of a network, participates in an IP fabric, you probably want to maximize the number of routing table entries it can store. In this case, you would choose a profile that allocates a higher percentage of memory to longest match prefixes. The QFX5200 switch supports a custom profile that allows you to partition the four available shared memory banks with a total of 128,000 entries among MAC addresses, Layer 3 host addresses, and LPM prefixes. Note: Support for QFX5200 switches was introduced in Junos OS Release 15.1x53-D30. The QFX5200 switch is not supported on Junos OS Release 16.1R1. Understanding the Allocation of MAC Addresses and Host AddressesAll five profiles are supported, each of which allocates different amounts of memory for Layer 2 or Layer 3 entries, enabling you choose one that best suits the needs of your network. The QFX5200 and QFX5210 switches, however, supports different maximum values for each profile from the other switches. For more information about the custom profile, see Configuring the Unified Forwarding Table on Switches. Note: The default profile is Note: Starting with Junos OS Release 18.1R1 on the QFX5210-64C switch, for all these profiles, except for the Note: Starting with Junos OS Release 18.3R1 on the QFX5120 and EX4650 switches, for all these profiles, except for the Note: On QFX5100, EX4600, EX4650, QFX5110, QFX5200, QFX5120, and QFX5210-64C switches, IPv4 and IPv6 host routes with ECMP next hops are stored in the host table. Best Practice: If the host or LPM table stores the maximum number of entries for any given type of entry, the entire shared table is full and is unable to accommodate any entries of any other type. Different entry types occupy different amounts of memory. For example, an IPv6 unicast address occupies twice as much memory as an IPv4 unicast address, and an IPv6 multicast address occupies four times as much memory as an IPv4 unicast address. Table 1 lists the profiles you can choose and the associated maximum values for the MAC address and host table entries on QFX5100 and EX4600 switches. Table 1: Unified Forwarding Table Profiles on QFX5100 and EX4600 Switches
Table 2 lists the profiles you can choose and the associated maximum values for the MAC address and host table entries on QFX5110 switches. Table 2: Unified Forwarding Table Profiles on QFX5110 Switches
Table 3 lists the LPM table size variations for the QFX5110 switch depending on the prefix entries. Table 3: LPM Table Size Variations on QFX5110 Switches
Table 4 lists the profiles you can choose and the associated maximum values for the MAC address and host table entries on QFX5200-32C switches. Table 4: Unified Forwarding Table Profiles on QFX5200-32C Switches
Table 5 lists the profiles you can choose and the associated maximum values for the MAC address and host table entries on QFX5200-48Y switches. Table 5: Unified Forwarding Table Profiles on QFX5200-48Y Switches
Table 6 lists the LPM table size variations for the QFX5200-48Y switch depending on the prefix entries. Table 6: LPM Table Size Variations on QFX5200-48Y Switches
Table 7 lists the profiles you can choose and the associated maximum values for the MAC address and host table entries on QFX5210-64C switches. Table 7: Unified Forwarding Table Profiles on QFX5210-64C Switches
Table 8 lists the profiles you can choose and the associated maximum values for the MAC address and host table entries on QFX5120 and EX4650 switches. Table 8: Unified Forwarding Table Profiles on QFX5120 and EX4650 Switches
Table 9 lists the LPM table size variations for the QFX5210-64C switch depending on the prefix entries. Table 9: LPM Table Size Variations on QFX5210-64C Switches
Table 10 lists the Layer 3 Defip table size variations for the QFX5120 and EX4650 switches depending on the changing IPv6/128 prefix entries. Table 10: LPM Table Size Variations on QFX5210-64C and EX4650 Switches
Unified Forwarding Table Profiles on QFX5130 and QFX5700 Switches for Junos OS Evolved ReleasesYou can configure a forwarding-profile for the unified forwarding table on QFX5130 and QFX5700 switches using the user@switch#set system packet-forwarding-options forwarding-profile ? Possible completions: + apply-groups Groups from which to inherit configuration data + apply-groups-except Don't inherit configuration data from these groups default-profile MAC: 32K L3-host: 32K LPM: 720K FP-Compression: 18K, restarts PFE host-acl-profile MAC: 160K L3-host: 160K LPM: 65K FP-Compression: 18K, restarts PFE host-profile MAC: 160K L3-host: 160K LPM: 72K FP-Compression: 0, restarts PFE lpm-profile MAC: 32K L3-host: 32K LPM: 1.24M ARP: 61K FP-Compression: 0 Tunnels: 0, restarts PFE Table 11: Unified Forwarding Table Profiles on QFX5130 and QFX5700 Switches
Note:
Understanding Ternary Content Addressable Memory (TCAM) and Longest Prefix Match EntriesYou can further customize non-LPM profiles by configuring the space available for ternary content addressable memory (TCAM) to allocate more memory for longest prefix match entries. You can change the number of entries allocated to these IPv6 addresses, essentially allocating more or less space for LPM IPv4 entries with any prefix length or IPv6 entries with prefix lengths of 64 of shorter. For more information about how to change the default parameters of the TCAM memory space for LPM entries, see Configuring the Unified Forwarding Table on Switches. Note: The option to adjust TCAM space is not supported on the longest prefix match (LPM) or custom profiles. However, for the LPM profile, you can configure TCAM space not to allocate any memory for IPv6 entries with prefix lengths of 65 or longer, thereby allocating that memory space only for IPv4 routes or IP routes with prefix lengths equal to or less than 64 or a combination of the two types of prefixes. Note: Starting with Junos OS Release 18.1R1 on QFX5210 switches, you can configure TCAM space to allocate a maximum of 8,000 IPv6 entries with prefix lengths of 65 or longer. The default value is 2,000 entries. Starting with Junos OS Release 13.2X51-D15, you can configure TCAM space to allocate a maximum of 4,000 IPv6 entries with prefix lengths of 65 or longer. The default value is 1,000 entries. Previous to Junos OS Release 13.2X51-D15, you could allocate only a maximum of 2,048 entries for IPv6 the IPv6 prefixes with lengths in the range /65 to /127 range. The default value was 16 entries for these types of IPv6 prefixes. On Junos OS Releases 13.2x51-D10 and 13.2x52D10, the procedure to change the default value of 16 entries differs from later releases, where the maximum and default values are higher. For more information about that procedure, see Configuring the Unified Forwarding Table on Switches Host Table Example for Profile with Heavy Layer 2 Traffic Table 12 lists various valid combinations that the host table can store if you use the Table 12: Example Host Table Combinations Using l2-profile-one on QFX5100 and EX4600 Switches
Example: Configuring a Unified Forwarding Table Custom ProfileTraditionally, forwarding tables have been statically defined and have supported only a fixed number of entries for each type of address. The Unified Forwarding Table (UFT) feature enables you to optimize how forwarding-table memory is allocated to best suit the needs of your network. This example shows how to configure a Unified Forwarding Table profile that enables you to partition four shared hash memory banks among three different types of forwarding-table entries: MAC addresses, Layer 3 host addresses, and longest prefix match (LPM). The UFT feature also supports five profiles that each allocate a specific maximum amount of memory for each type of forwarding table entry. Some profiles allocate more memory to Layer 2 entries, while other profiles allocate more memory to Layer 3 or LPM entries. The maximum values for each type of entry are fixed in these profiles. With the custom profile, you can designate one or more shared memory banks to store a specific type of forwarding-table entry. You can configure as few as one or as many as four memory banks in a custom profile. The custom profile thus provides even more flexibility in enabling you to allocate forwarding-table memory for specific types of entries.
RequirementsThis example uses the following hardware and software components:
Before you configure a custom profile, be sure you have:
OverviewThe Unified Forwarding Table custom profile enables you to allocate forwarding-table entries among four banks of shared hash tables with a total memory equal to 128,000 unicast IPv4 addresses, or 32,000 entries for each bank. Specifically, you can allocate one or more of these shared banks to store a specific type of forwarding-table entry. The custom profile does not affect the dedicated hash tables. Those tables remain fixed with 8,000 entries allocated to Layer 2 addresses, the equivalent of 8,000 entries allocated to IPv4 addresses, and the equivalent of 16,000 entries allocated to longest prefix match (LPM) addresses. In this example, you allocate two memory banks to Layer 3 host addresses, and two memory banks to LPM entries. This means that no shared hash table memory is allocated for Layer 2 addresses. Only the dedicated hash table memory is allocated for Layer 2 addresses in this scenario. ConfigurationTo configure a custom profile for the Unified Forwarding Table feature on a QFX5200 switch that allocates two shared memory banks for Layer 3 host address and two shared memory banks for LPM entries, perform these tasks:
CLI Quick ConfigurationTo
quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the CAUTION: When you configure and commit a profile, the Packet Forwarding Engine restarts and all the data interfaces on the switch go down and come back up. user@switch# set chassis forwarding-options custom-profile user@switch# set chassis forwarding-options custom-profile l2-entries num-banks 0 user@switch# set chassis forwarding-options custom-profile l3-entries num-banks 2 user@switch# set chassis forwarding-options custom-profile lpm-entries num-banks 2 Configuring the Custom ProfileStep-by-Step ProcedureTo create the custom profile:
Configuring the Allocation of Shared Memory BanksStep-by-Step ProcedureTo allocate memory for specific types of entries for the shared memory banks:
ResultsFrom configuration mode, confirm your configuration by entering the show chassis forwarding-options command. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration. user@switch# show chassis forwarding-profile custom-profile { l2-entries { num-banks 0; } l3-entries { num-banks 2; } lpm-entries { num-banks 2 } } If you are done configuring the switch, enter CAUTION: The Packet Forwarding Engine will restart and all the data interfaces on the switch will go down and come back up. VerificationConfirm that the configuration is working properly. Checking the Parameters of the Custom Profile
PurposeVerify that the custom profile is enabled. Actionuser@switch> show chassis forwarding-options UFT Configuration: custom-profile Configured custom scale: Entry type Total scale(K) L2(mac) 8 L3 (unicast & multicast) 72 Exact Match 0 Longest Prefix Match (lpm) 80 num-65-127-prefix = 1K -------------Bank details for various types of entries------------ Entry type Dedicated Bank Size(K) Shared Bank Size(K) L2 (mac) 8 32 * num shared banks L3 (unicast & multicast 8 32 * num shared banks Exact match 0 16 * num shared banks Longest Prefix match(lpm) 16 32 * num shared banks MeaningThe output shows that the custom profile is enabled as configured with two shared memory banks designated for Layer 3 host entries; two shared memory banks designated for LPM entries; and no shared memory allocated for Layer 2 entries. The total scale(K) field shows the total allocation of memory, that is, the amount allocated through the shared memory banks plus the amount allocated through the dedicated hash tables. The amount allocated through the dedicated hash tables is fixed and cannot be changed. Therefore, Layer 2 entries have 8K of memory allocated only through the dedicated hash table. Layer 3 host entries have 64K of memory allocated through two shared memory banks plus 8K through the dedicated hash table, for a total of 72K of memory. LPM entries have 64K of memory allocated through two shared memory banks plus 16K through the dedicated hash table, for a total of 80K of memory. Configuring the Unified Forwarding Table on SwitchesTraditionally, forwarding tables have been statically defined and have supported only a fixed number of entries for each type of address stored in the tables. The Unified Forwarding Table feature lets you optimize how your switch allocates forwarding-table memory for different types of addresses. You can choose one of five unified forwarding table profiles. Each profile allocates a different maximum amount of memory for Layer 2, Layer 3 host, and longest prefix match (LPM) entries. In addition to selecting a profile, you can also select how much additional memory to allocate for LPM entries. Two profiles allocate higher percentages of memory to Layer 2 addresses. A third profile allocates a higher percentage of memory to Layer 3 host address, while a fourth profile allocates a higher percentage of memory to LPM entries. There is a default profile configured that allocates an equal amount of memory to Layer 2 and Layer 3 host addresses with the remainder allocated to LPM entries. For a switch in a virtualized network that handles a great deal of Layer 2 traffic, you would choose a profile that allocates a higher percentage of memory to Layer 2 addresses. For a switch that operates in the core of the network, you would choose a profile that allocates a higher percentage of memory to LPM entries. On QFX5200 and QFX5210-64C switches only, you can also configure a custom profile that allows you to partition shared memory banks among the different types of forwarding table entries. On QFX5200 switches, these shared memory banks have a total memory equal to 128,000 IPv4 unicast addresses. On QFX5210 switches, these shared memory banks have a total memory equal to 256,000 IPv4 unicast addresses. For more information about configuring the custom profile, see Example: Configuring a Unified Forwarding Table Custom Profile.
Configuring a Unified Forwarding Table ProfileTo configure a unified forwarding table profile: Specify a forwarding-table profile. [edit chassis forwarding-options] user@switch# set profile-name For example, to specify the profile that allocates the highest percentage of memory to Layer 2 traffic: [edit chassis forwarding-options] user@switch# set l2-profile-one CAUTION: When you configure and commit a profile, in most cases the Packet Forwarding Engine automatically restarts and all the data interfaces on the switch go down and come back up (the management interfaces are unaffected). Starting with Junos OS Releases 14.1X53-D40, 15.1R5, and 16.1R3, for a Virtual Chassis or Virtual Chassis Fabric (VCF) comprised of EX4600 or QFX5100 switches, the Packet Forwarding Engine in member switches does not automatically restart upon configuring and committing a unified forwarding table profile change. This behavior avoids Virtual Chassis or VCF instability after the change propagates to member switches and multiple Packet Forwarding Engines automatically restart at the same time. Instead, a message is displayed at the CLI prompt and logged to the switch’s system log to notify you that the profile change does not take effect until the next time you reboot the Virtual Chassis or VCF. We recommend that you plan to make profile changes only when you can perform a Virtual Chassis or VCF system reboot immediately after committing the configuration update. Otherwise, the Virtual Chassis or VCF could become inconsistent if one or more members have a problem and restart with the new configuration before a planned system reboot activates the change on all members. Note: You can configure only one profile for the entire switch. Note: The Note: If the host table stores the maximum number of entries for any given type, the entire table is full and is unable to accommodate any entries of any other type. Keep in mind that an IPv6 unicast address occupies twice as much memory as an IPv4 unicast address, and an IPv6 multicast address occupies four times as much memory as an IPv4 unicast address.. Configuring the Memory Allocation for Longest Prefix Match EntriesIn addition to choosing a profile, you can further optimize memory allocation for longest prefix match (LPM) entries by configuring how many IPv6 prefixes to store with lengths from /65 through /127. The switch uses LPM entries during address lookup to match addresses to the most-specific (longest) applicable prefix. Prefixes of this type are stored in the space for ternary content addressable memory (TCAM). Changing the default parameters makes this space available for LPM entries. Increasing the amount of memory available for these IPv6 prefixes reduces by the same amount how much memory is available to store IPv4 unicast prefixes and IPv6 prefixes with lengths equal to or less than 64. The procedures for configuring the LPM table are different, depending on which version of Junos OS you are using. In the initial releases that UFT is supported, Junos OS Releases 13.2X51-D10 and 13.2X52-10, you can only increase the amount of memory
allocated to IPv6 prefixes with lengths from /65 through /127 for any profile, except for
Configuring the LPM Table With Junos OS Releases 13.2X51-D10 and 13.2X52-D10In Junos OS Releases 13.2x51-D10 and 13.2X52-D10, by default, the switch allocates memory for 16 IPv6 with prefixes with lengths in the range /65 through /127. You can configure the switch to allocate more memory for IPv6 prefixes with lengths in the range /65 through /127. To allocate more memory for IPv6 prefixes in the range /65 through /127:
Note: When you configure and commit the The Configuring the LPM Table With Junos OS Release 13.2x51-D15 and Later
Configuring Layer 2 and Layer 3 Profiles With Junos OS Release 13.2x51-D15 or LaterStarting in Junos OS Release 13.2X51-D15, you can configure the switch to
allocate forwarding table memory for as many as 4,000 IPv6 prefixes with lengths in the range /65 through /127 for any profile other than the To specify how much forwarding table memory to allocate for IPv6 prefixes with length in the range /65 through /127:
Starting with Junos OS Release 13.2X51-D15, you can use the Table 13: LPM Table Combinations for L2 and L3 profiles With Junos OS 13.2X51-D15 and Later
CAUTION: When you configure and commit a profile change with the However, starting with Junos OS Releases 14.1X53-D40, 15.1R5, and 16.1R3, Packet Forwarding Engines on switches in a Virtual Chassis or Virtual Chassis Fabric (VCF) do not automatically restart upon configuring a unified forwarding table profile change. This behavior avoids Virtual Chassis or VCF instability after the change propagates to member switches and multiple Packet Forwarding Engines automatically restart at the same time. Instead, a message is displayed at the CLI prompt and logged to the switch’s system log to notify you that the profile change does not take effect until the next time you reboot the Virtual Chassis or VCF. We recommend that you plan to make profile changes only when you can perform a Virtual Chassis or VCF system reboot immediately after committing the configuration update. Otherwise, the Virtual Chassis or VCF could become inconsistent if one or more members have a problem and restart with the new configuration before a planned system reboot activates the change on all members. Configuring the lpm-profile With Junos OS Release 13.2x51-D15 and LaterStarting with Junos OS Release 13.2X51-D15 you can configure the
Note: The memory allocated for each address type represents the maximum default value for all LPM memory. To configure the Specify to disable forwarding-table memory for IPv6 prefixes with lengths in the range /65 through /127. [edit chassis forwarding-options lpm-profile] user@switch# set prefix-65-127-disable
For example, on the QFX5100 and EX4600 switches only, if you use the
Note: On the QFX5200 switches, when you configure the
Configuring the lpm-profile With Junos OS Release 14.1x53-D30 and LaterStarting in Junos OS Release 15.1X53-D30, you can configure the Table 14: lpm-profile with unicast-in-lpm Option for QFX5100 and EX4600 Switches
Starting with Junos Release 18.1R1, you cannot set configure a prefix for the Table 15 lists the situations in which the Table 15: LPM Table Size Variations on QFX5200-48Y Switches
On QFX5120 and EX4600 switches, you cannot set configure a prefix for the Table 16 lists the situations in which the Table 16: LPM Table Size Variations on QFX5120 and EX4650 Switches
Note that all entries in each table share the same memory space. If a table stores the maximum number of entries for any given type, the entire shared table is full and is unable to accommodate any entries of any other type. For example, if you use the the To configure the
Configuring Non-LPM Profiles on QFX5120 and EX4650 SwitchesFor non-LPM profiles, each profile provides the option of reserving a portion of the 16K L3-defip table to store IPv6 Prefixes > 64. Because these are 128-bit prefixes, you can have maximum of 8k IPv6/128 entries in the l3-defip table.
Configuring Forwarding Mode on SwitchesBy default, packets packets are forwarded using store-and-forward mode. You can configure all the interfaces to use cut-through mode instead. To enable cut-through switching mode, enter the following statement: [edit forwarding-options] user@switch# set cut-through Disabling Layer 2 Learning and ForwardingDisabling dynamic MAC learning on an MX Series router or an EX Series switch prevents all the logical interfaces on the router or switch from learning source and destination MAC addresses. To disable MAC learning for an MX Series router or an EX Series switch, include the [edit protocols l2-learning] global-no-mac-learning; For information about how to configure a virtual switch, see Configuring a Layer 2 Virtual Switch . Release History Table 18.1R1 Starting with Junos OS Release 18.1R1 on the QFX5210-64C switch, for all these profiles, except for the 18.1R1 Starting with Junos OS Release 18.3R1 on the QFX5120 and EX4650 switches, for all these profiles, except for the 18.1R1 Starting with Junos OS Release 18.1R1 on QFX5210 switches, you can configure TCAM space to allocate a maximum of 8,000 IPv6 entries with prefix lengths of 65 or longer. The default value is 2,000 entries. 18.1R1 Starting with Junos Release 18.1R1, you cannot set configure a prefix for the 14.1X53-D40 Starting with Junos OS Releases 14.1X53-D40, 15.1R5, and 16.1R3, for a Virtual Chassis or Virtual Chassis Fabric (VCF) comprised of EX4600 or QFX5100 switches, the Packet Forwarding Engine in member switches does not automatically restart upon configuring and committing a unified forwarding table profile change. 13.2X51-D15 Starting with Junos OS Release 13.2X51-D15, you can configure TCAM space to allocate a maximum of 4,000 IPv6 entries with prefix lengths of 65 or longer. The default value is 1,000 entries. 13.2X51-D15 Starting with Junos OS Release 13.2X51-D15, you can also allocate either less or no memory for IPv6 prefixes with lengths in the range /65 through /127, depending on which profile is configured. 13.2X51-D15 Starting in Junos OS Release 13.2X51-D15, you can configure the switch to allocate forwarding table memory for as many as 4,000 IPv6 prefixes with lengths in the range /65 through /127 for any profile other than the 13.2X51-D15 Starting with Junos OS Release 13.2X51-D15, you can use the What does a layer 2 switch do when the destination MAC address of the received frame is not in the MAC table?Find the Destination MAC Address
If the destination MAC address is not in the table, the switch forwards the frame out all ports except the incoming port.
What will a layer 2 switch do when the destination MAC address of a received frame?If there is no entry for the destination host then switch will first learn the Mac address of the source host and then flood the frame through all its ports except the port on which the frame is received but if there is an entry for the destination host in Mac table of the switch then it will be unicast.
What happens when a switch receives a frame and the destination MAC address isn't in the table?When a frame enters the switch and the destination MAC addresses is unknown in the switch's MAC address table, the Switch will flood or forward copies of the frame out all ports, except the port on which the frame was received. On the other end, the unknown device receives the frame and sends a reply.
What does a layer 2 switch do with a received frame?A layer 2 switch is primarily responsible for transporting data on a physical layer and in performing error checking on each transmitted and received frame. A layer 2 switch requires MAC address of NIC on each network node to transmit data.
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