Unicast frame

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

Unicast is one-to-one network communication directed from one sender to one destination.

A unicast frame carries a network message from one sender to one destination host. Ethernet switches use the destination MAC address to decide where to send it. section 6.3.2

A switch handles unicast frames in two ways:

• Known unicast: the destination MAC address exists in the MAC address table, so the switch forwards the frame through the associated port. section 6.3.2
• Unknown unicast: the destination MAC address is absent, so the switch floods the frame through every port except the receiving port. section 6.3.2
• Switches learn source MAC addresses from received frames, then use those dynamic entries to forward later traffic more efficiently. section 6.6

Figure 6.4 PC1 sends a unicast frame to PC3, but neither SW1 nor SW2 have an entry for the destination MAC address in their MAC address table. (1) PC1 sends the frame, and SW1 learns PC1’s MAC address. (2) SW1 floods the frame. SW2 learns PC1’s MAC address. PC2 drops the frame. (3) SW2 floods the frame. PC4 drops the frame. PC3 receives and processes it.

Trace an unknown unicast from PC1 to PC3 as switches learn the source address, flood the frame, and deliver it to the destination. section 6.3.2

Why the first delivery may flood

When PC1 sends to PC3 before the switches know PC3’s MAC address, SW1 and SW2 flood the frame. PC2 and PC4 discard it; PC3 processes it. PC3’s reply lets both switches learn its MAC address, enabling direct forwarding afterward. section 6.3.2 section 6.6

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes

Defines known and unknown unicast behavior, explains MAC learning, and stops short of frame-format internals.

Examples

Uses PC1-to-PC2 known-unicast forwarding and PC1-to-PC3 unknown-unicast flooding across two switches.

What you can do after

Diagnose forwarding behavior with the show mac address-table command and distinguish forwarding from flooding.

How it connects

Connects unicast frames with destination MAC addresses, MAC address tables, MAC address learning, flooding, and ARP.

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Related concepts

• known unicast frameincludes · A known unicast frame is a unicast frame whose destination MAC address has an entry in the receiving switch's MAC address table.
• unknown unicast frameincludes · An unknown unicast frame is a unicast frame whose destination MAC address has no entry in the receiving switch's MAC address table.
• link-local addressincludes · An IPv6 link-local address is an IPv6 unicast address valid only for communication between devices on the same local network segment.
• anycastincludes · Anycast is a networking method in which one address is assigned to multiple endpoints and traffic is routed to one endpoint according to network routing.
• unique local addressused by · A unique local address is an IPv6 private unicast address intended for use within internal networks and not for communication over the public internet.
• neighbor advertisementincludes · A Neighbor Advertisement is an ICMPv6 message that communicates a neighbor's link-layer address or indicates that an IPv6 address is already in use.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 1 Fundamentals and Protocols. The section links point to the parts of the book it draws on.