+(65) 8344 4290 Ciscodumps.net@gmail.com Room 907, Block B, Baoneng Entrepreneurship Center, Guangrong Road, Hongqiao District, Tianjin

ThinkMo EDU Share – network 63.Handling of Unknown Data Frames by Switches and Routers

蒂娜 3 Comments 01/07/2023

ThinkMo EDU Share – network 63.Handling of Unknown Data Frames by Switches and Routers

The switch forwards according to the DMAC lookup device table

At this time, it will involve the processing methods for unknown unicast, unknown multicast, and unknown broadcast, receive unknown unicast data frames, flood and record the MAC on the interface, and generate MAC entries.

Receive unknown multicast, flooding has the following situations for multicast:

1. STP _

If the MAC address is 0180-c200-0000 for the STP protocol, it will be handed over to the STP protocol for processing.

2. ISIS _

If the MAC address is 0180-C200-0014 15, it will be handled by an ISIS protocol.

If the multicast data is received, if the PIM protocol is running, it will be forwarded according to the downstream interface of the (S, G) entry. If the PIM protocol is not running, the multicast data frame will be flooded.

3. IGMP _

If IGMP snooping is running, forward data according to the member ports in IGMP snooping. If IGMP proxy is running, multicast data will be forwarded according to the IGMP proxy entry.

4. Broadcast packet

After receiving the broadcast packet, the Layer 3 protocol will submit it to the corresponding protocol for processing according to the type field.

How does the router handle the unknown entry data packet?

1. Destination routing

The router looks up the table and forwards the packet according to the destination IP address of the packet. If there is no destination route, it directly discards it.

2. IP Purpose

If the received frame is a unicast frame, check that the destination MAC is itself and receive it, and perform corresponding protocol processing according to the type field.

If the destination IP of the IP header is itself, it is then handed over to the local protocol for processing according to the protocol field in the IP header. If the DIP is not itself, it will be routed and forwarded, and if no route is found, it will be discarded.

If the received data frame is broadcast, the corresponding protocol processing (such as DHCP, ARP, etc.) is performed according to the type field. If the received IP is multicast, the protocol (such as OSPF, PIM) is submitted.

3. Multicast routing

If a multicast data frame is received, and the multicast routing protocol is also running, the processing is as follows:

If there is no (S, G), the RPF check is performed. If the check is passed, an (S, G) entry is created and the RPF check is passed as the incoming interface. If the check fails, it is discarded.

If there is (S, G) and the receiving interface of the packet is consistent with the forwarding entry, the packet is forwarded to all downstream interfaces; if there is (S, G) but the received data interface is inconsistent with the RPF check interface, the interface will be checked. RPF check.

If the check result is consistent with the inbound interface of (S, G), the source is wrong and discarded; if it is inconsistent, it means that (S, G) is out of date, and the inbound interface is updated and forwarded to all downstream interfaces.

4. Switch identification

The switch identifies Layer 2 forwarding or Layer 3 forwarding, depending on whether the DMAC is the MAC corresponding to its own vlanif, whether it is Layer 3 forwarding or not Layer 2.

ThinkMo CCNA Dump exam information exchange group:

CCNA/CCNP/CCIE telegram study group:https://t.me/ccie_ei_lab
WAHTAPP:+65 83444290
WAHTAPP:+63 9750724648

ThinkMo CCNA 200-301 Tutorial VIP Exclusive:

The complete EVE_NG file, free learning PDF and PPT that can be used directly, as well as video explaining the technical points are all here!


  • Itís hard to come by well-informed people in this particular topic, but you sound like you know what youíre talking about! Thanks

  • Good post. I learn something totally new and challenging on blogs I stumbleupon on a daily basis. Its always useful to read content from other authors and practice something from their websites.

  • A motivating discussion is definitely worth comment. Theres no doubt that that you need to write more on this subject matter, it may not be a taboo matter but generally folks dont discuss such subjects. To the next! Kind regards!!


Leave a Reply