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cnetworkingnetwork-programmingmulticastdata-link-layer

understanding link layer multicasting

I am fairly new to network programming especially L2.

I am working on an implementation of LLDP(Link Layer Discovery Protocol) in C. I understand that multicasting over link layer works on directly connected LAN devices. My aim is to have it work across VLAN's (Eg: a device in 10.1.1.0 pool should be able to receive L2 multicast packets sent by a device in 192.168.1.0). Is there a possible way to achieve this ..or am I missing something vital here?

Wireshark wiki on LLDP says:

LLDP Data Units (LLDPDUs) are sent to the destination MAC address 01:80:c2:00:00:0e. This address is defined as the "LLDP_Multicast" address. This address is defined within a range of addresses reserved by the IEEE for protocols that are to be constrained to an individual LAN. AN LLDPDU will not be forwarded by MAC bridges (e.g. switches) that conform to IEEE Std 802.1D-2004.

  1. so does that mean that a switch or a router just ignores link layer multicast packets coming from another device in another subnet? does multicast over link layer work only on directly connected devices?

  2. what is the scope of a data link layer multicast address?

Solution

  • From your question, it seems you are mixing L2 and L3 technologies. If you are dealing L2 multicast, IP addresses have no meaning there.

    As for the questions:

    1) When talking about switches, you have to consider if a switch is 802.1D compliant or not. A dumb switch (a. la Generic $5 from a store next door) will treat your LLDP PDU as a broadcast frame and will distribute it to all other ports except for the one it received it from. This is because MAC address starts with 01 which indicates that the group address bit is set.

    A switch which is 802.1D compliant will not forward this frame anywhere. It might process it locally to learn about a device sending it. This will happen if the switch has LLDP protocol support enabled.

    A router will act in the same way as a 802.1D compliant switch. It will not forward the LLDP frame and might process it if the LLDP is enabled.

    A PC will act the same was as a router.

    2) Scope will depend on the endpoints of a link. Any smart device will terminate the journey of the LLDP frame. A dumb switch will treat it as a broadcast frame.