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Differences in VLAN and INT VLAN?

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  • 1.  Differences in VLAN and INT VLAN?

    Posted Apr 14, 2022 12:38 PM
    Previously, I had the VLAN and entered IP IGMP and NO IP IGMP QUERIER.

    Now, when I use VLAN 100, for example I need IP IGMP SNOOPING ENABLE, but if I use INTERFACE VLAN 100, I have to use IP IGMP ENABLE.   Also NO IGMP QUERIER works with INTERFACE VLAN 100, as well, but not with VLAN 100, at least in that form.

    What is the difference between the two?

    Shane Farmer

  • 2.  RE: Differences in VLAN and INT VLAN?

    Posted Apr 14, 2022 03:01 PM
    Hi Shane,

    'IGMP' and 'IGMP Snooping' are two different features and while they sound pretty similar they have slightly different purpose and application.

    IGMP protocol is a part of Multicast routing environment, this protocol is used by a multicast router (or L3 switch playing role of multicast router) to communicate with local hosts interested in receiving multicast traffic (multicast receivers). Clients announce which multicast groups they want to receive, subscribe to multicast groups, leave groups etc using IGMP protocol. For example if a host (like a Set-top box) wants to receive a certain TV channel, it sends a IGMP message with respective group address (the message is IGMP membership report). Or when a customer switches channel on her STB, the device sends two messages - 'leave group' for the old channel and 'membership report' for the new channel.

    IGMP Querier is a feature of L3 interface with IGMP enabled. IGMP querier works on one of L3 interfaces in the broadcast domain which is responsible for such 'MulticastRouter <-> Client' communication in the local network (subnet, broadcast domain). Also it is responsible for sending periodic IGMP queries looking for active receivers for all groups (IGMP General membership query) or specific groups (IGMP Group-specific membership query), thus multicast router keeps list of active receivers and sends multicast traffic only for the groups local host expect to receive. As for the 'no igmp querier' command, it definitely exists only on a SVI (L3 VLAN interface) because it is part of IGMP protocol which on its turn is part of Multicast routing. This command disables IGMP Querier function on the respective interface.

    IGMP Snooping is used on L2 network switches which placed between multicast router's L3 interface with IGMP enabled and hosts. Such switches being L2 switches don't have VLAN interfaces in respective VLAN, but still have a need to control multicast traffic distribution between Multicast Router's VLAN interface and end clients (multicast receivers). IGMP Snooping acts as a smart proxy that inspects IGMP messages exchanged between multicast router's L3 VLAN interface with IGMP enabled and multicast receivers. As a result of such inspection L2 switch with IGMP Snooping enabled creates a table which has at least three columns - VLAN, multicast group address and switchport number where multicast receiver is located. The purpose is to stop multicast traffic flooding in the VLAN and send multicast traffic only to end hosts interested in receiving such traffic. For example:

    VLAN |    Group       | Ports
    100    | | 1/1/1
    100    | | 1/1/2 ; 1/1/3

    In this example when a switch receives multicast traffic in VLAN 100 with destination IP address it will forward it only out of ports 1/1/2 and 1/1/3. Thus other ports won't be flooded with unneeded traffic. Same for the multicast traffic for group, it will be forwarded only to the host connected on 1/1/1.

    Check this article for IGMP - https://en.wikipedia.org/wiki/Internet_Group_Management_Protocol
    And this one about IGMP Snooping - https://en.wikipedia.org/wiki/IGMP_snooping

    P.S. Many L2 switches have a feature which is called 'IGMP Snooping Querier'. It is used when you don't have multicast routers in your network (so you don't have IGMP-enabled L3 interfaces acting as Querier), e.g. your network is pure L2, but you still have multicast traffic and you need to control its distribution to avoid flooding. Since IGMP Snooping cannot work without IGMP Querier (remember, snooping inspects IGMP message exchange between querier and clients), this features simulates a IGMP Querier in respective VLAN. It's quite typical for small networks. Or badly designed large networks.​

    Ivan Bondar

  • 3.  RE: Differences in VLAN and INT VLAN?

    Posted Apr 14, 2022 04:49 PM
    Thank you for the detailed information.  I didn't realize they were different things.  We have a L3 core switch and some other L3 switches in various locations, but really just utilize L2 functionality on them.  We're mostly buying L2 switches from here on out except for our core.

    That being said, we mainly do multicast imaging with ZENworks and back before we had VLANs and IGMP set up, it would cause a flood and cause wireless to drop connections.  Since moving to VLANs and IGMP, we can image fine.  Maybe I don't currently even have it set up correctly, and assume it was fine, but it's really not.  I assumed that VLANs helped the most, but definitely want to set up IGMP correctly so it will be that much more efficient.

    So I should really be using Snooping on the VLANs to actually do what I want, and point to the L3 switch?  And set the L3 switch up to be the querier?  Anything else that I should do as part of that?

    It's been a little confusing going back and forth between Procurve and ArubaOS-CX, as well.  We are swapping out all switches for CX at some point, but will be a few years.  Hoping to get a base config kind of set, then only change ports as needed, so want to get this kind of stuff done right.

    For reference our core is an HP 3800 and the rest are a mix of CX6100 and HP 2920/2530/2910.

    Shane Farmer

  • 4.  RE: Differences in VLAN and INT VLAN?

    Posted Apr 17, 2022 10:37 AM

    Yes, if you have several VLANs with multicast traffic, then definitely setup should be the following:

    L3 switch:

    Multicast routing enabled

    PIM enabled on Vlan L3 interfaces (SVI)

    IGMP enabled on Vlan L3 interfaces (SVI)

    L2 switches:
    IGMP Snooping enabled on all VLANs with multicast traffic
    IGMP Snooping should be set to filter unknown multicast by default (on ArubaOS this is enabled by configuration command "igmp filter-unknown-mcast" per each VLAN. Reboot is required.)

    Please, keep in mind that IGMP version on L3 switch should be the same as IGMP Snooping version on L2 switches. Otherwise IGMP Snooping may incorrectly interpret IGMP messages exchanged between clients and L3 switch and fail to construct multicast traffic distribution table.

    Ivan Bondar