ttn022191ttn022191 Registered Users Posts: 2 ■■□□□□□□□□
What is the difference between VLAN trucking protocol and Link Aggregation Control protocol. Doesn't both allow you to connect VLANs from different switches?


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    Welly_59Welly_59 Member Posts: 431
    They do totally different things. VTP automatically propogates VLAN information over trunks. LACP allows you to bundle seperate physical interfaces into one logical unit
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    Cisco InfernoCisco Inferno Member Posts: 1,034 ■■■■■■□□□□
    First youre going to want to understand why you create VLANs and what they do, they break up broadcast domains on layer 2 of the OSI model. A VLAN is a subnet. Since your switch will have multiple VLANs, there has to be a way to identify each frame.
    Data frames traveling across the network are tagged with an 802.1q VLAN identifier tag that tells the switch which VLAN the frame is from or going to. Depending on a switch’s ingress or egress traffic, the switch will make forwarding decisions to the right logical group based (VLAN) based on this 802.1q tag.

    Now that we know that, we have to replicate this throughout our network if VLANs are going to be used. A switch has to have a VLAN on it to understand where to send incoming traffic. If SWITCH1 has for instance VLAN 10 on it and sends a frame tagged as such to switch 2, How will SWITCH2 know which port it needs to send it out of? VLAN information is locally significant and does not pass to other switches. One way of solving this is by manually configuring VLAN 10 on SWITCH2. But what if you have many switches in your LAN? An inconsistency may cause an outage or not allow the proper communication to hosts on that switch. This is where VTP comes in.

    VTP is not actually used for trunking. It is a protocol that allows for VLAN configuration on 1 switch to be propagated to other switches in your LAN without manual configuration. All you need to know is that there is one switch where you make your VLAN configs; this is called a VTP Server. Your other switches will receive their VLAN information(and your changes) from the VTP Server. These other switches are called VTP Clients. All switches must at least be configured with the same VTP Domain name and password if used. You can also have switches that do not participate in VTP. They are in Transparent Mode. Using transparent mode gives you very precise control of which VLANs are available on which switch and makes the network administrator think very carefully. This is useful in a larger switched network where critical resources cannot risk downtime by some mismatch made by another switch.

    The next thing you need to know about it called Port Channeling, or in Cisco's verbiage, EtherChannel. Microsoft also calls this NIC Teaming. EtherChannel is the practice of bundling links together. I think the max is 8. You can have multiple pairs of switch ports act as a single link. The default behavior of a switch is to block redundant links to prevent loops which could bring your network down (Spanning Tree Protocol). There are times where you need to aggregate the bandwidth of more than one links such as interconnecting switches, or a connection to a server or other storage infrastructure. Imagine a 1Gbps port times 4! Now you have a 4Gbps link that is treated as its own connection.

    You could statically create a Port Channel on two ports but there are concerns with this. In order to form a Port Channel on a Cisco switch for instance, all of the interfaces in each EtherChannel must be the same speed, duplex, VLANs, and trunking configuration. Ok so thats fine, you make sure everything matches and that there is no mismatch, but what if there was an easier way?

    To help this, you use a Port Channel Protocol such as Link Aggregation Control Protocol. It does exactly what it sounds like. It monitors and makes sure that before a port channel comes up, that everything matches and is compatible. This avoids undesirable behavior in your Port Channel. If a change happens to a port that is a mismatch, it will let you know as opposed to not detecting it with a static config. One of the other unheard benefits of LACP is that it monitors your channel with hello messages. Think of pings. If for instance you have a Static Port Channel and there is an intermediate device between your switches such as a media converter that fails, the switch won't detect an issue with the link (due to the ports being fine). LACP is the control mechanism that will detect a loss in it's beacons (PDU's protocol data unit) and tell your switch that there actually is a problem with your link.

    So ya, you listed two totally different things that can deal with trunks or not. VTP doesn't do any trunking, but allows two switches to have the same VLAN data. With this you can create a trunk link. A trunk is just a port or link that carries frames from multiple VLANs. Remember those 802.1q tags? Frames traverse this type of link and are ID'd with their VLAN tag. Non tagged traffic from ports not a member of a certain VLAN can also traverse a trunk link. It uses the concept of a Native VLAN. Outgoing frames are tagged with the Native VLAN tag. For a trunk link, the Native VLAN must match, otherwise unrecognized traffic will be dropped. Trunks are used between switches, switches to routers, servers or anywhere you need multiple subnet traffic on. Port Channels do not have to be trunk links but often are.

    I hope that helps. Let me know if that cleared it up.
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