VLAN limit

dave330idave330i Member Posts: 2,091 ■■■■■■■■■■
in Off-Topic
When I was learning CCNA, the training video mentioned that while you could create 4096 different VLANs, Cisco recommend ~500 due to limitations of the switch. Is this still the case?
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  • KelkinKelkin Member Posts: 261 ■■■□□□□□□□
    Actually really depends on the switch and code and configuration. For instance.. In the older cisco 3125x (Blade Enclosure Switch) They only support up to 128 Spanning tree instances which if you know STP/RSTP each vlan can be a spanning-tree instance.
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  • networker050184networker050184 Mod Posts: 11,962 Mod
    As Kelkin said, it's really STP instances that start to get in your way. The most common work around for this in me experience is MST.
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  • DevilWAHDevilWAH Member Posts: 2,997 ■■■■■■■■□□
    This is much like the recommendation of <250 IP per subnet. AS routers/Nic have got more powerful and networks more "intelligent", this limit is not quite so important these days. However its worth knowing the reasons it was put forward in the first place. Some types of networks lean them selves to lots of background chatter that having small subnets help to mitigate the affect. While other subnets can handle several thousands with out issue.

    If you ever see a recommended limit, its always based on some assumptions, so its worth knowing what they are so you can make an informed choice.
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  • NetworkVeteranNetworkVeteran Member Posts: 2,338 ■■■■■■■■□□
    networker050184 wrote: »
    As Kelkin said, it's really STP instances that start to get in your way. The most common work around for this in me experience is MST.
    +1. It's worth noting many vendors implement STP with CST (common spanning tree for all VLANs) as per the 802.1D standard, rather than Cisco's PVST (per-vlan-spanning-trees) idea. Those vendors don't hit these scaling limits.
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  • xXErebuSxXErebuS Member Posts: 230
    networker050184 wrote: »
    As Kelkin said, it's really STP instances that start to get in your way. The most common work around for this in me experience is MST.

    Of course; MST is the devil..... don't forget to mention that =D
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  • NetworkVeteranNetworkVeteran Member Posts: 2,338 ■■■■■■■■□□
    xXErebuS wrote: »
    Of course; MST is the devil..... don't forget to mention that =D
    That's one (of many) reasons carriers who need plenty of VLANs may simply go with another vendor. (I'm quite proficient with MST, but it's newer code and yet another protocol for carriers to learn and train their engineers about.)
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  • it_consultantit_consultant Member Posts: 1,903
    All of my Brocades are limited to 4096 precisely because of the PVSTP memory requirements.
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  • NetworkVeteranNetworkVeteran Member Posts: 2,338 ■■■■■■■■□□
    it_consultant wrote: »
    All of my Brocades are limited to 4096 precisely because of the PVSTP memory requirements.
    While I'm not disputing the possibility of your statement, most switches Brocade or otherwise are limited to 4096 VLANs not because of memory/processor limitations, but rather because the VlanID portion of an 802.1Q tag is 12 bits long, and 2^12 = 4096.

    Also Brocade devices normally can't support 4096 spanning trees, bur rather closer to 255. Which is plenty for just about any practical application. That's probably why they stopped there. They scale to 4096 VLANs without MST precisely because they're capable of running with a CST as opposed to PVST, an IEEE-compliant mode they call "single spanning tree mode".
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  • shabeermshabeerm Member Posts: 29 ■□□□□□□□□□
    dave330i wrote: »
    When I was learning CCNA, the training video mentioned that while you could create 4096 different VLANs, Cisco recommend ~500 due to limitations of the switch. Is this still the case?

    Maximum VLANs supported by 802.1Q is 4094 and Maximum VLAN supported by ISL is 1000
    For CCNA / CCNP notes visit my blog http://sysnetnotes.blogspot.in/
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  • mistabrumley89mistabrumley89 Member Posts: 356 ■■■□□□□□□□
    NetworkVeteran wrote: »
    Also Brocade devices normally can't support 4096 spanning trees, bur rather closer to 255. Which is plenty for just about any practical application. That's probably why they stopped there. They scale to 4096 VLANs without MST precisely because they're capable of running with a CST as opposed to PVST, an IEEE-compliant mode they call "single spanning tree mode".

    Depending on the device model they normally max out at 255 or 512 virtual interfaces and vlans by default. You can change both vlan max and virtual-interface maxes by using a system-max {virtual-interface/vlan} DECIMAL at global config. Which will allow you to utilize 4096.
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  • NetworkVeteranNetworkVeteran Member Posts: 2,338 ■■■■■■■■□□
    mistabrumley89 wrote: »
    You can change both vlan max and virtual-interface maxes by using a system-max {virtual-interface/vlan} DECIMAL at global config.
    Nice tip! :)
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