Big Network Cisco

[alfredobosca]

Hello,

I am inspecting a big network that has at least 300 switches and all is Layer2 domain :( 

-  I have detected that exists default native vlan 1 and another different native vlan. If control traffic is handled by native vlan, could exists 2 tree of Spannig Tree?

                       *BPDU packets are also handled by native vlan

Thanks in advance!

[Julio E. Moisa]

Hi Alfredo,

I think 2 spanning tree's could be coexist on a same network but it is not a good practice and it could generate many problems on your network. Instead you could select Rapid per vlan STP or MST in order to improve your layer 2 network. 

About the native vlan it should be other than vlan 1 for security purposes, also the vlan 1 should be shutdown. 

The following links could be useful:

https://en.wikibooks.org/wiki/Local_Area_Network_design/Rapid_Spanning_Tree_Protocol#Coexistence_of_STP_and_RSTP

Coexistence of STP and RSTP

If a bridge not supporting RSTP is introduced into the network, on receiving Configuration BPDUs with Type equal to 0 they are able to automatically switch to STP mode, but this has some side effects:

• because of a single bridge not supporting RSTP, the whole network goes into STP mode and thus fast convergence times are lost;
• if the single bridge not supporting RSTP faults or is disconnected from the network, the other bridges keep working in STP mode, and explicit manual configuration should be taken on every single bridge.

A bridge can be configured so as to work in RSTP mode on some ports, and in STP mode on other ports → the network is split in two portions working with different spanning tree protocol versions. However this may lead to network instability because of transient loops due to the fact that the RSTP portion enables the forwarding of data frames earlier than the STP portion.

For a seamless coexistence of RSTP and non-RSTP bridges within the same network, Multiple Spanning Tree Protocol, standardized as IEEE 802.1s (2002), should be used: the network portions working with RSTP and the ones working with STP are separated in different domains.

About Native vlan:

http://etherealmind.com/basics-cisco-ios-native-vlans/

Hope it is useful

:-)

[Ganesh Hariharan]

Hello,
I am inspecting a big network that has at least 300 switches and all is Layer2 domain :( 
-  I have detected that exists default native vlan 1 and another different native vlan. If control traffic is handled by native vlan, could exists 2 tree of Spannig Tree?                      *BPDU packets are also handled by native vlan
Thanks in advance!

Hi,

Lets have look on native VLAN ,Generally the IEEE 802.1q TRUNKING encapsulation standard says the NATIVE VLAN represents traffic sent and received on an interface running 802.1q   encapsulation that does not have a tag. 

So NATIVE VLAN exists also on access ports, its role is relevenat only on trunk ports.

NATIVE VLAN can be modified on a per-port basis or it can be "disabled",you can configure some higher-end switches to tag all frames, so there is not NATIVE VLAN.

So w.r.t your query ..If native VLAN is VLAN1 then following behaviour will happen..

• VLAN1 standard STP BPDU is sent untagged
• VLAN1 PVST+ BPDU is sent untagged
• Other VLAN's PVST+ BPDUs are sent tagged with their appropriate VLAN

If native VLAN is different from VLAN1 like lan 999 or 99 then following behaviour will happen..

• VLAN1 standard STP BPDU is sent untagged
• VLAN1 PVST+ BPDU is sent tagged with VLAN1
• Other VLAN's PVST+ BPDUs are sent tagged accordingly (the one for the native VLAN will be untagged)

In short, the standard STP BPDU is always derived from VLAN1 and is always sent untagged. The PVST+ BPDUs are derived from their appropriate VLANs and are tagged according to the native VLAN on the trunk.

Hope it Helps..

-GI