Understanding traffic flow in a switched environment .....

Answered Question
Mar 12th, 2007

Hi,

I need an expert to confirm if my understanding of Spanning-tree is correct. If an access port which is connected say to vlan 10 wants to send data, it will send to the root port (say gi1/1) on the access switch which in turn will forward the data to the primary building distribution switch where the

root bridge is located since I configured it with 'spanning-tree root primary vlan 10' ? Is this understanding correct ?

In other words traffic will flow from :-

Edge Port --> Root Port on Access Switch (Gi1/1) --> Root Bridge (Distribution Switch) --> and it goes to where it is suppose to go .....and the reverese if data is flowing through it ?

[2] Is there a command to see traffic flow in a switched environment

...something equivalent to

traceroute ?

Hmmmm ... I am beginnnig to enjoy spanning-tree.

Pls advice,

- sn -

I have this problem too.
0 votes
Correct Answer by dgahm about 9 years 9 months ago

That entry says mac address 0002.55eb.3116 is in VLAN 140 and can be reached via interface 4/13.

If you are pinging across VLANs, the host originating the ping will ARP for the mac address of the default gateway. Once it has the mac it will send an ethernet frame to that mac address. The switches will forward the frame switch to switch until it gets to the router interface. The router will strip the mac, arp for the host mac if on a connected VLAN, build the frame and send it on its way. The switches in that VLAN will forward the frame to its destination.

Here is a good review document on transparent bridging:

http://www.cisco.com/en/US/tech/tk1330/tsd_technology_support_technical_reference_chapter09186a00807597b8.html

Please rate helpful posts.

Dave

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Jon Marshall Mon, 03/12/2007 - 07:13

Hi Sanjay

The answer is it depends.

if you have an client on an access port in vlan 10 on switch A and it wants to send data to a client in the same vlan and on the same switch then no traffic will not go via the root bridge.

if you have an intermediate switch between the root switch and your switch and the same conditions apply about the clients and vlans then again the traffic will not go via the root bridge.

Where the scenario you describe will usually happen is when you are going from one vlan to another and your root bridge and layer 3 interfaces are on the same switch(es). Then yes it will go to the root bridge get routed onto the destination vlan and switched from there.

It really entirely depends on the topology of your switch infrastructure and also if you are routing between vlans the placement of your active layer 3 gateways in relation to the root bridge.

If there are any specific examples let me know.

HTH

Jon

dgahm Mon, 03/12/2007 - 08:06

Sanjay,

You really should think of spanning tree as separate from the layer 2 frame forwarding process. Spanning tree does of course dictate the topology by blocking redundant paths to the root, but when forwarding traffic the switch does not care where the root is, it simply looks in it's forwarding table to find which active port to send the traffic to. The rules of transparent bridging apply. If you built the same topology manually (disconnect the ports that STP would block) and disabled spanning tree, the traffic forwarding would be identical in both cases.

In IOS switches a show mac-address address XXXX.XXXX.XXXX will tell you what port traffic to a specific MAC address will forward to. Cat OS is show cam XX-XX-XX-XX-XX-XX

Please rate helpful posts.

Dave

sanjaynadarajah Mon, 03/12/2007 - 08:24

Hi Dave,

If I have an entry as the following :-

140 0002.55eb.3116 dynamic ip FastEthernet4/13

which is an access port and say that I do a ping to vlan 141, this means the mac address above will forward it out to fa4/13 to the presumably the uplink of the switch,ie gi1/1 heading towards the distribution switch/distribtion router (6500 hybrid IOS) and it will route it to vlan 141 before going to the chose route right ?

Thanks,

-sn-

Correct Answer
dgahm Mon, 03/12/2007 - 09:49

That entry says mac address 0002.55eb.3116 is in VLAN 140 and can be reached via interface 4/13.

If you are pinging across VLANs, the host originating the ping will ARP for the mac address of the default gateway. Once it has the mac it will send an ethernet frame to that mac address. The switches will forward the frame switch to switch until it gets to the router interface. The router will strip the mac, arp for the host mac if on a connected VLAN, build the frame and send it on its way. The switches in that VLAN will forward the frame to its destination.

Here is a good review document on transparent bridging:

http://www.cisco.com/en/US/tech/tk1330/tsd_technology_support_technical_reference_chapter09186a00807597b8.html

Please rate helpful posts.

Dave

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