Vlan-Bridging

[Rupesh Kashyap]

Hi, I have one L2 switch having two vlan 4 & 5. I want communication between host of both vlans. How it can possible? Although I have configured Vlan-Bridge, but it is not working

interface Vlan4

bridge-group 1

!

interface Vlan5

bridge-group 1

!

bridge 1 protocol vlan-bridge

[Giuseppe Larosa]

Hello Rupesh,

vlan bridging can help if users on vlan4 and vlan5 are in the same IP subnet otherwise you need a router.

IPv4 doesn't allow two hosts in two different IP subnets to talk to each other directly.

To be noted that IPv6 allows this.

About vlan bridging each vlan needs to have at least one L2 port in STP forwarding state.

Hope to help

Giuseppe

[jox198501]

Hi. Can you help me about his one:

I'm having trouble understanding this document

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/11072-inter-vlan-11072.html#protocol

My topology is

pc1----(e0)S1(e1)-----(e1)S2(e0)----pc2

pc1 2.2.2.10/24 2.2.2.1

pc2 3.3.3.10/24 3.3.3.1

S1                                                                    S2

eo in vlan2 access mode                                  eo in vlan3 access mode

e1 in trunking mode                                          e1 in trunking mode

int vlan 2 2.2.1/24                                            int vlan 2 2.2.2/24

bridge-group 10                                              bridge-group 10

int vlan 3 3.3.3/24                                           int vlan 3 3.3.1/24

bridge-group 10                                              bridge-group 10

bridge 10 protocol vlan-bridge                        bridge 10 protocol vlan-bridge

I wanted to explore this configuration , but 1st problem is i keep getting this message on S1:

%TBRIDGE-4-INVALIDMEDIA: UNKNOWN received on Vlan2 - invalid media for transparent bridging

What does it mean?

p.s. on trunk link these are frames (root S2 is sending bpdus)

So there are instances of STP (pvst)for vlan1, vlan2, vlan3, and instance(vlan-bdrige) for bridge10, and there is always this "spanning-tree for bridges" frame which is untagged.

I guess that is basic 802.1d STP, but i dont see its instance running in show span-tree 

What it its purpose ?

Thanks

[Giuseppe Larosa]

Hello @jox198501 ,

>> I guess that is basic 802.1d STP, but i dont see its instance running in show span-tree 

Yes, it is but 802.1D STP has a single instance for all vlans.

Cisco switches with default settings will send out also an untagged 802.1D STP equivalent to PVST instance for Vlan 1.

This is done for backward compatibillity with third party devices that speak 802.1D STP.

>>

%TBRIDGE-4-INVALIDMEDIA: UNKNOWN received on Vlan2 - invalid media for transparent bridging

What does it mean?

This is more difficult to understand . My guess is that the inter vlan bridging is not compatible with the configuration of SVI interfaces and this looks like reasonable as SVI interfaces are used for inter vlan routing.

Again if you bridge between two Vlans the two hosts should be in the same IP subnet otherwise they can not communicate.

In other words your lab setup is good for inter vlan routing tests not for bridging between Vlans.

PC1 and Pc2 should be in the same IP subnet but in different Vlans to make a test of bridging.

Hope to help

Giuseppe

[rakesh.hegde]

Hi Rupesh,

Fallback bridging bridges traffic that the switch can not route. For example, you can bridge IPV6 on a 3550 since it can not route IPV6.

HTH,

-Rakesh

[Giuseppe Larosa]

Hello Rakesh,

other platforms like C6500 can perform vlan bridging for ipv4 by adding

bridge-group 1 bridge ip

C3550 and probably other C3x50 behave like you have described

Hope to help

Giuseppe

[rakesh.hegde]

Hi Giuseppe ,

You are right. 6500 behaves more like a router when it comes to bridging. Unlike Fallback bridging, we need to enable IRB to get it working. The command bridge-group <> bridge ip is enabled by default.

-Rakesh