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daudparvez
Beginner

MLS SWITCHING

Hi All,

Consider the following scenario:

                  

MLS Email.jpg

An IGP is running between D-SW1 and D-SW2. They both advertise the Routes 10.0.0.0/24 and 20.0.0.0/24 to each other. However, since these are also their CONNECTED ROUTES, therefore, because of lower AD, both switches prefer the Connected Routes.

Now suppose that the Layer 2 Trunk between A-SW2 and D-SW2 goes down.

A host X, connected to A-SW2 in VLAN 10 wants to send some data to another host Y connected to same switch SW-2 in VLAN 20.

Therefore, X sends it’s data to D-SW1. However, D-SW1 has the route 20.0.0.0/24 as Connected, so that it does not send these packets to D-SW2 and thus the packets are dropped.

My question is that is this a recommended arrangement? i.e, should we use extended VLANs, i.e, VLANs configured on different switches while we are using MLS for interVLAN routing?

Another question is that what can we do to make this arrangement to work? A solutions coming into my mind is to Use Layer 2 Trunk between D-SW1 and D-SW2. However, as per CCNP Book, it is not recommended to use Layer 2 Links between Distribution Layer Switches to limit Broadcasts.

Any recommendations/comments ??

Best Regards,

Daud Parvez

1 REPLY 1
Lee Smitherman
Beginner

Hi,

Therefore, X sends it’s data to D-SW1. However, D-SW1 has the route 20.0.0.0/24 as Connected, so that it does not send these packets to D-SW2 and thus the packets are dropped.

Why are the packets dropped?  D-SW1 has a Connected route to 20.0.0.0/24 as you mentioned, so it will send it back down to A-SW2 down vlan 20(D-SW1 will route it between vlan 10 and 20).