IP Forwarding: MLS Routed Ports

Routed Ports on Multilayer Switches are switchports that have been turned into Layer 3 ports that you can assign an IP address to and route with.  Using the ‘no switchport’ command on an interface makes that port a Routed Port.

You can apply an IP address to it, however you cannot create sub-interfaces on that port.  That interface is not placed into any USER DEFINED VLAN…(internal usage VLANs are created for each routed port).  The switch does not keep any layer 2 information for the interface, the adjacency table lists the outgoing physical interface which means that Layer 2 switching logic is not required for that port.

Ethernet Port Channels can be configured as routed ports as well.  To do so you mush configure the no switchport command on all participating interfaces and then add those ports to the channel group.  The Port Channel that’s created will inherit the properties of the ports that have been assigned and an IP address can then be configured on the port channel interface.  Existing layer 2 port channels cannot be converted to layer 3 and vice versa without completely removing the port channel first.

Internal Usage VLANs

These VLANs are created on behalf of a routed port, the normal way Switches Forward frames is by using VLANs, so when a routed port is configured, that port is bound to an internal VLAN the switch uses to pass traffic unto.  These VLANs are hidden and not stored in the VLAN database file.  Assignment of internal usage VLANs to routed ports is done at runtime.  Conflicts can come up as a result if an admin tries creating a VLAN ID with an already used internal usage VLAN. To prevent this you must configure the Internal Usage VLAN Allocation policy to assign VLANs in order of ascending or descending opposite of the range in which you have VLANs configured.  for example your extended VLANs are all in the low 1000’s, you’ll want to change the allocation policy to descending so internal usage VLANs are allocated starting at 4000 going down to avoid VLAN overlap.



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