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New Member

Native IP packet forwarding on LSR

If a native IP packet arrives at an LSR (P router) is a label imposed and packet label switched (assuming a label exists for the destination)or is the packet forwarded natively using standard IP routing.

Is the label imposition / native forwarding behaviour the same for all types of packet, eg OSPF (non bcast), BGP, SNMP, Telnet.

Reading RFC 3031 it looks as though labels should be imposed, however reading the cisco MPLS FAQ it indicates native IP packets are forwarded natively.

Thanks for any inout


Re: Native IP packet forwarding on LSR

if a native IP packet arrive at a P device which has a label towards the destination then it shall get label switched. Label imposition is normally done on the PE device i.e. the vpn label but there are mpls networks out there that do not have PE's as such i.e. mpls is just turned on but no vpns or no MPBGP is used within the network.


Re: Native IP packet forwarding on LSR

The assumption with a P router is that it is not connected to a customer router hence it should not recieve native ip packet. But if one way or the other, it gets a natve packet, it will impose a label, provided that it has a label for it. This is what is unusual, because it implies that the FIB of the provider routers do not have the same entry. MPLS will not work well in this scenario. The only situation I can think of is when the packet originates from the P router, in a ping situation for example.

New Member

Re: Native IP packet forwarding on LSR

If this is a P router for this IP flow it would look to swap the label - a shim layer process. Since there is no label, I believe it would simply be routed natively.


Cisco Employee

Re: Native IP packet forwarding on LSR

Note that if an IP packet reaches the LSR and that there is a label for the destination, label imposition will take place and the LSR functionally becomes an E-LSR as well as an LSR.

In the same way, an E-LSR can functionally become an LSR if it performs label swapping on certain packets.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
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Re: Native IP packet forwarding on LSR

The native ip packet will be labeled if the outgoing interface is MPLS enabled