Subject: [mpls] basic doubt about LERs in MPLS

From: "shilpa goel" <[email-masked]>
Date: Wed, 17 Jan 2007 16:38:42 +0530

Hi,

 I have one basic doubt regarding LERs in IP/MPLS networks.

How does a LER decide whether it should do IP routing (i.e. IP Forwarding
Table lookup) or labelling (i.e. LFIB lookup) of the unlabeled IP packets
that it receives at an interface?

Linked to this I have another doubt that how are control/protocol packets
(which are IP) routed in a router i.e. are they label switched along the
LSPs that exist for the FECs/destinations to which they are addressed or by
default IP routed?

regards,
Shilpa
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From: "Andrew Walding" <[email-masked]>
Date: Wed, 17 Jan 2007 11:38:07 -0600

Hi Shilpa,
As you can imagine, different OS systems do things slightly differently
(Cisco vs Juniper).  So let's stay generic.
 
First, MPLS is usually activated by interface on the platform.  This means
that a LER will have one interface (physical or logical) that is MPLS and
one that is not - that is what makes it an LER.  We would expect that on the
interface that is not MPLS enabled to receive packets and frames that do not
have MPLS labels.  The layer 2 PID or ethertype will clearly identify the
contents of the arriving frame (usually 0x08000 is IP).  Based on this the
lookup will be to the IP forwarding table.
 
The other alternative a packet arriving on the MPLS enabled interface.  This
type of interface usually can receive both MPLS encapsulated and non MPLS
encapsulated.  Again, we depend on the Layer 2 PID or ethertype to tell us
(MPLS encap Unicast IP = 0x08847, MPLS encap Multicast IP - 0x08848).  With
either of these, we look up the LFIB.
 
With regards to transport between routers for protocol messaging, since IP
and MPLS encaps can be received on an interface, either can be used.  There
are reserved labels (0-15) though not all are specified.  Label value of 1
is reserved for router to router messaging - similar to the ILMI or LMI
channel in Frame and ATM.
 
Does that help?

Andy
 <http://www.cellstream.com/> http://www.cellstream.com

  _____  

From: shilpa goel [mailto:[email-masked]] 
Sent: Wednesday, January 17, 2007 5:09 AM
To: [email-masked]; [email-masked]; [email-masked]
Subject: [MPLS-OPS]: basic doubt about LERs in MPLS

Hi,

regards,
Shilpa