Solved: MPLS Principles

jaighobahi · ‎01-14-2013

Hi All,

Please, bear with me. I am trying to understand the basics of MPLS. I do not wish to gloss over the topic and carry on. The Cisco document that I am reading says:

"A label distribution protocol (LDP) advertises the bindings between routes and labels. These bindings are checked against the routing table. If the route (prefix/mask and next hop) learned from the LDP matches the route learned from IGP in the routing table, an entry is created in the label that forwards information bases (LFIB) on the LSR".

I do not understand the above statement. So, I ask the following questions:

1. what generates the labels in the LSR?

2. I hear there is also a CEF table. What are the contents of this table?

3. What performs the binding between a label and an IP address?

4. What is the role of the LFIB in the LSR?

Please, provide answers if you can or point me in the direction of the answers.

Thank you.

Peter Paluch · ‎01-14-2013

Hello,

1.  what generates the labels in the LSR?

It is the LSR itself. The LSR takes its routing table and enumerates its entries. These values are the label values bound to individual networks known by the LSR. The label bindings, i.e. the correspondence of networks and their labels, is advertised by LDP. There are also other protocols capable of carrying these label mappings - BGP and RSVP.

2.  I hear there is also a CEF table.  What are the contents of this table?

The CEF is an optimized copy of the routing table, designed for fast lookups - more efficient than the classical routing table. A part of this CEF is also what we call an adjacency table. The adjacency table contains prepared frame headers for packets routed through particular neighbors - note that the neighbors (next hops) are known from the IP routing table and their Layer2 addresses (MAC, DLCI, etc.) are known from these L3/L2 mapping tables (ARP, IP/DLCI, ...), so you can construct the frame headers using the existing information before the packets start flowing.

The CEF is important for Cisco implementation of MPLS because if the next hop also advertised a label value for a particular network, the adjacency table will prepare the frame header including the appropriate label value. Thanks to this, the CEF facilitates actual labeling of incoming packets - if a particular next hop is identified for a network, that next hop's label value for this destination network will be imposed onto the packet, as the so-called rewrite information is already prepared within the CEF structures.

3.  What performs the binding between a label and an IP address?

This binding is generate by a LSR and is advertised by LDP or BGP.

4.  What is the role of the LFIB in the LSR?

The LFIB is to MPLS what routing table is to a router, or what MAC address table is to a switch. The LFIB contains information how to forward labeled packets. In short, the LFIB has entries of the form "if an incoming labeled packet carries the label N, replace it with label M and send it out this interface towards this neighbor". It is very similar to Frame Relay or ATM switching table, by the way.

Feel welcome to ask further, as MPLS is a vast topic!

Best regards,

Peter

View solution in original post

Peter Paluch · ‎01-14-2013

Hello,

1.  what generates the labels in the LSR?

It is the LSR itself. The LSR takes its routing table and enumerates its entries. These values are the label values bound to individual networks known by the LSR. The label bindings, i.e. the correspondence of networks and their labels, is advertised by LDP. There are also other protocols capable of carrying these label mappings - BGP and RSVP.

2.  I hear there is also a CEF table.  What are the contents of this table?

The CEF is an optimized copy of the routing table, designed for fast lookups - more efficient than the classical routing table. A part of this CEF is also what we call an adjacency table. The adjacency table contains prepared frame headers for packets routed through particular neighbors - note that the neighbors (next hops) are known from the IP routing table and their Layer2 addresses (MAC, DLCI, etc.) are known from these L3/L2 mapping tables (ARP, IP/DLCI, ...), so you can construct the frame headers using the existing information before the packets start flowing.

The CEF is important for Cisco implementation of MPLS because if the next hop also advertised a label value for a particular network, the adjacency table will prepare the frame header including the appropriate label value. Thanks to this, the CEF facilitates actual labeling of incoming packets - if a particular next hop is identified for a network, that next hop's label value for this destination network will be imposed onto the packet, as the so-called rewrite information is already prepared within the CEF structures.

3.  What performs the binding between a label and an IP address?

This binding is generate by a LSR and is advertised by LDP or BGP.

4.  What is the role of the LFIB in the LSR?

The LFIB is to MPLS what routing table is to a router, or what MAC address table is to a switch. The LFIB contains information how to forward labeled packets. In short, the LFIB has entries of the form "if an incoming labeled packet carries the label N, replace it with label M and send it out this interface towards this neighbor". It is very similar to Frame Relay or ATM switching table, by the way.

Feel welcome to ask further, as MPLS is a vast topic!

Best regards,

Peter

jaighobahi · ‎01-14-2013

Thanks for this response. MPLS is starting to make sense to me. I need to assimilate the points you made, going forward.