Implementing Secure Converged Wide Area Networks (ISCW)

1. Implementing Secure Converged Wide Area Networks (ISCW) Module 4: Frame Mode MPLS Implementation

2. Module 4: Frame Mode MPLS Implementation Lesson 4.2: Assigning MPLS Labels to Packets

3. Objectives • Describe the steps in label allocation and distribution in a frame mode MPLS network. • Describe packet propagation across an MPLS network. • Describe Penultimate Hop Popping (PHP) on Edge LSRs. • Compare and contrast MPLS networks with PHP and without PHP.

4. Label Allocation in a Frame Mode MPLS Environment • Label allocation and distribution in a frame mode MPLS network follows these steps: • IP routing protocols build the IP routing table. • Each LSR independently assigns a label to every destination in the IP routing table. • LSRs announce their assigned labels to all other LSRs. • Every LSR builds LIB, LFIB, and FIB data structures based on the received labels.

5. Building the IP Routing Table • IP routing protocols are used to build IP routing tables on all LSRs. • FIBs are built based on IP routing tables, initially with no labeling information.

6. Allocating Labels • Every LSR allocates a label for every destination in the IP routing table. • Labels have local significance. • Label allocations are asynchronous.

7. LIB and LFIB Setup • LIB and LFIB structures have to be initialized on the LSR that is allocating the label. • Untagged action removes the label from the frame and causes the router to send a pure IP packet.

8. Label Distribution and Advertisement • The allocated label is advertised to all neighbor LSRs, regardless of whether the neighbors are upstream or downstream LSRs for the destination.

9. Receiving Label Advertisement • Every LSR stores the received label in the LSR’s LIB. • Edge LSRs that receive the label from their next hop also store the label information in the FIB.

10. Interim Packet Propagation • Forwarded IP packets are labeled only on the path segments where the labels have already been assigned.

11. Further Label Allocation • Every LSR will eventually assign a label for every destination.

12. Receiving Label Advertisement • Every LSR stores received information in its LIB. • LSRs that receive their label from their next-hop LSR also populate the IP forwarding table.

13. Populating the LFIB Table • Router B has already assigned a label to Network X and created an entry in the LFIB. • The outgoing label is inserted in the LFIB after the label is received from the next-hop LSR.

14. Packet Propagation Across an MPLS Network

15. Penultimate Hop Popping (PHP) • PHP optimizes MPLS performance by reducing CPU effort on Edge LSRs. • The Edge LSR advertises a pop or implicit null label (value of 3) to a neighbor. • The pop tells the neighbor to use PHP.

16. MPLS Without PHP • A double lookup is required.

17. A B C D MPLS with PHP • A label is removed on the router that is located before the last hop within an MPLS domain (the penultimate router).

18. Summary • There are four steps for label allocation and distribution in a Unicast IP routing network and MPLS functionality, including label allocation and distribution. The following steps detail what happens: • The routers exchange information using routing protocol. • Local labels are generated. • Local labels are propagated to adjacent routers. • Every LSR builds data structures based on received labels. • When a router receives an IP packet, the lookup done is an IP lookup. When a router receives a labeled packet, the lookup is done in the LFIB table of the router. • Using Penultimate Hop Popping (PHP), an LSR removes the outermost label of an MPLS-tagged packet before passing the packet to an adjacent Edge LSR. The process reduces the load on the Edge LSR.

19. Q and A

20. Resources • MPLS FAQ For Beginners • http://www.cisco.com/en/US/partner/tech/tk436/tk428/technologies_q_and_a_item09186a00800949e5.shtml • MPLS Technology Support • http://www.cisco.com/en/US/partner/tech/tk436/tsd_technology_support_category_home.html