Label Assignment and Distribution Introducing Typical Label Distribution

Outline • Overview • Propagating Labels Across a Network? • What Are LSPs? • Propagating Labels Using PHP • What Is the Impact of IP Aggregation on LSPs? • Allocating Labels in a Frame Mode MPLS Network • Distributing and Advertising Labels • Populating the LFIB • Propagating Packets Across an MPLS Network • Detecting Frame Mode Loops • Allocating Per Platform Labels • Summary © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 2

MPLS Unicast IP Routing Architecture • MPLS introduces a label field that is used forwarding decisions. • Although labels are locally significant, they have to be advertised to directly reachable peers. – One option would be to include this parameter in existing IP routing protocols. – The other option is to create a new protocol to exchange labels. • The second option has been used because there are too many existing IP routing protocols that would have to be modified to carry labels. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 3

MPLS Unicast IP Routing Architecture (Cont. ) © 2006 Cisco Systems, Inc. All rights

Label Switched Path • An LSP is a sequence of LSRs that forwards labeled packets of a certain forwarding equivalence class. – MPLS unicast IP forwarding builds LSPs based on the output of IP routing protocols. – LDP advertises labels only for individual segments in the LSP. • LSPs are unidirectional. – Return traffic uses a different LSP (usually the reverse path because most routing protocols provide symmetrical routing). • An LSP can take a different path from the one chosen by an IP routing protocol (MPLS TE). © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 7

LSP Building The IP routing protocol determines the path. © 2006 Cisco Systems, Inc.

LSP Building (Cont. ) LDP propagates labels to convert the path to an LSP.

PHP: Before • Double lookup is not an optimal way of forwarding labeled packets.

PHP: After A label is removed on the router before the last hop within

PHP • Penultimate hop popping optimizes MPLS performance (one less LFIB lookup). • PHP does not work on ATM. (virtual path identifier/virtual channel identifier cannot be removed. ) • The pop or implicit null label uses a reserved value when being advertised to a neighbor. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 12

Impact of IP Aggregation on LSPs • IP aggregation breaks an LSP into two segments. • Router C is forwarding packets based on Layer 3 information. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 13

Impact of IP Aggregation on LSPs (Cont. ) • IP aggregation breaks an LSP into two segments. • Aggregation should not be used where end to end. LSPs are required, such as with: – MPLS VPNs – MPLS TEs – MPLS enabled ATM network – Transit BGP where core routers are not running BGP © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 14

Label Allocation in a Frame Mode MPLS Network Label allocation and distribution in a frame mode MPLS network follows these steps: • IP routing protocols build the IP routing table. • Each LSR assigns a label to every destination in the IP routing table independently. • LSRs announce their assigned labels to all other LSRs. • Every LSR builds its LIB, LFIB, and FIB data structures based on received labels. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 15

Label Allocation in a Frame Mode MPLS Network: Building the IP Forwarding Table • IP routing protocols are used to build IP routing tables on all LSRs. • FIBs are initially built based on IP routing tables with no labeling information. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 16

Label Allocation in a Frame Mode MPLS Network: Allocating Labels • Every LSR allocates a label for every destination in the IP routing table. • Labels have local significance. • Label allocations are asynchronous. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 17

Label Allocation in a Frame Mode MPLS Network: LIB and LFIB Setup LIB and LFIB structures have to be initialized on the LSR allocating the label. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 18

Label Allocation in a Frame Mode MPLS Network: Labels and Table Setup • Router

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. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 20

Label Distribution and Advertisement: Receiving Label Advertisement • Every LSR stores the received label in its LIB. • Edge LSRs that receive the label from their next hop also store the label information in the FIB. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 21

Label Distribution and Advertisement: Interim Packet Propagation Forwarded IP packets are labeled only on the path segments where the labels have already been assigned. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 22

Label Distribution and Advertisement: Further Label Allocation Every LSR will eventually assign a label

Label Distribution and Advertisement: Receiving Label Advertisement • Every LSR stores received information in its LIB. • LSRs that receive their label from their next hop LSR will also populate the IP forwarding table. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 24

Populating the LFIB • 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. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 25

Packet Propagation Across an MPLS Network © 2006 Cisco Systems, Inc. All rights reserved.

Loop Detection • LDP relies on loop detection mechanisms built into IGPs that are used to determine the path. • If, however, a loop is generated (that is, misconfiguration with static routes), the TTL field in the label header is used to prevent indefinite looping of packets. • TTL functionality in the label header is equivalent to TTL in the IP headers. • TTL is usually copied from the IP headers to the label headers (TTL propagation). © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 27

Normal TTL Operation • Cisco routers have TTL propagation enabled by default. • On ingress: TTL is copied from IP header to label header. • On egress: TTL is copied from label header to IP header. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 28

TTL and Loop Detection Labeled packets are dropped when the TTL is decreased to

Disabling TTL Propagation • TTL propagation can be disabled. • The IP TTL value is not copied into the TTL field of the label, and the label TTL is not copied back into the IP TTL. • Instead, the value 255 is assigned to the label header TTL field on the ingress LSR. • Disabling TTL propagation hides core routers in the MPLS domain. • Traceroute across an MPLS domain does not show any core routers. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 30

Traceroute with Disabled TTL Propagation • The first traceroute packet (ICMP or UDP) that reaches the network is dropped on router A. • An ICMP TTL exceeded message is sent to the source from router A. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 31

Traceroute with Disabled TTL Propagation (Cont. ) • The second traceroute packet that reaches the network is dropped on router D. • An ICMP TTL exceeded message is sent to the source from router D. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 32

Impact of Disabling TTL Propagation • Traceroute across an MPLS domain does not show core routers. • TTL propagation has to be disabled on all label switch routers. • Mixed configurations (some LSRs with TTL propagation enabled and some with TTL propagation disabled) could result in faulty traceroute output. • TTL propagation can be enabled forwarded traffic only—traceroute from LSRs does not use the initial TTL value of 255. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 33

Per Platform Label Allocation • An LFIB on a router usually does not contain an incoming interface. • The same label can be used on any interface—per platform label allocation. • LSR announces a label to an adjacent LSR only once, even if there are parallel links between them. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 34

Per Platform Label Allocation: Benefits and Drawbacks of Per Platform Label Allocation Benefits: • Smaller LFIB • Faster label exchange © 2006 Cisco Systems, Inc. All rights reserved. Drawback: • Insecure: Any neighbor LSR can send packets with any label in the LFIB. MPLS v 2. 2— 2 35

Summary • Labels are propagated across a network either by extending the functionality of existing routing protocols or by creating a new protocol that is dedicated to exchanging labels. • An LSP is a sequence of LSRs that forward labeled packets of a certain forwarding equivalence class. • Penultimate hop popping optimizes MPLS performance (one less LFIB lookup). • IP aggregation can break an LSP into two segments. • Every LSR assigns a label for every destination in the IP routing table. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 36

Summary (Cont. ) • Although labels are locally significant, they have to be advertised to directly reachable peers. • Outgoing labels are inserted in the LFIB after the label is received from the next hop LSR. • Packets are forwarded using labels from the LFIB table rather than the IP routing table. • If TTL propagation is disabled, traceroute across an MPLS domain does not show core routers. • LSR announces a label to an adjacent LSR only once, even if there are parallel links between them. © 2006 Cisco Systems, Inc. All rights reserved. MPLS v 2. 2— 2 37