© 2006 cisco systems, inc. all rights reserved. mpls v2.2—8-1 mpls te overview understanding mpls...

© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-1

MPLS TE Overview

Understanding MPLS TE Components


Outline

• Overview

• Traffic Tunnels: Concepts

• Traffic Tunnels: Characteristics

• Traffic Tunnels: Attributes

• Network Links and Link Attributes

• Constraint-Based Path Computation

• TE Processes

• Role of RSVP in Path Setup and Trunk Admission Control

• Forwarding Traffic to a Tunnel

• Summary


The concept of MPLS TE traffic tunnels was introduced to overcome the limitations of hop-by-hop IP routing:

• A tunnel is an aggregation of traffic flows that are placed inside a common MPLS label-switched path.

• Flows are then forwarded along a common path within a network.

Traffic Tunnels: Concepts


Traffic Tunnels: Concepts (Cont.)

• Unidirectional single class of service model encapsulates all of the traffic between an ingress and an egress router.

• Different classes of service model assigns traffic into separate tunnels with different characteristics.


Traffic Tunnels – Characteristics

• A traffic tunnel is distinct from the MPLS LSP through which it traverses:– More than one TE tunnel can be defined between two

points:

• Each tunnel may pick the same or different paths through the network

• Each tunnel will use different MPLS labels

– A traffic tunnel can be moved from one path onto another based on resources in the network.

• A traffic tunnel is configured by defining its required attributes and characteristics.


Traffic Tunnels – Attributes

• Attributes are explicitly assigned through administrative action.

• A traffic tunnel is characterized by:

– Its ingress (headend) and egress (tailend) label switch routers

– The forwarding equivalence class that is mapped onto it

– A set of attributes that determine its characteristics


Traffic Tunnels–Attributes (Cont.)

The administrator enters the relevant information (attributes) at the headend of the traffic tunnel:• Traffic parameter—Resources required for tunnel (for example,

required bandwidth)

• Generic path selection and management—Path can be administratively specified or computed by the IGP

• Resource class affinity—Include or exclude certain links for certain traffic tunnels

• Adaptability—Should the traffic tunnel be reoptimized?

• Priority and preemption—Importance of a traffic tunnel and possibility for a preemption of another tunnel

• Resilience—Desired behavior under fault conditions


Network Links and Link Attributes

Resource attributes (link availability) are configured locally on the router interfaces:• Maximum bandwidth– The amount of bandwidth available

• Link affinity string – To allow the operator to administratively include or exclude

links in path calculations

• Constraint-based specific metric– TE default metric


Constraint-Based Path Computation

• Constraint-based routing is demand-driven.

• Resource-reservation-aware routing paradigm:

–Based on criteria including, but not limited to, network topology

–Calculated at the edge of a network:• Modified Dijkstra’s algorithm at tunnel headend (CSPF

[Constraint-based SPF] or PCALC [path calculation]).

• Output is a sequence of IP interface addresses (next-hop routers) between tunnel endpoints.


Constraint-Based Path Computation (Cont.)

• Constraint-based routing takes into account:– Policy constraints associated with the tunnel and physical links

– Physical resource availability

– Network topology state

• Two types of tunnels can be established across those links with matching attributes:– Dynamic—Using the least-cost path computed by OSPF or IS-IS

– Explicit—Definition of a path by using Cisco IOS configuration commands


Traffic Engineering Processes

• Information distribution

• Path selection and calculation

• Path setup

• Trunk admission control

• Forwarding traffic on to tunnel

• Path maintenance


• When the path has been determined, a signaling protocol is needed:– To establish and maintain label-switched paths (LSPs) for

traffic tunnels

– For creating and maintaining resource reservation states across a network (bandwidth allocation)

• The Resource Reservation Protocol (RSVP) was adopted by the MPLS workgroup of the IETF.

Role of RSVP in Path Setup Procedures


Path Setup with RSVP

• When the path has been calculated, it must be signaled across the network.– Reserve any bandwidth to avoid “double booking” from

other TE reservations.

– Priority can be used to preempt low priority existing tunnels.

• RSVP is used to set up TE LSP.– PATH message (from head to tail) carries LABEL_REQUEST.

– RESV message (from tail to head) carries LABEL.

• When RESV messages reaches headend, tunnel interface is up.

• RSVP messages exist for LSP teardown and error signaling.


RSVP and Trunk Admission Control

• On receipt of PATH message:– Router checks whether there is bandwidth available to

honor the reservation.

– If bandwidth is available, then RSVP is accepted.

• On receipt of a RESV message:– Router actually reserves the bandwidth for the TE LSP.

– If preemption is required, lower priority LSPs are torn down.

• OSPF or IS-IS updates are triggered.


Forwarding Traffic to a Tunnel

• IP routing is separate from LSP routing and does not see internal details of the LSP.

• The traffic has to be mapped to the tunnel:– Static routing—The static route in the IP routing table

points to an LSP tunnel interface.

– Policy routing—The next-hop interface is an LSP tunnel.

– Autoroute—SPF enhancement:

• The headend sees the tunnel as a directly connected interface (for modified SPF only).

• The default cost of a tunnel is equal to the shortest IGP metric regardless of the used path.


• Autoroute feature enables the headend to see the LSP as a directly connected interface:– This feature is used only for the SPF route determination,

not for the constraint-based path computation.

– All traffic directed to prefixes topologically behind the tunnel endpoint (tailend) is forwarded onto the tunnel.

• Autoroute affects the headend only; other routers on the LSP path do not see the tunnel.

• Tunnel is treated as a directly connected link to the tailend:– When tunnel tail is seen in PATH list during IGP SPF,

replace outgoing physical interface with tunnel interface.

– Inherit tunnel to all downstream neighbors of tailend.

IP Forwarding Database Modification with Autoroute


Autoroute Topology (OSPF and ISIS)

Tunnel1: R1 R2 R3 R4 R5 Tunnel2: R1 R6 R7 R4


Autoroute Topology (OSPF and ISIS)

From R1 Router Perspective: Next hop to R5 is Tunnel1.

Next hop to R4 and R8 is Tunnel2. All nodes behind tunnel are routed via tunnel.

2020


Summary

• Traffic tunnels are configured with a set of resource requirements, such as bandwidth and priority.

• CSPF augments the link cost by considering other factors such as bandwidth availability or link latency when choosing a path.

• RSVP with TE extensions is used for establishing and maintaining LSPs.

• TE tunnels do not appear in the IP routing table.

© 2006 cisco systems, inc. all rights reserved. mpls v2.2—8-1 mpls te overview understanding mpls...

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