Selecting Domain Paths in Inter-Domain MPLS-TP and MPLS-TE Networks
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-1 MPLS TE Overview Understanding MPLS...
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Transcript of © 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
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-2
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
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-3
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
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-4
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.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-5
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.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-6
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
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-7
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
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-8
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
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-9
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.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-10
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
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-11
Constraint-Based Path Computation (Cont.)
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-12
Constraint-Based Path Computation (Cont.)
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-13
Traffic Engineering Processes
• Information distribution
• Path selection and calculation
• Path setup
• Trunk admission control
• Forwarding traffic on to tunnel
• Path maintenance
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-14
• 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
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-15
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.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-16
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.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-17
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.
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-18
• 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
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-19
Autoroute Topology (OSPF and ISIS)
Tunnel1: R1 R2 R3 R4 R5 Tunnel2: R1 R6 R7 R4
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-20
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
© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—8-21
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—8-22