MPLS Configuration Guide for Cisco NCS 560 Series Routers, Cisco IOS XR Release 7.0.x
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Traditional IP routing
emphasizes on forwarding traffic to the destination as fast as possible. As a
result, the routing protocols find out the least-cost route according to its
metric to each destination in the network and every router forwards the packet
based on the destination IP address and packets are forwarded hop-by-hop. Thus,
traditional IP routing does not consider the available bandwidth of the link.
This can cause some links to be over-utilized compared to others and bandwidth
is not efficiently utilized. Traffic Engineering (TE) is used when the problems
result from inefficient mapping of traffic streams onto the network resources.
Traffic engineering allows you to control the path that data packets follow and
moves traffic flows from congested links to non-congested links that would not
be possible by the automatically computed destination-based shortest path.
Multiprotocol Label
Switching (MPLS) with its label switching capabilities, eliminates the need for
an IP route look-up and creates a virtual circuit (VC) switching function,
allowing enterprises the same performance on their IP-based network services as
with those delivered over traditional networks such as Frame Relay or
Asynchronous Transfer Mode (ATM). MPLS traffic engineering (MPLS-TE) relies on
the MPLS backbone to replicate and expand upon the TE capabilities of Layer 2
ATM and Frame Relay networks.
MPLS-TE learns the
topology and resources available in a network and then maps traffic flows to
particular paths based on resource requirements and network resources such as
bandwidth. MPLS-TE builds a unidirectional tunnel from a source to a
destination in the form of a label switched path (LSP), which is then used to
forward traffic. The point where the tunnel begins is called the tunnel headend
or tunnel source, and the node where the tunnel ends is called the tunnel
tailend or tunnel destination. A router through which the tunnel passes is
called the mid-point of the tunnel.
MPLS uses extensions
to a link-state based Interior Gateway Protocol (IGP), such as Intermediate
System-to-Intermediate System (IS-IS) or Open Shortest Path First (OSPF). MPLS
calculates TE tunnels at the LSP head based on required and available resources
(constraint-based routing). If configured, the IGP automatically routes the
traffic onto these LSPs. Typically, a packet that crosses the MPLS-TE backbone
travels on a single LSP that connects the ingress point to the egress point.
MPLS TE automatically establishes and maintains the LSPs across the MPLS
network by using the Resource Reservation Protocol (RSVP).
Overview of MPLS-TE
Features
In MPLS traffic
engineering, IGP extensions flood the TE information across the network. Once
the IGP distributes the link attributes and bandwidth information, the headend
router calculates the best path from head to tail for the MPLS-TE tunnel. This
path can also be configured explicitly. Once the path is calculated, RSVP-TE is
used to set up the TE LSP (Labeled Switch Path).
To forward the
traffic, you can configure autoroute, forward adjacency, or static routing. The
autoroute feature announces the routes assigned by the tailend router and its
downstream routes to the routing table of the headend router and the tunnel is
considered as a directly connected link to the tunnel.
If forward adjacency
is enabled, MPLS-TE tunnel is advertised as a link in an IGP network with the
link's cost associated with it. Routers outside of the TE domain can see the TE
tunnel and use it to compute the shortest path for routing traffic throughout
the network.
MPLS-TE provides
protection mechanism known as fast reroute to minimize packet loss during a
failure. For fast reroute, you need to create back up tunnels. The autotunnel
backup feature enables a router to dynamically build backup tunnels when they
are needed instead of pre-configuring each backup tunnel and then assign the
backup tunnel to the protected interfaces.
DiffServ Aware
Traffic Engineering (DS-TE) enables you to configure multiple bandwidth
constraints on an MPLS-enabled interface to support various classes of service
(CoS). These bandwidth constraints can be treated differently based on the
requirement for the traffic class using that constraint.
The MPLS traffic engineering autotunnel mesh feature allows you to set up full mesh of TE tunnels automatically with a minimal
set of MPLS traffic engineering configurations. The MPLS-TE auto bandwidth feature allows you to automatically adjusts bandwidth
based on traffic patterns without traffic disruption.
The MPLS-TE interarea
tunneling feature allows you to establish TE tunnels spanning multiple Interior
Gateway Protocol (IGP) areas and levels, thus eliminating the requirement that
headend and tailend routers should reside in a single area.
MPLS-TE Nonstop Routing (NSR) is enabled by default without any user configuration and cannot be disabled.
MPLS-TE NSR means the application is in hot-standby mode and standby MPLS-TE instance is ready to take over from the active
instance quickly on RP failover.
Note that the MPLS-TE does not do routing. If there is standby card available then the MPLS-TE instance is in a hot-standby
position.
The following output shows the status of MPLS-TE NSR:
Router#show mpls traffic-eng nsr status
TE Process Role : V1 Active
Current Status : Ready
Ready since : Tue Nov 01 10:42:34 UTC 2022 (1w3d ago)
IDT started : Tue Nov 01 03:28:48 UTC 2022 (1w3d ago)
IDT ended : Tue Nov 01 03:28:48 UTC 2022 (1w3d ago)
Previous Status : Not ready
Not ready reason : Collaborator disconnected
Not ready since : Tue Nov 01 10:42:34 UTC 2022 (1w3d ago)
During any issues with the MPLS-TE, the NSR on the router gets affected which is displayed in the show redundancy output as
follows:
Router#show mpls traffic-eng nsr status details
.
.
.
Current active rmf state: 4 (I_READY)
All standby not-ready bits clear - standby should be ready
Current active rmf state for NSR: Not ready
<jid> <node> <name> Reason for standby not NSR-ready
1082 0/RP0/CPU0 te_control TE NSR session not synchronized
Not ready set Wed Nov 19 17:28:14 2022: 5 hours, 23 minutes ago
1082 0/RP1/CPU0 te_control Standby not connected
Not ready set Wed Nov 19 17:29:11 2022: 5 hours, 22 minutes ago
How MPLS-TE
Works
MPLS-TE automatically
establishes and maintains label switched paths (LSPs) across the backbone by
using RSVP. The path that an LSP uses is determined by the LSP resource
requirements and network resources, such as bandwidth. Available resources are
flooded by extensions to a link state based Interior Gateway Protocol (IGP).
MPLS-TE tunnels are calculated at the LSP headend router, based on a fit
between the required and available resources (constraint-based routing). The
IGP automatically routes the traffic to these LSPs. Typically, a packet
crossing the MPLS-TE backbone travels on a single LSP that connects the ingress
point to the egress point.
The following
sections describe the components of MPLS-TE:
Tunnel
Interfaces
From a Layer 2
standpoint, an MPLS tunnel interface represents the headend of an LSP. It is
configured with a set of resource requirements, such as bandwidth and media
requirements, and priority. From a Layer 3 standpoint, an LSP tunnel interface
is the headend of a unidirectional virtual link to the tunnel destination.
MPLS-TE Path
Calculation Module
This calculation
module operates at the LSP headend. The module determines a path to use for an
LSP. The path calculation uses a link-state database containing flooded
topology and resource information.
RSVP with TE
Extensions
RSVP operates at
each LSP hop and is used to signal and maintain LSPs based on the calculated
path.
MPLS-TE Link
Management Module
This module operates
at each LSP hop, performs link call admission on the RSVP signaling messages,
and keep track on topology and resource information to be flooded.
Link-state
IGP
Either Intermediate
System-to-Intermediate System (IS-IS) or Open Shortest Path First (OSPF) can be
used as IGPs. These IGPs are used to globally flood topology and resource
information from the link management module.
Label Switching
Forwarding
This forwarding
mechanism provides routers with a Layer 2-like ability to direct traffic across
multiple hops of the LSP established by RSVP signaling.
Configuring
MPLS-TE
MPLS-TE requires
co-ordination among several global neighbor routers. RSVP, MPLS-TE and IGP are
configured on all routers and interfaces in the MPLS traffic engineering
network. Explicit path and TE tunnel interfaces are configured only on the
head-end routers. MPLS-TE requires some basic configuration tasks explained in
this section.
Building MPLS-TE
Topology
Building MPLS-TE
topology, sets up the environment for creating MPLS-TE tunnels. This procedure
includes the basic node and interface configuration for enabling MPLS-TE. To
perform constraint-based routing, you need to enable OSPF or IS-IS as IGP
extension.
Before You
Begin
Before you start to
build the MPLS-TE topology, the following pre-requisites are required:
Stable router
ID is required at either end of the link to ensure that the link is successful.
If you do not assign a router ID, the system defaults to the global router ID.
Default router IDs are subject to change, which can result in an unstable link.
Enable RSVP on
the port interface.
Example
This example enables
MPLS-TE on a node and then specifies the interface that is part of the MPLS-TE.
Here, OSPF is used as the IGP extension protocol for information distribution.
This example enables
MPLS-TE on a node and then specifies the interface that is part of the MPLS-TE.
Here, IS-IS is used as the IGP extension protocol for information distribution.
Creating an MPLS-TE
tunnel is a process of customizing the traffic engineering to fit your network
topology. The MPLS-TE tunnel is created at the headend router. You need to
specify the destination and path of the TE LSP.
To steer traffic through the tunnel, you can use the following ways:
Static Routing
Autoroute Announce
Forwarding Adjacency
From the 7.1.1 release, IS-IS autoroute announce function is enhanced to redirect traffic from a source IP address prefix
to a matching IP address assigned to an MPLS-TE tunnel destination interface.
Before You
Begin
The following
prerequisites are required to create an MPLS-TE tunnel:
You must have a
router ID for the neighboring router.
Stable router
ID is required at either end of the link to ensure that the link is successful.
If you do not assign a router ID to the routers, the system defaults to the
global router ID. Default router IDs are subject to change, which can result in
an unstable link.
Configuration
Example
This example
configures an MPLS-TE tunnel on the headend router with a destination IP
address 192.168.92.125. The bandwidth for the tunnel, path-option, and
forwarding parameters of the tunnel are also configured. You can use static
routing, autoroute announce or forwarding adjacency to steer traffic through
the tunnel.
Verify the
configuration of MPLS-TE tunnel using the following command.
RP/0/# show mpls traffic-engineering tunnels brief
Signalling Summary:
LSP Tunnels Process: running
RSVP Process: running
Forwarding: enabled
Periodic reoptimization: every 3600 seconds, next in 2538 seconds
Periodic FRR Promotion: every 300 seconds, next in 38 seconds
Auto-bw enabled tunnels: 0 (disabled)
TUNNEL NAME DESTINATION STATUS STATE
tunnel-te1 192.168.92.125 up up
Displayed 1 up, 0 down, 0 recovering, 0 recovered heads
Automatic Modification Of An MPLS-TE Tunnel’s Metric
If the IGP calculation on a router results in an equal cost multipath (ECMP) scenario where next-hop interfaces are a mix
of MPLS-TE tunnels and physical interfaces, you may want to ensure that a TE tunnel is preferred. Consider this topology:
Figure 1. MPLS-TE Tunnel
All links in the network have a metric of 5.
To offload a congested link between R3 and R4, an MPLS-TE tunnel is created from R3 to R2.
If the metric of the tunnel is also 5, traffic from R3 to R5 is load-balanced between the tunnel and the physical R3-R4 link.
To ensure that the MPLS-TE tunnel is preferred in such scenarios, configure the autoroute metric command on the tunnel interface. The modified metric is applied in the routing information base (RIB), and the tunnel is
preferred over the physical path of the same metric. Sample configuration:
The autoroute metric command syntax is autoroute metric {absolute|relative} value
absolute enables the absolute metric mode, for a metric range between 1 and 2147483647.
relative enables the relative metric mode, for a metric range between -10 and 10, including zero.
Note
Since the relative metric is not saved in the IGP database, the advertised metric of the MPLS-TE tunnel remains 5, and doesn't affect SPF calculation
outcomes on other nodes.
Fast reroute (FRR)
provides link protection to LSPs enabling the traffic carried by LSPs that
encounter a failed link to be rerouted around the failure. The reroute decision
is controlled locally by the router connected to the failed link. The headend
router on the tunnel is notified of the link failure through IGP or through
RSVP. When it is notified of a link failure, the headend router attempts to
establish a new LSP that bypasses the failure. This provides a path to
reestablish links that fail, providing protection to data transfer. The path of
the backup tunnel can be an IP explicit path, a dynamically calculated path, or
a semi-dynamic path. For detailed conceptual information on fast reroute, see
MPLS-TE Features - Details
Before You
Begin
The following
prerequisites are required to create an MPLS-TE tunnel:
You must have a
router ID for the neighboring router.
Stable router
ID is required at either end of the link to ensure that the link is successful.
If you do not assign a router ID to the routers, the system defaults to the
global router ID. Default router IDs are subject to change, which can result in
an unstable link.
Configuration
Example
This example
configures fast reroute on an MPLS-TE tunnel. Here, tunnel-te 2 is configured
as the back-up tunnel. You can use the
protected-by command to configure path protection for
an explicit path that is protected by another path.
The MPLS Traffic
Engineering Auto-Tunnel Backup feature enables a router to dynamically build
backup tunnels on the interfaces that are configured with MPLS TE tunnels
instead of building MPLS-TE tunnels statically.
The MPLS-TE
Auto-Tunnel Backup feature has these benefits:
Backup tunnels are
built automatically, eliminating the need for users to pre-configure each
backup tunnel and then assign the backup tunnel to the protected interface.
Protection is
expanded—FRR does not protect IP traffic that is not using the TE tunnel or
Label Distribution Protocol (LDP) labels that are not using the TE tunnel.
The TE attribute-set
template that specifies a set of TE tunnel attributes, is locally configured at
the headend of auto-tunnels. The control plane triggers the automatic
provisioning of a corresponding TE tunnel, whose characteristics are specified
in the respective attribute-set.
Configuration
Example
This example
configures Auto-Tunnel backup on an interface and specifies the attribute-set
template for the auto tunnels. In this example, unused backup tunnels are
removed every 20 minutes using a timer and also the range of tunnel interface
numbers are specified.
This example shows a
sample output for automatic backup tunnel configuration.
RP/0/# show mpls traffic-eng tunnels brief
TUNNEL NAME DESTINATION STATUS STATE
tunnel-te0 200.0.0.3 up up
tunnel-te1 200.0.0.3 up up
tunnel-te2 200.0.0.3 up up
tunnel-te50 200.0.0.3 up up
*tunnel-te60 200.0.0.3 up up
*tunnel-te70 200.0.0.3 up up
*tunnel-te80 200.0.0.3 up up
The backup tunnels
that bypass only a single link of the LSP path are referred as Next Hop (NHOP)
backup tunnels because they terminate at the LSP's next hop beyond the point of
failure. They protect LSPs, if a link along their path fails, by rerouting the
LSP traffic to the next hop, thus bypassing the failed link.
Configuration
Example
This example
configures next hop backup tunnel on an interface and specifies the
attribute-set template for the auto tunnels. In this example, unused backup
tunnels are removed every 20 minutes using a timer and also the range of tunnel
interface numbers are specified.
Shared Risk Link
Groups (SRLG) in MPLS traffic engineering refer to situations in which links in
a network share common resources. These links have a shared risk, and that is
when one link fails, other links in the group might fail too.
OSPF and IS-IS flood
the SRLG value information (including other TE link attributes such as
bandwidth availability and affinity) using a sub-type length value (sub-TLV),
so that all routers in the network have the SRLG information for each link.
MPLS-TE SRLG feature
enhances backup tunnel path selection by avoiding using links that are in the
same SRLG as the interfaces it is protecting while creating backup tunnels.
Configuration
Example
This example creates
a backup tunnel and excludes the protected node IP address from the explicit
path.
MPLS-TE Flexible
Name-based Tunnel Constraints provides a simplified and more flexible means of
configuring link attributes and path affinities to compute paths for the
MPLS-TE tunnels.
In traditional TE,
links are configured with attribute-flags that are flooded with TE link-state
parameters using Interior Gateway Protocols (IGPs), such as Open Shortest Path
First (OSPF).
MPLS-TE Flexible
Name-based Tunnel Constraints lets you assign, or map, up to 32 color names for
affinity and attribute-flag attributes instead of 32-bit hexadecimal numbers.
After mappings are defined, the attributes can be referred to by the
corresponding color name.
Configuration
Example
This example shows
assigning a how to associate a tunnel with affinity constraints.
RP/0/# configure
RP/0/(config)# mpls traffic-eng
RP/0/(config-mpls-te)# affinity-map red 1
RP/0/(config-mpls-te)# interface HundredGigabitEthernet0/0/0/3RP/0/(config-mpls-te-if)# attribute-names red
RP/0/(config)# interface tunnel-te 2
RP/0/(config-if)# affinity include red
RP/0/(config)# commit
Configuring
Automatic Bandwidth
Automatic bandwidth
allows you to dynamically adjust bandwidth reservation based on measured
traffic. MPLS-TE automatic bandwidth monitors the traffic rate on a tunnel
interface and resizes the bandwidth on the tunnel interface to align it closely
with the traffic in the tunnel. MPLS-TE automatic bandwidth is configured on
individual Label Switched Paths (LSPs) at every headend router.
The following table specifies the parameters that can be configured as part of automatic bandwidth configuration.
Table 1. Automatic Bandwidth Parameters
Bandwidth Parameters
Description
Application frequency
Configures how often the tunnel bandwidths changed for each tunnel. The default value is 24 hours.
Bandwidth limit
Configures the minimum and maximum automatic bandwidth to set on a tunnel.
Bandwidth collection frequency
Enables bandwidth collection without adjusting the automatic bandwidth. The default value is 5 minutes.
Overflow threshold
Configures tunnel overflow detection.
Adjustment threshold
Configures the tunnel-bandwidth change threshold to trigger an adjustment.
Configuration
Example
This example enables automatic bandwidth on MPLS-TE tunnel interface and configure the following automatic bandwidth variables.
Application frequency
Bandwidth limit
Adjustment threshold
Overflow detection
RP/0/# configure
RP/0/(config)# interface tunnel-te 1
RP/0/(config-if)# auto-bw
RP/0/(config-if-tunte-autobw)# application 1000
RP/0/(config-if-tunte-autobw)# bw-limit min 30 max 1000
RP/0/(config-if-tunte-autobw)# adjustment-threshold 50 min 800
RP/0/(config-if-tunte-autobw)# overflow threshold 100 limit 1
RP/0/(config)# commit
Verification
Verify the automatic
bandwidth configuration using the
show mpls
traffic-eng tunnels auto-bw brief command.
RP/0/# show mpls traffic-eng tunnels auto-bw brief
Tunnel LSP Last appl Requested Signalled Highest Application
Name ID BW(kbps) BW(kbps) BW(kbps) BW(kbps) Time Left
-------------- ------ ---------- ---------- ---------- ---------- --------------
tunnel-te1 5 500 300 420 1h 10m
The MPLS-TE
auto-tunnel mesh (auto-mesh) feature allows you to set up full mesh of TE
Point-to-Point (P2P) tunnels automatically with a minimal set of MPLS traffic
engineering configurations. You can configure one or more mesh-groups and each
mesh-group requires a destination-list (IPv4 prefix-list) listing destinations,
which are used as destinations for creating tunnels for that mesh-group.
You can configure
MPLS-TE auto-mesh type attribute-sets (templates) and associate them to
mesh-groups. Label Switching Routers (LSRs) can create tunnels using the tunnel
properties defined in this attribute-set.
Auto-Tunnel mesh
configuration minimizes the initial configuration of the network. You can
configure tunnel properties template and mesh-groups or destination-lists on TE
LSRs that further creates full mesh of TE tunnels between those LSRs. It
eliminates the need to reconfigure each existing TE LSR in order to establish a
full mesh of TE tunnels whenever a new TE LSR is added in the network.
Configuration
Example
This example
configures an auto-tunnel mesh group and specifies the attributes for the
tunnels in the mesh-group.
Verify the
auto-tunnel mesh configuration using the
show mpls
traffic-eng auto-tunnel mesh command.
RP/0/# show mpls traffic-eng auto-tunnel mesh
Auto-tunnel Mesh Global Configuration:
Unused removal timeout: 1h 0m 0s
Configured tunnel number range: 1000-2000
Auto-tunnel Mesh Groups Summary:
Mesh Groups count: 1
Mesh Groups Destinations count: 3
Mesh Groups Tunnels count:
3 created, 3 up, 0 down, 0 FRR enabled
Mesh Group: 10 (3 Destinations)
Status: Enabled
Attribute-set: 10
Destination-list: dl-65 (Not a prefix-list)
Recreate timer: Not running
Destination Tunnel ID State Unused timer
---------------- ----------- ------- ------------
192.168.0.2 1000 up Not running
192.168.0.3 1001 up Not running
192.168.0.4 1002 up Not running
Displayed 3 tunnels, 3 up, 0 down, 0 FRR enabled
Auto-mesh Cumulative Counters:
Last cleared: Wed Oct 3 12:56:37 2015 (02:39:07 ago)
Total
Created: 3
Connected: 0
Removed (unused): 0
Removed (in use): 0
Range exceeded: 0
Configuring an MPLS Traffic Engineering Interarea Tunneling
The MPLS TE Interarea Tunneling feature allows you to establish MPLS TE tunnels that span multiple Interior Gateway Protocol
(IGP) areas and levels. This feature removes the restriction that required the tunnel headend and tailend routers both to
be in the same area. The IGP can be either Intermediate System-to-Intermediate System (IS-IS) or Open Shortest Path First
(OSPF).To configure an inter-area tunnel, you specify on the headend router a loosely routed explicit path for the tunnel
label switched path (LSP) that identifies each area border router (ABR) the LSP should traverse using the next-address loose
command. The headend router and the ABRs along the specified explicit path expand the loose hops, each computing the path
segment to the next ABR or tunnel destination.
Configuration Example
This example configures an IPv4 explicit path with ABR configured as loose address on the headend router.
Fast Reroute (FRR) is a mechanism for protecting MPLS TE LSPs from link and node failures by locally repairing the LSPs at
the point of failure, allowing data to continue to flow on them while their headend routers try to establish new end-to-end
LSPs to replace them. FRR locally repairs the protected LSPs by rerouting them over backup tunnels that bypass failed links
or node.
Note
If FRR is greater than 50ms, it might lead to a loss of traffic.
Backup tunnels that
bypass only a single link of the LSP’s path provide link protection. They
protect LSPs if a link along their path fails by rerouting the LSP’s traffic to
the next hop (bypassing the failed link). These tunnels are referred to as
next-hop (NHOP) backup tunnels because they terminate at the LSP’s next hop
beyond the point of failure.
The following figure
illustrates link protection.
Figure 2. Link
Protection
FRR provides node
protection for LSPs. Backup tunnels that bypass next-hop nodes along LSP paths
are called next-next-hop (NNHOP) backup tunnels because they terminate at the
node following the next-hop node of the LSP paths, bypassing the next-hop node.
They protect LSPs if a node along their path fails by enabling the node
upstream of the failure to reroute the LSPs and their traffic around the failed
node to the next-next hop. NNHOP backup tunnels also provide protection from
link failures, because they bypass the failed link and the node.
The following figure
illustrates node protection.
Figure 3. Node
Protection
MPLS-TE
Forwarding Adjacency
MPLS TE forwarding
adjacency allows you to handle a TE label-switched path (LSP) tunnel as a link
in an Interior Gateway Protocol (IGP) network that is based on the Shortest
Path First (SPF) algorithm. Both Intermediate System-to-Intermediate System
(IS-IS) and Open Shortest Path First (OSPF) are supported as the IGP. A
forwarding adjacency can be created between routers regardless of their
location in the network. The routers can be located multiple hops from each
other.
As a result, a TE
tunnel is advertised as a link in an IGP network with the tunnel's cost
associated with it. Routers outside of the TE domain see the TE tunnel and use
it to compute the shortest path for routing traffic throughout the network. TE
tunnel interfaces are advertised in the IGP network just like any other links.
Routers can then use these advertisements in their IGPs to compute the SPF even
if they are not the headend of any TE tunnels.
Automatic
Bandwidth
Automatic bandwidth
allows you to dynamically adjust bandwidth reservation based on measured
traffic. MPLS-TE automatic bandwidth is configured on individual Label Switched
Paths (LSPs) at every headend router. MPLS-TE automatic bandwidth monitors the
traffic rate on a tunnel interface and resizes the bandwidth on the tunnel
interface to align it closely with the traffic in the tunnel.
MPLS-TE automatic
bandwidth can perform these functions:
Monitors periodic polling of
the tunnel output rate
Resizes the tunnel bandwidth
by adjusting the highest rate observed during a given period.
For every
traffic-engineered tunnel that is configured for an automatic bandwidth, the
average output rate is sampled, based on various configurable parameters. Then,
the tunnel bandwidth is readjusted automatically based on either the largest
average output rate that was noticed during a certain interval, or a configured
maximum bandwidth value.
While re-optimizing
the LSP with the new bandwidth, a new path request is generated. If the new
bandwidth is not available, the last good LSP remains used. This way, the
network experiences no traffic interruptions. If minimum or maximum bandwidth
values are configured for a tunnel, the bandwidth, which the automatic
bandwidth signals, stays within these values.
The output rate on a
tunnel is collected at regular intervals that are configured by using the
application command in MPLS-TE auto bandwidth
interface configuration mode. When the application period timer expires, and
when the difference between the measured and the current bandwidth exceeds the
adjustment threshold, the tunnel is re-optimized. Then, the bandwidth samples
are cleared to record the new largest output rate at the next interval. If a
tunnel is shut down, and is later brought again, the adjusted bandwidth is
lost, and the tunnel is brought back with the initially configured bandwidth.
When the tunnel is brought back, the application period is reset.
MPLS Traffic
Engineering Interarea Tunneling
The MPLS-TE
interarea tunneling feature allows you to establish TE tunnels spanning
multiple Interior Gateway Protocol (IGP) areas and levels, thus eliminating the
requirement that headend and tailend routers reside in a single area.
Interarea support
allows the configuration of a TE LSP that spans multiple areas, where its
headend and tailend label switched routers (LSRs) reside in different IGP
areas. Customers running multiple IGP area backbones (primarily for scalability
reasons) requires Multiarea and Interarea TE . This lets you limit the amount
of flooded information, reduces the SPF duration, and lessens the impact of a
link or node failure within an area, particularly with large WAN backbones
split in multiple areas.
The following
figure shows a typical interarea TE network using OSPF.
Figure 4. Interarea
(OSPF) TE Network Diagram
The following
figure shows a typical interlevel (IS-IS) TE Network.
Figure 5. Interlevel
(IS-IS) TE Network Diagram
As shown in the
Interlevel
(IS-IS) TE Network Diagram,
R2, R3, R7, and R4 maintain two databases for routing and TE information. For
example, R3 has TE topology information related to R2, flooded through Level-1
IS-IS LSPs plus the TE topology information related to R4, R9, and R7, flooded
as Level 2 IS-IS Link State PDUs (LSPs) (plus, its own IS-IS LSP).
Loose hop
optimization allows the re-optimization of tunnels spanning multiple areas and
solves the problem which occurs when an MPLS-TE LSP traverses hops that are not
in the LSP's headend's OSPF area and IS-IS level. Interarea MPLS-TE allows you
to configure an interarea traffic engineering (TE) label switched path (LSP) by
specifying a loose source route of ABRs along the path. Then it is the
responsibility of the ABR (having a complete view of both areas) to find a path
obeying the TE LSP constraints within the next area to reach the next hop ABR
(as specified on the headend router). The same operation is performed by the
last ABR connected to the tailend area to reach the tailend LSR.
You must be aware
of these considerations when using loose hop optimization:
You must specify the router
ID of the ABR node (as opposed to a link address on the ABR).
When multiarea is deployed
in a network that contains subareas, you must enable MPLS-TE in the subarea for
TE to find a path when loose hop is specified.
You must specify the
reachable explicit path for the interarea tunnel.
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