MPLS TE Fast
Reroute Link and Node Protection
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.
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
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
Services Traffic Engineering
Services Aware Traffic Engineering (DS-TE) is an extension of the regular
MPLS-TE feature. Regular traffic engineering does not provide bandwidth
guarantees to different traffic classes. A single bandwidth constraint is used
in regular TE that is shared by all traffic. To support various classes of
service (CoS), you can configure multiple bandwidth constraints. These
bandwidth constraints can be treated differently based on the requirement for
the traffic class using that constraint.
Cisco IOS XR
software supports two DS-TE modes: pre-standard and IETF. The pre-standard
DS-TE mode uses the Cisco proprietary mechanisms for RSVP signaling and IGP
advertisements. This DS-TE mode does not interoperate with third-party vendor
equipment. Pre-standard DS-TE is enabled only after configuring the sub-pool
bandwidth values on MPLS-enabled interfaces. Pre-standard DS-TE mode supports a
single bandwidth constraint model a Russian Doll Model (RDM) with two bandwidth
pools: global-pool and sub-pool. TE class map is not used with Pre-standard
IETF DS-TE mode uses
IETF-defined extensions for RSVP and IGP. This mode inter-operates with
third-party vendor equipment. IETF mode supports multiple bandwidth constraint
models, including RDM and Maximum Allocation Bandwidth Constraint Model (MAM),
both with two bandwidth pools. In an IETF DS-TE network, identical bandwidth
constraint models must be configured on all nodes. TE class map is used with
IETF DS-TE mode and must be configured the same way on all nodes in the
The MAM constraint
model has the following characteristics:
- Easy to use and intuitive.
- Isolation across class types.
- Simultaneously achieves
isolation, bandwidth efficiency, and protection against QoS degradation.
The RDM constraint
model has these characteristics:
- Allows greater sharing of
bandwidth among different class types.
- Ensures bandwidth efficiency
simultaneously and protection against QoS degradation of all class types.
- Specifies that it is used
with preemption to simultaneously achieve isolation across class-types such
that each class-type is guaranteed its share of bandwidth, bandwidth
efficiency, and protection against QoS degradation of all class types.
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
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.
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.
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.
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.
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.
Engineering Interarea Tunneling
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.
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.
figure shows a typical interarea TE network using OSPF.
Figure 4. Interarea
(OSPF) TE Network Diagram
figure shows a typical interlevel (IS-IS) TE Network.
Figure 5. Interlevel
(IS-IS) TE Network Diagram
As shown in the
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).
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.