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Configuring and Verifying a Primary Path

Purpose

Primary paths are optional and when configured, limit the RSVP calculation of the complete path to the routers specified in the primary Explicit Route Object (ERO) list, which determines the physical path for the LSP. When primary paths are not configured, the ingress router determines the path to the egress router. Only one primary path is permitted per LSP, as shown in Figure 1.

Figure 1: Primary Path

Image g016746.gif

Within the configuration of the primary physical path, you can specify strict or loose ERO values and parameters that affect only the primary physical path, such as bandwidth or priority. The ERO list for the primary path includes an address for each transit router. Specifying the ingress and/or egress routers is optional. For each router address, you can specify the type, which can be one of the following:

  • Strict—The route taken from the previous router to this router is a direct path and cannot include any other routers. This is the default. If the address is an interface address, this router also ensures that the incoming interface is the one specified. Specifying the incoming interface is important when there are parallel links between the previous router and this router, and because it ensures that routing can be enforced on a per-link basis.

    For strict addresses, you must ensure that the router immediately preceding the router you are configuring has a direct connection to that router. The address can be a loopback interface address, in which case the incoming interface is not checked.

  • Loose—The route taken from the previous router to this router need not be a direct path, can include other routers, and can be received on any interface. The address can be any interface address or the address of the loopback interface.

If you are listing more than one address, specify the addresses in order, starting with the ingress router (optional) or the first transit router, and continuing sequentially along the path up to the egress router (optional) or the router immediately before the egress router. You need to specify only one address per router hop. If you specify more than one address for the same router, only the first address is used; the additional addresses are ignored and truncated.

When configuring a primary path, you can specify the bandwidth and priority values associated with that primary path.

The bandwidth value is included in the sender’s Tspec field in RSVP path setup messages. You specify the bandwidth value in bits per second, with a higher value implying a greater user traffic volume. The default bandwidth is 0 bits per second. A nonzero bandwidth requires transit routers to reserve capacity along the outbound links for the path. The RSVP reservation scheme is used to reserve this capacity. Any failure in bandwidth reservation (such as failures at RSVP policy control or admission control) might cause the LSP setup to fail.

The priority value is composed of two distinct values: a setup and a hold priority. The setup priority value is used to determine if there is enough bandwidth available at that priority level to establish the primary path. The priority level is between 0 (best) and 7 (worst).The hold priority value is used by an established primary path to retain its bandwidth reservations in the network. If insufficient link bandwidth is available during session establishment, the setup priority is compared to the hold priorities of other established sessions to determine whether some of them should be preempted to accommodate the new session. Sessions with worse hold priorities are preempted.

To configure and verify a primary path, follow these steps:

  1. Configure a Primary Path
  2. Verify That the Primary Path Is Operational

Configure a Primary Path

Action

To configure a primary path with an ERO list, bandwidth, and priority, follow these steps:

  1. In configuration mode, go to the following hierarchy level:
    [edit]user@host# edit protocols mpls
  2. Configure the primary ERO list:
    [edit protocols mpls]user@host# set path path-name address strict

    For example:

    [edit protocols mpls]user@R1# set path via-r2 10.0.12.14 strict user@R1# set path via-r2 10.0.24.2 strict
  3. Configure the LSP:
    [edit protocols mpls]user@host# set label-switched-path lsp-path-name to destination;

    For example:

    [edit protocols mpls]user@R1# set label-switched-path r1-to-r5 to 192.168.5.1;
  4. Configure the primary path:
    [edit protocols mpls label-switched-path lsp-path-name] user@host# set primary primary-name

    For example:

    [edit protocols mpls label-switched-path r1-to-r5]user@R1# set primary via-r2
  5. Configure the bandwidth:
    [edit protocols mpls label-switched-path lsp-path-name]user@host# set primary primary-name bandwidth bandwidth

    For example:

    [edit protocols mpls label-switched-path r1-to-r5]user@R1# set primary via-r2 bandwidth 35m
  6. Configure the priority value:
    [edit protocols mpls label-switched-path lsp-path-name]user@host# set primary primary-name priority reservation-priority setup-priority

    For example:

    [edit protocols mpls label-switched-path r1-to-r5]user@R1# set primary via-r2 priority 6 6
  7. Verify and commit the configuration:
    [edit protocols mpls label-switched-path lsp-path-name]user@host# show user@host# commit

Sample Output

The sample output below illustrates the configuration of the primary path on ingress router R1 in the network shown in Primary Path.

[edit protocols mpls]
user@R1# show  
label-switched-path r1-to-r5 {
    to 192.168.5.1;
    primary via-r2 {   #  Bandwidth and priority configured at the primary path 
        bandwidth 35m; #  level of the hierarchy
        priority 6 6; #  Priority setup and hold values
    }
}
path via-r2 { #  Primary ERO list
    10.0.12.14 strict;
    10.0.24.2 strict;
[...Output truncated...]

[edit protocols mpls]
user@R1# commit  
commit complete

Meaning

The sample output shows a label-switched path (LSP) with bandwidth and priority applied to only one primary path. The same parameters specified one level up in the hierarchy, at the [edit protocols mpls label-switched-path lsp-path-name] hierarchy level, affect all paths.

The path, via-r2, specifies the complete strict path from the ingress to the egress routers through 10.0.12.14, 10.0.24.2, in that order. There cannot be any intermediate routers except the ones specified. However, there can be intermediate routers between 10.0.24.2 and the egress router because the egress router is not specifically listed in the path statement. To prevent intermediate routers before egress, configure the egress router as the last router, with a strict type.

For more information on configuring a primary path, see the JUNOS MPLS Applications Configuration Guide.


Verify That the Primary Path Is Operational

Purpose

Primary paths must always be used in the network if they are available, therefore an LSP always moves back to the primary path after a failure, unless the configuration is adjusted. For more information on adjusting the configuration to prevent a failed primary path from reestablishing, see Preventing Use of a Path That Previously Failed.

Action

To verify that the primary path is operational, enter the following JUNOS command-line interface (CLI) operational mode commands:

user@host> show mpls lsp extensive ingress user@host> show rsvp interface

Sample Output 1

user@R1> show mpls lsp extensive ingress
Ingress LSP: 1 sessions

192.168.5.1
  From: 192.168.1.1, State: Up, ActiveRoute: 0, LSPname: r1-to-r5
   ActivePath: via-r2 (primary)
  LoadBalance: Random
  Encoding type: Packet, Switching type: Packet, GPID: IPv4
 *Primary   via-r2           State: Up
     Priorities: 6 6
     Bandwidth: 35Mbps
    SmartOptimizeTimer: 180
     Computed ERO (S [L] denotes strict [loose] hops): (CSPF metric: 11)
 10.0.12.14 S 10.0.24.2 S 
     Received RRO  (ProtectionFlag 1=Available 2=InUse 4=B/W 8=Node 10=SoftPreempt):
           10.0.12.14 10.0.24.2
    5 Apr 29 14:40:43 Selected as active path
    4 Apr 29 14:40:43 Record Route:  10.0.12.14 10.0.24.2
    3 Apr 29 14:40:43 Up
    2 Apr 29 14:40:43 Originate Call
    1 Apr 29 14:40:43 CSPF: computation result accepted
  Standby   via-r7           State: Dn
    SmartOptimizeTimer: 180
        No computed ERO.
  Created: Sat Apr 29 14:40:43 2006
Total 1 displayed, Up 1, Down 0

Sample Output 2

user@R1> show rsvp interface
RSVP interface: 3 active
                  Active Subscr- Static      Available   Reserved    Highwater
Interface   State resv   iption  BW          BW          BW          mark
fe-0/1/0.0  Up         2   100%   100Mbps      100Mbps     0bps        0bps       
fe-0/1/1.0  Up         1   100%   100Mbps      100Mbps     0bps        0bps       
so-0/0/3.0  Up         1   100%   155.52Mbps   155.52Mbps  0bps        0bps

Meaning

Sample output 1 shows that the LSP is operational and is using the primary path (via-r2) with R2 (10.0.12.14) and R4 (10.0.24.2) as transit routers. The priority values are the same for setup and hold, 6 6. Priority 0 is the highest (best) priority and 7 is the lowest (worst) priority. The JUNOS software default for setup and hold priority is 7:0. Unless some LSPs are more important than others, preserving the default is a good practice. Configuring a setup priority that is better than the hold priority is not allowed, resulting in a failed commit in order to avoid preemption loops.


Published: 2010-01-30

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