Description

This article describes the default behavior of the Constrained Shortest Path First (CSPF) path selection in the ingress/head end of the LSP on the two available LSP paths, where one LSP path is within the same OSPF area and other LSP path is in a different OSPF area.

Symptoms

The following diagram describes this behavior.                              

                                                                  

                                Area 3000
           +----------------------------------------------------+
           |                                                    |
           |  ge-0/2/0                                          |
      +----+----+                                         +-----+---+
      |         |                                         |         |
      |  R1     |                                         |    R4   |
      |         |                                         |         |
      +--+----+-+                                         +--+----+-+
         |    |                                              |    |
         |    |    ge-0/3/0                                  |    |
 ge-0/3/2|    |                                              |    |
         |    |                   Area 3000                  |    |
         |    |                                              |    |
         |    |      +-------+           +-------+           |    |
         |    |      |       |           |       |           |    |
         |    +------+   R2  +-----------+   R3  +-----------+    |
         |           +-------+           +-------+                |
         |                                                        |
         |                                                        |
         |                                                        |
         |                                                        |
         |    +-------+            +-------+           +-------+  |
         +----+       |            |       |           |       |  |
              |  R5   +------------+   R6  +-----------+  R7   +--+
              +-------+            +-------+           +-------+
                            
                                 Area 3001


In MPLS LSP path selection, if CSPF is enabled with loose path, the path selection is expected to be in the same area. However, the LSP is getting selected via the interface that is in a different area during the link failure on the primary LSP path.

  • As per this topology, an LSP is formed between R1 loopback IP and R2 loopback IP with loose path.
  • When all links are up, the LSP selects interface ge-0/3/0 that is directly connected to R2 and in Area 3000.
  • When this interface ge-0/3/0 goes down, the LSP is signaled via interface ge-0/3/2 that is in Area 3001.
  • One more thing to notice here is that R5 does not have R2 loopback IP in its routing table or in the OSPF database.

R1 configuration:

mpls {
 
    label-switched-path r1-r2 {
        to 172.18.24.1;
        fast-reroute;
        primary r1-r2;
    }
 path r1-r2 {
        172.18.24.1 loose;
    }

}

ospf {
    traffic-engineering;
    preference 20;
          
    area 0.0.0.0 {
        interface lo0.0 {
            passive;
        }
    }
    area 3.0.0.0 {
        stub;
        interface ge-0/2/0.3000 {
            interface-type p2p;
            metric 1000;
           
        }
        interface ge-0/3/0.3000 {
            interface-type p2p;
            metric 10;
          
        }
    }
    area 3.0.0.1 {
        stub;
            interface ge-0/3/2.3001 {
            interface-type p2p;
            metric 10;
          
        }
    }
}
 

When all links are up, from R1 if you use the show route command  to that LSP endpoint IP, you see the following output:

[edit]
lab@MX-104-1# run show route 172.18.24.1 

inet.0: 31 destinations, 31 routes (31 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

172.18.24.1/32     *[OSPF/20] 00:15:00, metric 10
                    > to 172.18.64.10 via ge-0/3/0.3000

inet.3: 30 destinations, 38 routes (23 active, 0 holddown, 13 hidden)
+ = Active Route, - = Last Active, * = Both

172.18.24.1/32     *[RSVP/7/1] 00:14:47, metric 10
                    > to 172.18.64.10 via ge-0/3/0.3000, label-switched-path r1.rbr.hfa-to-R1.R3000.S0573
                      to 172.18.64.26 via ge-0/2/0.3001, label-switched-path r1.rbr.hfa-to-R1.R3000.S0573
 

We can see that the detour LSP is already selected via interface ge-0/3/2 in Area 3001.

[edit]
lab@MX-104-1# run show rsvp session lsp name r1-r2 extensive
Ingress RSVP: 6 sessions

172.18.24.1
  From: 172.18.255.46, LSPstate: Up, ActiveRoute: 0
  LSPname: r1.rbr.hfa-to-R1.R3000.S0573, LSPpath: Primary
  LSPtype: Static Configured
  Suggested label received: -, Suggested label sent: -
  Recovery label received: -, Recovery label sent: 3
  Resv style: 1 FF, Label in: -, Label out: 3
  Time left:    -, Since: Mon Oct 20 14:22:14 2014
  Tspec: rate 0bps size 0bps peak Infbps m 20 M 1500
  Port number: sender 23 receiver 37387 protocol 0
  FastReroute desired
  PATH rcvfrom: localclient 
  Adspec: sent MTU 1500
  Path MTU: received 1500
  PATH sentto: 172.18.64.10 (ge-0/3/0.3000) 11 pkts
  RESV rcvfrom: 172.18.64.10 (ge-0/3/0.3000) 7 pkts
  Explct route: 172.18.64.10 
  Record route: <self> 172.18.64.10  
    Detour is Up
    Detour Tspec: rate 0bps size 0bps peak Infbps m 20 M 1500
    Detour adspec: sent MTU 1500
    Path MTU: received 1500             
    Detour PATH sentto: 172.18.64.34 (ge-0/3/2.3001) 7 pkts
    Detour RESV rcvfrom: 172.18.64.34 (ge-0/3/2.3001) 4 pkts
    Detour Explct route: 172.18.64.34 172.18.64.38 172.18.64.42 172.18.64.46
    172.18.64.21 172.18.64.17 172.18.64.13 
    Detour Record route: <self> 172.18.64.34 172.18.64.38 172.18.64.42
    172.18.64.46 172.18.64.21 172.18.64.17 172.18.64.13  
    Detour Label out: 302560
Total 1 displayed, Up 1, Down 0
 

When we disable interface ge-0/3/0, the LSP is signaled via interface ge-0/3/2, which is in area 3001.

lab@MX-104-1# set interfaces ge-0/3/0 disable 
Oct 14 10:27:43

[edit]
lab@MX-104-1# commit 
Oct 14 10:27:46
re0: 
configuration check succeeds

lab@MX-104-1> show route 172.18.24.1        
Oct 14 10:28:46

inet.0: 31 destinations, 31 routes (31 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

172.18.24.1/32     *[OSPF/1] 00:00:30, metric 1030
                    > to 172.18.64.2 via ge-0/2/0.3000

inet.3: 6 destinations, 6 routes (6 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

172.18.24.1/32     *[RSVP/7/1] 00:00:25, metric 1030
                    > to 172.18.64.34 via ge-0/3/2.3001, label-switched-path r1.rbr.hfa-to-R1.R3000.S0573
 

Now to reach destination IP 172.18.24.1 from R1, the inet.0 table shows the interface in Area 3000 (ge-0/2/0), and inet.3 table shows the interface in Area 3001 (ge-0/3/2).

Solution

This is happening because:

  1. When CSPF is enabled, the path selection is totally based on the Traffic Engineering Database (TED) entries.
  2. When the primary link goes down, CSPF checks the TED database to signal the LSP, not the OSPF entries.
  3. In this setup from the R1 router which is an ABR, the CSPF checks the TED database and finds the interface ge-0/3/2 with the lower metric to signal the LSP.
  4. The other interface ge-0/2/0 has a higher metric, hence it is not selected.
  5. CSPF is able to reach 172.18.24.1 based on the SPF calculation with the TED database on each router until the LSP endpoint. In this case, the best path is selected based on the metric seen in the TED database.
  
[edit]
lab@MX-104-1# run show ted database extensive 172.18.255.46 
TED database: 0 ISIS nodes 8 INET nodes
NodeID: 172.18.255.46
  Type: Rtr, Age: 126 secs, LinkIn: 3, LinkOut: 3
  Protocol: OSPF(0.0.0.0)
  Protocol: OSPF(3.0.0.0)
    To: 172.18.255.47, Local: 172.18.64.1, Remote: 172.18.64.2
      Local interface index: 332, Remote interface index: 0
      Color: 0 <none>
      Metric: 1000
      Static BW: 1000Mbps
      Reservable BW: 1000Mbps
      Available BW [priority] bps:
          [0] 1000Mbps     [1] 1000Mbps    [2] 1000Mbps    [3] 1000Mbps    
          [4] 1000Mbps     [5] 1000Mbps    [6] 1000Mbps    [7] 1000Mbps    
      Interface Switching Capability Descriptor(1):
        Switching type: Packet
        Encoding type: Packet
        Maximum LSP BW [priority] bps:
          [0] 1000Mbps     [1] 1000Mbps    [2] 1000Mbps    [3] 1000Mbps    
          [4] 1000Mbps     [5] 1000Mbps    [6] 1000Mbps    [7] 1000Mbps    
  Protocol: OSPF(3.0.0.1)
    To: 172.18.24.4, Local: 172.18.64.33, Remote: 172.18.64.34
      Local interface index: 339, Remote interface index: 0
      Color: 0 <none>                   
      Metric: 10
      Static BW: 1000Mbps
      Reservable BW: 1000Mbps
      Available BW [priority] bps:
          [0] 1000Mbps     [1] 1000Mbps    [2] 1000Mbps    [3] 1000Mbps    
          [4] 1000Mbps     [5] 1000Mbps    [6] 1000Mbps    [7] 1000Mbps    
      Interface Switching Capability Descriptor(1):
        Switching type: Packet
        Encoding type: Packet
        Maximum LSP BW [priority] bps:
          [0] 1000Mbps     [1] 1000Mbps    [2] 1000Mbps    [3] 1000Mbps    
          [4] 1000Mbps     [5] 1000Mbps    [6] 1000Mbps    [7] 1000Mbps    
    To: 172.18.255.47, Local: 172.18.64.25, Remote: 172.18.64.26
      Local interface index: 333, Remote interface index: 0
      Color: 0 <none>
      Metric: 1000
      Static BW: 1000Mbps
      Reservable BW: 1000Mbps
      Available BW [priority] bps:
          [0] 1000Mbps     [1] 1000Mbps    [2] 1000Mbps    [3] 1000Mbps    
          [4] 1000Mbps     [5] 1000Mbps    [6] 1000Mbps    [7] 1000Mbps    
      Interface Switching Capability Descriptor(1):
        Switching type: Packet          
        Encoding type: Packet
        Maximum LSP BW [priority] bps:
          [0] 1000Mbps     [1] 1000Mbps    [2] 1000Mbps    [3] 1000Mbps    
          [4] 1000Mbps     [5] 1000Mbps    [6] 1000Mbps    [7] 1000Mbps

This is an expected behavior of CSPF path selection. When fast reroute (FRR) is configured for the LSP, CSPF selects the detour path based on the Traffic Engineering Database. Even the detour path will be signaled on the interface in Area 3001. It is based on the metric seen in the TED database.

If you need LSP to be signaled in the same Area 3000, specify "no-cspf" under LSP and disable the FRR configuration. Otherwise, with CSPF enabled you need to use other constraints such as strict ERO, admin groups, or changing the interface IGP metric to signal the LSP on the selected interfaces.

Former Article Id

RSVP detour LSP not signaled within the same OSPF area

Modification History

2024-08-22: Updated Data Category