Description

For a MPLS LSP, when link or node-link protection is configured and there are redundant links in the network, bypass LSPs are created on transit routers. Sometimes when the primary path fails, traffic shifts to bypass LSP. This article will help you determine which LSP is forwarding the traffic over bypass LSP.

Solution

This example shows the output from transit router only to explain which LSP is forwarding traffic over bypass LSP. 

The following output shows bypass LSP Bypass -> 1.1.23.2 is in BackupActive state, which means bypass LSP is UP and Forwarding Traffic.  

labroot@MX80-r002# run show mpls lsp bypass logical-system r2
Apr 24 13:10:21
Ingress LSP: 1 sessions
To                     From               State               Rt Style Labelin Labelout LSPname
192.168.1.103   192.168.1.102   BackupActive  0 1 SE       -   299824     Bypass->1.1.23.2  < when the bypass is BackupActive it means its up and forwarding traffic
Total 1 displayed, Up 1, Down 0
 

In the extensive output "Number of data route tunnel through" field gives the number of LSP using the bypass to forward traffic. 

labroot@MX80-r002# run show rsvp session name Bypass->1.1.23.2 extensive logical-system r2
Apr 24 13:59:50
Ingress RSVP: 2 sessions
 
192.168.1.103
  From: 192.168.1.102, LSPstate: Up, ActiveRoute: 0
  LSPname: Bypass->1.1.23.2
  LSPtype: Static Configured
  Suggested label received: -, Suggested label sent: -
  Recovery label received: -, Recovery label sent: 299824
  Resv style: 1 SE, Label in: -, Label out: 299824
  Time left:    -, Since: Wed Apr 24 09:24:07 2019
  Tspec: rate 0bps size 0bps peak Infbps m 20 M 1500
  Port number: sender 1 receiver 15931 protocol 0
  Type: Bypass LSP
    Number of data route tunnel through: 1      <-- 1 LSP is using this bypass to forward traffic, it can be 0 when bypass LSP is only UP (not BackupActive)
    Number of RSVP session tunnel through: 1    <-- Only 1 LSP is protected on this bypass path
  Enhanced FRR: Enabled (Downstream)
  PATH rcvfrom: localclient
  Adspec: sent MTU 1500
  Path MTU: received 1500
  PATH sentto: 1.1.24.2 (xe-0/0/0.24) 1 pkts    <-- this is the interface on which bypass path is signaled
       outgoing message state: refreshing, Message ID: 97, Epoch: 15610269
  RESV rcvfrom: 1.1.24.2 (xe-0/0/0.24) 1 pkts, Entropy label: Yes
       incoming message handle: R-42/1, Message ID: 43, Epoch: 15610266
  Explct route: 1.1.24.2 1.1.43.2
 

When you check the LSPs signaled through the interface on which bypass LSP is signaled, you will find ingress LSP on that interface which should be transit LSP on this router.  

labroot@MX80-r002# run show rsvp session interface xe-0/0/0.24 logical-system r2   <-- you can see r1-to-r5 is showing as ingress LSP on xe-0/0/0.24 which is protecting xe-0/0/0.23
Apr 24 13:51:52
Ingress RSVP: 2 sessions
To              From            State  Rt Style Labelin Labelout   LSPname
192.168.1.103   192.168.1.102   Up     0  1 SE   -      299824     Bypass->1.1.23.2
192.168.1.105   1.1.24.1        Up     0  1 SE   -      300080     r1-to-r5  <-- transit LSP showing as ingress LSP on protected interface and ‘from’ address is interface address
Total 2 displayed, Up 2, Down 0

Now if you will check the extensive output for that LSP you will find out that LSP Type is " Backup LSP at Point-of-Local-Repair" that means this LSP is currently using the bypass path on that interface. 

labroot@MX80-r002# run show rsvp session ingress name r1-to-r5 logical-system r2 extensive < if you check here  
Apr 24 13:12:28
Ingress RSVP: 2 sessions
 
192.168.1.105
  From: 1.1.24.1, LSPstate: Up, ActiveRoute: 0
  LSPname: r1-to-r5
  LSPtype: Static Configured
  Suggested label received: -, Suggested label sent: -
  Recovery label received: -, Recovery label sent: 300080
  Resv style: 1 SE, Label in: -, Label out: 300080
  Time left:    -, Since: Wed Apr 24 13:10:05 2019
  Tspec: rate 0bps size 0bps peak Infbps m 20 M 1500
  Port number: sender 6 receiver 45048 protocol 0
  Type: Backup LSP at Point-of-Local-Repair   <-- this is the LSP which is actively using the bypass path 
  PATH rcvfrom: localclient
  Adspec: sent MTU 1500
  Path MTU: received 1500
  PATH sentto: 1.1.43.2 (xe-0/0/0.24) 2 pkts
       outgoing message state: refreshing, Message ID: 305, Epoch: 15610269
  RESV rcvfrom: 192.168.1.103 (xe-0/0/0.24) 1 pkts, Entropy label: Yes
       incoming message handle: R-85/1, Message ID: 124, Epoch: 15610266
  Explct route: 1.1.43.2 1.1.35.2
  Record route: <self> 1.1.43.2 192.168.1.105 (node-id) 1.1.35.2 
Total 1 displayed, Up 1, Down 0

The same LSP will show as down on primary path because of the failure on that path, but traffic will be forwarded via bypass LSP. This LSP will show up on ingress. 

labroot@MX80-r002# run show rsvp session transit logical-system r2                                   
Transit RSVP: 4 sessions
To              From            State   Rt  Style Labelin Labelout LSPname
192.168.1.101   192.168.1.105   Up       0  1 FF  300160        3   r5-to-r1
192.168.1.101   192.168.1.104   Up       0  1 FF  300112        3   r4-to-r1
192.168.1.104   192.168.1.101   Up       0  1 FF  300144        3   r1-to-r4
192.168.1.105   192.168.1.101   Dn       0  0  -  300192        -   r1-to-r5  <-- same LSP is showing down as transit LSP

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