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[M/MX/T-series] Example - How to prevent an L2circuit from using LDP when the RSVP LSP fails

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Article ID: KB29319 KB Last Updated: 23 Jun 2020Version: 2.0
Summary:

If a configured Resource Reservation Protocol (RSVP) LSP is being used for l2circuit traffic and it fails, the l2circuit will fall back to the Label Distribution Protocol (LDP). There is no Junos OS command to prevent this behavior, so a workaround is to use a policy to influence the forwarding behavior, which will cause the l2circuit to remain down without modifying the Label Distribution Protocol (LDP).

This article details the workaround.

 

Symptoms:

How to prevent an l2circuit from falling back to LDP when the RSVP-signaled LSP goes down, without removing the LDP configuration

 

Solution:

You can use a policy applied under [forwarding-options] to have the l2circuit fall back to the desired LSP, without falling back to the LDP:

  1. Configure the l2circuit with a community:

set protocols l2circuit neighbor 3.3.3.3 interface ge-1/1/0.0 psn-tunnel-endpoint 3.3.3.3
set protocols l2circuit neighbor 3.3.3.3 interface ge-1/1/0.0 virtual-circuit-id 123
set protocols l2circuit neighbor 3.3.3.3 interface ge-1/1/0.0 community test_comm
  1. Configure a policy that matches the protocol and community. Have the policy install the desired LSP as a next-hop strict. This would cause the policy to install a discard next hop if the LSP is not up: 

set policy-options policy-statement NO-LDP term 1 from protocol l2circuit
set policy-options policy-statement NO-LDP term 1 from community test_comm
set policy-options policy-statement NO-LDP term 1 then install-nexthop strict lsp TEST
set policy-options community test_comm members 11:11
  1. Apply the policy to forwarding-options as an export policy.

    Note: If you also have another forwarding-table policy, for example ECMP (load balancing) related, with a terminating "then" clause in it, make sure to place this policy before it):

set routing-options forwarding-table export NO-LDP

After the configuration is applied, the l2circuit route will always point to the LSP in the forwarding table. If the LSP is in a down state, the next hop will be discarded and the l2circuit will not come up:

lab@router> show l2circuit connections
Neighbor: 3.3.3.3 
    Interface                 Type  St     Time last up          # Up trans
    ge-1/1/0.0(vc 123)        rmt   Up     Jun 11 18:35:22 2014           4
      Remote PE: 3.3.3.3, Negotiated control-word: Yes (Null)
      Incoming label: 299776, Outgoing label: 299776
      Negotiated PW status TLV: No
      Local interface: ge-1/1/0.0, Status: Up, Encapsulation: VLAN

The route will still be on inet.3 via LDP and the route on l2circuit.0 will show a next-hop discard:

lab@router> show route
...
inet.3: 2 destinations, 3 routes (2 active, 0 holddown, 0 hidden)
3.3.3.3/32         *[RSVP/7/1] 00:00:06, metric 2
                    > to 10.1.2.2 via lt-0/0/0.1, label-switched-path TEST
                    [LDP/9] 01:28:22, metric 2
                    > to 10.1.2.2 via lt-0/0/0.1, Push 299776
...
l2circuit.0: 2 destinations, 2 routes (2 active, 0 holddown, 0 hidden)
3.3.3.3:CtrlWord:4:123:Local/96                
                   *[L2CKT/7] 00:00:06, metric2 2
                    > to 10.1.2.2 via lt-0/0/0.1, label-switched-path TEST
3.3.3.3:CtrlWord:4:123:Remote/96                
                   *[LDP/9] 01:28:15
                      Discard

For additional examples on using communities with the l2circuit, refer to KB22700 - [M/MX/T] How to map a L2circuit traffic to take a particular RSVP LSP if there are multiple equal-cost RSVP LSP's available to reach the L2circuit neighbor?.

 

Modification History:

2020-06-23: Made some configuration syntax corrections

 

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