MPLS TE Autoroute Announce

MPLS Traffic Engineering (TE) autoroute is one of the routing options you can use to send traffic down an MPLS TE tunnel. When you create a TE tunnel, it connects but won’t be used until you configure routing.

With physical interfaces or GRE tunnels, you have to enable an IGP on the interface to establish a neighbor adjacency and to advertise the tunnel interface. We don’t do this with TE tunnels because:

  • TE tunnels are unidirectional. For an IGP neighbor adjacency, you need to be able to send and receive traffic on the tunnel.
  • Originally, TE tunnels were only used within an OSPF or ISIS area. Since these IGPs know what the entire area’s topology looks like, it doesn’t make sense to advertise any information through the tunnel. Nowadays, it is possible to configure interarea TE tunnels, though.

Autoroute is a dynamic option to use the TE tunnel. Autoroute announce modifies the shortest path first (SPF) algorithm by adding the TE tunnel to the SPF tree. When the IGP runs SPF and encounters a destination on or behind the tailend router, we install the TE tunnel as the next hop. These routes are IGP routes, but they are not advertised to other routers. This is only a local change to the routing table. 

It is possible to advertise these routes to other routers with forwarding adjacency.

Configuration

Let’s see how to configure autoroute announce. This is the topology I’ll use:

Mpls Te Pe C Router Topology Tunnel

Routers PE1, P1, P2, P3, and PE2 run MPLS TE. There is a tunnel where PE1 is the headend and PE2 is the tailend router. I use IOSv Software (VIOS-ADVENTERPRISEK9-M), Version 15.9(3)M4.

  • Configurations
  • CE1
  • CE2
  • P1
  • P2
  • P3
  • PE1
  • PE2

Want to take a look for yourself? Here you will find the startup configuration of each device.

Let’s check our tunnel:

PE1#show mpls traffic-eng tunnels Tunnel 1

Name: PE1_t1                              (Tunnel1) Destination: 6.6.6.6
  Status:
    Admin: up         Oper: up     Path: valid       Signalling: connected
    path option 1, type dynamic (Basis for Setup, path weight 20)

  Config Parameters:
    Bandwidth: 750      kbps (Global)  Priority: 7  7   Affinity: 0x0/0xFFFF
    Metric Type: TE (default)
    AutoRoute:  enabled   LockDown: disabled  Loadshare: 750      bw-based
    auto-bw: disabled
  Active Path Option Parameters:
    State: dynamic path option 1 is active
    BandwidthOverride: disabled  LockDown: disabled  Verbatim: disabled


  InLabel  :  - 
  OutLabel : GigabitEthernet0/1, 26
  RSVP Signalling Info:
       Src 2.2.2.2, Dst 6.6.6.6, Tun_Id 1, Tun_Instance 18
    RSVP Path Info:
      My Address: 192.168.23.2   
      Explicit Route: 192.168.23.3 192.168.36.3 192.168.36.6 6.6.6.6 
      Record   Route:   NONE
      Tspec: ave rate=750 kbits, burst=1000 bytes, peak rate=750 kbits
    RSVP Resv Info:
      Record   Route:   NONE
      Fspec: ave rate=750 kbits, burst=1000 bytes, peak rate=750 kbits
  Shortest Unconstrained Path Info:
    Path Weight: 20 (TE)
    Explicit Route: 192.168.23.2 192.168.23.3 192.168.36.3 192.168.36.6 
                    6.6.6.6 
  History:
    Tunnel:
      Time since created: 6 days, 21 hours, 41 minutes
      Time since path change: 6 days, 21 hours, 41 minutes
      Number of LSP IDs (Tun_Instances) used: 18
    Current LSP:
      Uptime: 6 days, 21 hours, 41 minutes

Above, we see the tunnel is connected and goes through P1. Currently, we don’t use it:

PE1#show ip route isis

      1.0.0.0/32 is subnetted, 1 subnets
i L2     1.1.1.1 [115/20] via 192.168.12.1, 00:00:07, GigabitEthernet0/0
      3.0.0.0/32 is subnetted, 1 subnets
i L2     3.3.3.3 [115/20] via 192.168.23.3, 00:00:07, GigabitEthernet0/1
      4.0.0.0/32 is subnetted, 1 subnets
i L2     4.4.4.4 [115/20] via 192.168.24.4, 00:00:07, GigabitEthernet0/2
      5.0.0.0/32 is subnetted, 1 subnets
i L2     5.5.5.5 [115/30] via 192.168.24.4, 00:00:07, GigabitEthernet0/2
      6.0.0.0/32 is subnetted, 1 subnets
i L2     6.6.6.6 [115/30] via 192.168.23.3, 00:00:01, GigabitEthernet0/1
      7.0.0.0/32 is subnetted, 1 subnets
i L2     7.7.7.7 [115/40] via 192.168.23.3, 00:00:01, GigabitEthernet0/1
i L2  192.168.36.0/24 [115/20] via 192.168.23.3, 00:00:01, GigabitEthernet0/1
i L2  192.168.45.0/24 [115/20] via 192.168.24.4, 00:00:07, GigabitEthernet0/2
i L2  192.168.56.0/24 [115/30] via 192.168.24.4, 00:00:01, GigabitEthernet0/2
                      [115/30] via 192.168.23.3, 00:00:01, GigabitEthernet0/1
i L2  192.168.67.0/24 [115/30] via 192.168.23.3, 00:00:01, GigabitEthernet0/1

In the output above, you only see physical interfaces. Time to enable autoroute:

PE1(config)#interface Tunnel 1
PE1(config-if)#tunnel mpls traffic-eng autoroute announce

Now take another look at the routing table:

PE1#show ip route isis

      1.0.0.0/32 is subnetted, 1 subnets
i L2     1.1.1.1 [115/20] via 192.168.12.1, 00:00:28, GigabitEthernet0/0
      3.0.0.0/32 is subnetted, 1 subnets
i L2     3.3.3.3 [115/20] via 192.168.23.3, 00:00:28, GigabitEthernet0/1
      4.0.0.0/32 is subnetted, 1 subnets
i L2     4.4.4.4 [115/20] via 192.168.24.4, 00:00:28, GigabitEthernet0/2
      5.0.0.0/32 is subnetted, 1 subnets
i L2     5.5.5.5 [115/30] via 192.168.24.4, 00:00:28, GigabitEthernet0/2
      6.0.0.0/32 is subnetted, 1 subnets
i L2     6.6.6.6 [115/30] via 6.6.6.6, 00:00:23, Tunnel1
      7.0.0.0/32 is subnetted, 1 subnets
i L2     7.7.7.7 [115/40] via 6.6.6.6, 00:00:23, Tunnel1
i L2  192.168.36.0/24 [115/20] via 192.168.23.3, 00:00:23, GigabitEthernet0/1
i L2  192.168.45.0/24 [115/20] via 192.168.24.4, 00:00:28, GigabitEthernet0/2
i L2  192.168.56.0/24 [115/30] via 192.168.24.4, 00:00:23, GigabitEthernet0/2
                      [115/30] via 6.6.6.6, 00:00:23, Tunnel1
i L2  192.168.67.0/24 [115/30] via 6.6.6.6, 00:00:23, Tunnel1

We now see that all destinations on or behind the tailend router are routed through the tunnel interface. A quicker way to see what destinations are routed through the tunnel is show mpls forwarding-table:

PE1#show mpls forwarding-table
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
16         No Label   1.1.1.1/32       0             Gi0/0      192.168.12.1
17         Pop Label  4.4.4.4/32       0             Gi0/2      192.168.24.4
18         Pop Label  192.168.45.0/24  0             Gi0/2      192.168.24.4
19         22         5.5.5.5/32       0             Gi0/2      192.168.24.4
20         23         192.168.56.0/24  0             Gi0/2      192.168.24.4
      [T]  No Label   192.168.56.0/24  0             Tu1        point2point 
21         Pop Label  3.3.3.3/32       0             Gi0/1      192.168.23.3
22         Pop Label  192.168.36.0/24  0             Gi0/1      192.168.23.3
23    [T]  Pop Label  6.6.6.6/32       0             Tu1        point2point 
24    [T]  No Label   7.7.7.7/32       0             Tu1        point2point 
25    [T]  No Label   192.168.67.0/24  0             Tu1        point2point 

[T]     Forwarding through a LSP tunnel.
        View additional labelling info with the 'detail' option

The [T] tells us which destinations go through the tunnel. If you want to see more detail, add the detail parameter:

PE1#show mpls forwarding-table 7.7.7.7 detail 
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
24         No Label   7.7.7.7/32       0             Tu1        point2point 
        MAC/Encaps=14/18, MRU=1500, Label Stack{26}, via Gi0/1
        5254001A70435254001F16328847 0001A000
        No output feature configured