Re: show mpls forwarding-table exact-route

Amirmahdi.M · ‎07-07-2025

Hi

Following "Another Discussion " i have this problem which the output of "show mpls forwarding-table exact-route" is not correct.

i tested it on both ASR903(15.6(2)SP) and CSR1000V(15.4(3)S)

this is the output of my csr1000v

it says that its going to use Vlan 20 for outgoing traffic but i captured the traffic and found out its now working as expected.(used vlan 10)

this causes an imbalance of traffic on my links.

i would be pleased if anyone can help or explain to me why this is happenning and how can i resolve this problem.

PE.LEFT#show ip route 172.16.1.3
Routing entry for 172.16.1.3/32
  Known via "isis", distance 115, metric 20, type level-1
  Redistributing via isis test
  Last update from 192.168.12.6 on GigabitEthernet2.20, 01:41:09 ago
  Routing Descriptor Blocks:
    192.168.12.6, from 172.16.1.3, 01:41:09 ago, via GigabitEthernet2.20
      Route metric is 20, traffic share count is 1
  * 192.168.12.2, from 172.16.1.3, 01:41:09 ago, via GigabitEthernet2.10
      Route metric is 20, traffic share count is 1
PE.LEFT#show ip cef 172.16.1.3
172.16.1.3/32
  nexthop 192.168.12.2 GigabitEthernet2.10 label 18
  nexthop 192.168.12.6 GigabitEthernet2.20 label 18
PE.LEFT#show run | s router isis test
 ip router isis test
 ip router isis test
router isis test
 net 49.0000.1720.1600.1001.00
 is-type level-1
 metric-style wide
 passive-interface Loopback0
 maximum-paths 32
PE.LEFT#show mpls l2 vc det    
Local interface: Gi3 up, line protocol up, Ethernet:1 up
  Destination address: 172.16.1.3, VC ID: 21, VC status: up
    Output interface: Gi2.10, imposed label stack {18 21}
    Preferred path: not configured  
    Default path: active
    Next hop: 192.168.12.2
  Create time: 01:37:02, last status change time: 01:37:02
    Last label FSM state change time: 01:37:02
  Signaling protocol: LDP, peer 172.16.1.3:0 up
    Targeted Hello: 172.16.1.1(LDP Id) -> 172.16.1.3, LDP is UP
    Graceful restart: not configured and not enabled
    Non stop routing: not configured and not enabled
    Status TLV support (local/remote)   : enabled/supported
      LDP route watch                   : enabled
      Label/status state machine        : established, LruRru
      Last local dataplane   status rcvd: No fault
      Last BFD dataplane     status rcvd: Not sent
      Last BFD peer monitor  status rcvd: No fault
      Last local AC  circuit status rcvd: No fault
      Last local AC  circuit status sent: No fault
      Last local PW i/f circ status rcvd: No fault
      Last local LDP TLV     status sent: No fault
      Last remote LDP TLV    status rcvd: No fault
      Last remote LDP ADJ    status rcvd: No fault
    MPLS VC labels: local 18, remote 21 
    Group ID: local 0, remote 0
    MTU: local 1500, remote 1500
    Remote interface description: 
  Sequencing: receive disabled, send disabled
  Control Word: On (configured: autosense)
  SSO Descriptor: 172.16.1.3/21, local label: 18
  Dataplane:
    SSM segment/switch IDs: 4106/4105 (used), PWID: 4
  VC statistics:
    transit packet totals: receive 991, send 3797
    transit byte totals:   receive 152030, send 402298
    transit packet drops:  receive 0, seq error 0, send 0


PE.LEFT#show mpls fo                                 
PE.LEFT#show mpls forwarding-table 
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
16         Pop Label  172.16.1.2/32    0             Gi2.10     192.168.12.2
           Pop Label  172.16.1.2/32    0             Gi2.20     192.168.12.6
17         18         172.16.1.3/32    0             Gi2.10     192.168.12.2
           18         172.16.1.3/32    0             Gi2.20     192.168.12.6
18         No Label   l2ckt(4)         157948        Gi3        point2point 
19         Pop Label  192.168.23.0/30  0             Gi2.10     192.168.12.2
           Pop Label  192.168.23.0/30  0             Gi2.20     192.168.12.6PE.LEFT#show mpls fo
PE.LEFT#show mpls forwarding-table ex
PE.LEFT#show mpls forwarding-table exact-route l
PE.LEFT#show mpls forwarding-table exact-route label 17 bo
PE.LEFT#show mpls forwarding-table exact-route label 17 bottom-label 21
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
17         18         172.16.1.3/32    0             Gi2.20     192.168.12.6
PE.LEFT#

Thanks

Amir

MHM Cisco World · ‎07-07-2025

You have TWO path to destiantion

So what you see I think normal

MHM

MHM Cisco World · ‎07-07-2025

PE.LEFT#show mpls forwarding-table exact-route 172.16.1.3/32

Do this you will see output of two path

MHM

Amirmahdi.M · ‎07-07-2025

Hi @MHM Cisco World

yes i do see 2 paths.

normally, the traffic of each vc should pass through one link. and if there are multiple links and vc s, they should get balanced.

PE.LEFT#show mpls forwarding-table 172.16.1.3
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
17         18         172.16.1.3/32    0             Gi2.10     192.168.12.2
           18         172.16.1.3/32    0             Gi2.20     192.168.12.6
PE.LEFT#show mpls forwarding-table exact-route label 17 bottom-label 20
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
17         18         172.16.1.3/32    0             Gi2.10     192.168.12.2
PE.LEFT#show mpls forwarding-table exact-route label 17 bottom-label 21
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
17         18         172.16.1.3/32    0             Gi2.20     192.168.12.6
PE.LEFT#show mpls forwarding-table exact-route label 17 bottom-label 22
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
17         18         172.16.1.3/32    0             Gi2.10     192.168.12.2
PE.LEFT#show mpls forwarding-table exact-route label 17 bottom-label 23
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
17         18         172.16.1.3/32    0             Gi2.20     192.168.12.6
PE.LEFT#show mpls forwarding-table exact-route label 17 bottom-label 24
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
17         18         172.16.1.3/32    0             Gi2.10     192.168.12.2
PE.LEFT#show mpls forwarding-table exact-route label 17 bottom-label 25
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
17         18         172.16.1.3/32    0             Gi2.20     192.168.12.6
PE.LEFT#

the problem is that for example i have a label stack of {17 25} so this should pass through Gi2.20 but it doesnt.

the router forwards all the traffic through gi2.10 and doesnt act as it says in the show exact-route command.

i can verify this by seeing both the wireshark capture of the link and by seeing the traffic rate passing through the links.

Thanks

Amir

MHM Cisco World · ‎07-07-2025

Sorry this P or PE ?

I think the prefix must insert with latter ""V"" in table and with label

MHM

Amirmahdi.M · ‎07-07-2025

The one i just posted is the PE but i tried on both of them and they are the same.

the "v" in the mpls forwarding table is for l3vpn vrf prefixes.

in my scenario, there is only l2vpn traffic. (which i have the problem)

I think there is a bug or maybe a misconfiguration but i want documentation about it.

Amir

MHM Cisco World · ‎07-07-2025

Ok' try use in command only bottom label

Let upper label change according to select interface

MHM

Harold Ritter · ‎07-07-2025

Hi @Amirmahdi.M ,

The interface displayed by the "show mpls for label X bottom-label Y" and the output interface displayed in the "show mpls l2 vc det" should match. I suspect the reason they don't match in your case is because of the older level of code you are running.

Ultimately, you should rely on the output interface displayed in the "show mpls l2 vc det".

PE.LEFT#show mpls l2 vc det    
Local interface: Gi3 up, line protocol up, Ethernet:1 up
  Destination address: 172.16.1.3, VC ID: 21, VC status: up
    Output interface: Gi2.10, imposed label stack {18 21}

You can always use mpls traffic engineering if you want the pseudowire to steer the traffic via a specific interface.

Regards,
Harold Ritter, CCIE #4168 (EI, SP)

Amirmahdi.M · ‎07-07-2025

Hi @Harold Ritter

You are right about the PE side. I forgot that it always sends the traffic through Gi2.10 (no FAT or Entropy)

I played with the vc ids and finally got the P router which should do the balance in MPLS core, do the bottom label lookup and it sent the traffic through Gi2.20 (CSR1000V)

So i realized that it works correct on the CSR1000V but i dont know why it doesnt act the same on the ASR903

The ASR903 is acting as a P router for a mikrotik PE

There are tons of vc ids on the mikrotik so the Cisco P router should be able to balance the traffic on the links it has with the mikrotik but this doesnt happen.

like 98% of the traffic is on link 1 and only 2% of the traffic is on link2

The scenario is just like the one i tried with CSR1000V

Thanks

Amir

MHM Cisco World · ‎07-07-2025

From what I read in cisco guide' asr900 mpls load balance is per flow

I.e. if source destiantion is same then router will use same label stack

MHM

Amirmahdi.M · ‎07-12-2025

Hello again @Harold Ritter and @MHM Cisco World

I guessed that maybe the reason why the asr903 (P router) wouldnt do the load balancing was the lack of control word on the mikrotik PE side.

I did enable the control word on mikrotik so the P routers would know that this is l2vpn traffic not l3vpn but it didnt make any change.(verfied CW on both PEs)

I still see about all the traffic passing through link 1.

I also found out that the little traffic that was passing through link 2, was the P router's own xconnect which was shown in the output interface of "show mpls l2 vc det". I also tried using "traceroute mpls nil-fec" to test the load balancing but because of the explicit null on the bottom of the stack (i think nil-fec forces it), i couldnt do much testing.

Do you think this is some kind of bug or does the asr903 lack the feature of load balancing l2vpn traffic by the bottom label (VC label)?

Also because mikrotik lacks the proper implementation of ISIS i cant really do MPLS TE to manually balance the traffic.(we dont use ospf)

Thanks

Amir