MPLS data forwarding

[Ratheesh mv]

Hi Team...

Please help me to clear my doubt ..

Topology is attached . 

PE2 is receiving 11.11.11.11/32 from PE1 as VPNV4 route .

Next hop 1.1.1.1(PE1 loopback IP) is reachable via three different labelled path... As below out put 

PE-2#sh mpls forwarding-table 1.1.1.1

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop    

tag    tag or VC   or Tunnel Id      switched   interface              

503    408         1.1.1.1/32        0          Fa1/0      192.168.80.1 

       307         1.1.1.1/32        0          Fa0/0      192.168.70.1 

       208         1.1.1.1/32        0          Fa0/1      192.168.60.1 

PE-2#

In corresponding CEF table of VRF on PE2 , top label(MPLS label) is not being appeared and  only appearing VPN label(Bottom label) when 3 different labeled paths are available.As below out put ......

PE2 is receiving 11.11.11.11/32 from PE1 as VPNV4 route ...

PE-2#sh ip cef vrf central_srvr 11.11.11.11

11.11.11.11/32, version 9, epoch 0, per-destination sharing

0 packets, 0 bytes

  tag information set

    local tag: VPN-route-head

    fast tag rewrite with 

Recursive rewrite via 1.1.1.1/32, tags imposed {102}

  via 1.1.1.1, 0 dependencies, recursive

    next hop 192.168.60.1, FastEthernet0/1 via 1.1.1.1/32

    valid adjacency

    tag rewrite with 

Recursive rewrite via 1.1.1.1/32, tags imposed {102}

  Recursive load sharing using 1.1.1.1/32.

PE-2#

In above out put top label is missed ..

If I shutdown 2 LDP neighbours ,top label (MPLS label) and bottom label (VPN) is present in CEF table ..As below output ...

Output of MPLS forwarding table post shutdown 2 LDP neighbours 

-----------------------------------------------------------------------------------------

PE-2#show mpls forwarding-table 1.1.1.1

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop    

tag    tag or VC   or Tunnel Id      switched   interface              

503    408         1.1.1.1/32        0          Fa1/0      192.168.80.1 

PE-2#

CEF table .post shutdown 2 LDP neighbours

------------------------------------------------------------

PE-2#sh ip cef vrf central_srvr 11.11.11.11

11.11.11.11/32, version 9, epoch 0, cached adjacency 192.168.80.1

0 packets, 0 bytes

  tag information set

    local tag: VPN-route-head

    fast tag rewrite with Fa1/0, 192.168.80.1, tags imposed: {408 102}

  via 1.1.1.1, 0 dependencies, recursive

    next hop 192.168.80.1, FastEthernet1/0 via 1.1.1.1/32

    valid cached adjacency

    tag rewrite with Fa1/0, 192.168.80.1, tags imposed: {408 102}

PE-2#

why is not appearing top label (MPLS) in CEF table when multiple labelled paths are available in MPLS forwarding table???? .

[Hector Gustavo Serrano Gutierrez]

It looks to me cosmetic in the sense of how the CEF output presents the information when you have more than one path available.

What IOS version are you running on your device?

[Giuseppe Larosa]

Hello Ratheesh,

I agree with Hector that this is cosmetic.

the reason why it does not show you the outer label when multiple equal cost paths are available is an implementation choice because the labels are different on each path.

However, it is telling you that uses multiple paths when available

Recursive rewrite via 1.1.1.1/32, tags imposed {102}

 >> Recursive load sharing using 1.1.1.1/32.

You need to combine two show outputs to know that two labels are used

You can try to use a different command like the following

show ip cef exact-route vrf central_srvr  <source-address> <destination-address>

because the exact path is chosen using also the source address and not only the destination address.

Hope to help

Giuseppe