question for the output from cef table

[gongya001]

R-3735(config)#do show ip cef 6.6.6.6
6.6.6.6/32, version 25, epoch 0, cached adjacency 192.168.1.2
0 packets, 0 bytes
  tag information set
    local tag: 30
    fast tag rewrite with Fa0/0, 192.168.1.2, tags imposed: {31}
  via 192.168.1.2, FastEthernet0/0, 0 dependencies
    next hop 192.168.1.2, FastEthernet0/0
    valid cached adjacency
    tag rewrite with Fa0/0, 192.168.1.2, tags imposed: {31}

PE-2(config)#do show ip cef 6.6.6.6 detail
6.6.6.6/32, epoch 0, per-destination sharing
  local label info: global/31
  nexthop 10.10.23.3 Ethernet0/1 label 24
  nexthop 10.10.28.8 Ethernet0/2 label 21

The outputs above are different, is that because of different version ? or something I did not enable ?

thanks !!

[Deepak Kumar]

Hi,

Based on the output, router R-3735 is an MPLS POP router. 

[gongya001]

thanks !!

[Giuseppe Larosa · Accepted solution]

Hello @gongya001 ,

both routers R-3735 and PE-2 have MPLS Label switched path(s) to destination 6.6.6.6/32.

The output are different but the meaning is the same:

R-3735 to send a packet to destination 6.6.6.6 needs to do the following:

take the packet and put it in an MPLS frame with label 31 and a L2 header that is provided by the adjacency table (ARP) destination MAC = MAC address of next-hop 192.168.1.2 source MAC address that of outgoing interface fas0/0.

Router PE-2 is on the path to 6.6.6.6/32 and we can see that the local label binding is actually 31:

local label info: global/31
  nexthop 10.10.23.3 Ethernet0/1 label 24
  nexthop 10.10.28.8 Ethernet0/2 label 21

This means that R-3735 is likely using PE-2 as next-hop to destination as it is using label 31.

PE-2 behaves like an internal LSR router and performs label swap it receives an MPLS frame with label 31 it can either sends out eth0/1 withj label 24 or out eth0/2 with label 21.

Being the outgoing label values different we can also say that there are other two different routers on the other end of each ethernet interface.

R-3735 is an edge LSR for this destination 6.6.6.6/32 and PE-2 is on the path on the LSP to destination. It performs label imposition or push.

To be noted the MPLS label changes at each router hop and reflects the binding performed on the upstream router and received via LDP.

The MPLS label has a global meaning a device will advertise the same binding over different interfaces if multiple parallel links exist between two routers.

In the case of PE-2 its ethernet interfaces connect to different devices as the outgoing label is different for the same destination.

Hope to help

Giuseppe

[gongya001]

thanks !

I am watching a few videos and tried to lab them. I noticed the outputs from the same command are different. I am afraid I missed something.

[Giuseppe Larosa]

Hello @gongya001 ,

this show command can have a different formatting on different platforms / IOS / IOS XE versions.

However, each device provide the output from its own point of view and for simple MPLS forwarding based on LSPs the MPLS label value is the one chosen by the upstream device(s) that are MPLS enabled routers in the path to the destination as I have explained in my previous post in this thread.

Hope to help

Giuseppe

[gongya001]

thanks so much !!