LFIB Entry for Directly Connected Interfaces in a LSR

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Dec 11th, 2013
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I have a loopback interface in my LSR. The directly connected interface and the loopback interface (which is again a directly connected interface) doesn't have a LFIB entry in my router. Although there is a LIB entry as imp-null for the directly connected interfaces. Am looking for pointers to understand this behaviour.


Output:


PE2#sh ip int brief

Interface                       IP-Address      OK? Method   Status              Protocol

FastEthernet0/0            172.16.56.6     YES NVRAM  up                    up

FastEthernet0/1            192.168.1.2     YES NVRAM  up                    up

FastEthernet1/0            192.168.2.2     YES NVRAM  up                    up

FastEthernet2/0            unassigned      YES NVRAM  administratively down down

Loopback0                    6.6.6.6            YES NVRAM  up                    up


PE2#sh mpls ldp bindings 6.6.6.6 32

  tib entry: 6.6.6.6/32, rev 4

        local binding:  tag: imp-null

        remote binding: tsr: 5.5.5.5:0, tag: 17


PE2#sh mpls forwarding-table

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop

tag    tag or VC   or Tunnel Id      switched   interface

16     16              4.4.4.4/32           0          Fa0/0      172.16.56.5

17     Pop tag      5.5.5.5/32           0          Fa0/0      172.16.56.5

18     18              172.16.34.0/24    0          Fa0/0      172.16.56.5

19     Pop tag      172.16.45.0/24    0          Fa0/0      172.16.56.5

20     20              3.3.3.3/32           0          Fa0/0      172.16.56.5

21     Untagged    8.8.8.8/32[V]      1242       Fa1/0      192.168.2.1

22     Aggregate   192.168.2.0/24[V] 0

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Peter Paluch Wed, 12/11/2013 - 02:34
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Prabu,


The behavior you are describing is normal. The LFIB handles labeled packets, i.e. it tells the router what to do with a packet carrying a particular label. However, for directly connected networks, you expect to receive unlabeled packets because of Penultimate Hop Popping (that is why you have allocated an imp-null label for them and advertised it to your neighbors). So when your neighbors forward an unlabeled packet for any of your directly connected networks, you already receive it without any label, so there is no information in this packet LFIB could use to decide the next process for this packet. Instead, you must simply handle it in your routing table (or CEF to be more precise). That is why you do not have any antries for your directly connected networks in your LFIB - because LFIB only handles labeled packets while packets towards your directly connected networks arrive unlabeled.


Best regards,

Peter

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