What is TTL value for VC Label

Good OEQs:

1.What is TTL value for VC Label?

2.what is difference between L3 and L2 FEC?

3. wat are pseudowire types for FR, ATM, Eth Port, Eth Vlan, HDLC, and PPP?

4.wat is size for Group ID field?

5.what is label adv mode for Targetted LDP?


  • Q1 What is TTL value for VC Label

    ANS : For HDLC and PPP , The ingress LSR, R1, SHOULD set the TTL field of the VC label to a  value of 2.

     For  ATM, If a packet was received by the Edge LSR as an unlabeled packet, the "incoming TTL" comes from the IP header.

    If a packet was received by the Edge LSR as a labeled packet,  "incoming TTL" comes from the entry at the top of the label stack. 

    if a packet is associated with label binding, the "outgoing TTL" is set to the larger of 

    (a) 0 

    (b) the difference between the incoming TTL and the hop count.

    If a hop count has not been associated with the label  binding that is used when the packet is forwarded, the "outgoing TTL"  is set to the larger of 

    (a) 0 

    (b) one less than the incoming TTL.

    Hope i am right 


    knowledge itself is a power 
  • Cool. Not sure about Cell Mode, but Frame Mode That is 2

  • is atm part correct for vc label ttl value

  •    "MPLS Pseudowire Type".  This type has 15-bit values.
       PW Type values 1 through 30 are specified as below
       and PW Type values 31 through 1024 are to be assigned by IANA,
       PW Type values 1025 through 4096 and 32767 are allocated by IETF                                        
       PW Type values 4097 through 32766 are reserved for vendor-    proprietary extensions and are to be assigned by IANA
       Pseudowire Type value allocations are specified below:
       PW type Description                                      
       0x0001  Frame Relay DLCI ( Martini Mode )                
       0x0002  ATM AAL5 SDU VCC transport                       
       0x0003  ATM transparent cell transport                   
       0x0004  Ethernet Tagged Mode                             
       0x0005  Ethernet                                         
       0x0006  HDLC                                             
       0x0007  PPP                                              
       0x0008  SONET/SDH Circuit Emulation Service Over MPLS    
       0x0009  ATM n-to-one VCC cell transport                  
       0x000A  ATM n-to-one VPC cell transport                  
       0x000B  IP Layer2 Transport                              
       0x000C  ATM one-to-one VCC Cell Mode                     
       0x000D  ATM one-to-one VPC Cell Mode                     
       0x000E  ATM AAL5 PDU VCC transport                       
       0x000F  Frame-Relay Port mode                            
       0x0010  SONET/SDH Circuit Emulation over Packet          
       0x0011  Structure-agnostic E1 over Packet                
       0x0012  Structure-agnostic T1 (DS1) over Packet          
       0x0013  Structure-agnostic E3 over Packet                
       0x0014  Structure-agnostic T3 (DS3) over Packet          
       0x0015  CESoPSN basic mode                               
       0x0016  TDMoIP AAL1 Mode                                 
       0x0017  CESoPSN TDM with CAS                             
       0x0018  TDMoIP AAL2 Mode                                 
       0x0019  Frame Relay DLCI                                 


  • Cool. Long history in place of 5-6 words, but good elaboration, thank you, wat about other questions.

  • downstream on- demand label advertisement mode.  for targeted LDP sessions 



  • 4.wat is size for Group ID field? 

    with respect to what part of topic is the question releated 

    i need clear idea 


    and please correct me if any of the above answer are not correct 

  • if this is with respect to the PW ID FEC TLV, then Group id is a 1 byte field that contains the the group of PWs.  All ACs on the same interface get the same Group ID.

    GroupID can be used to withdraw all VC labels that are associated with that group in 1 Label Withdraw message. This is also referred to as "Wild Card Label Withdraw".


  • Hi, For question number 5 as per MPLS fundamentals Page 388, the correct anwser should be downstream unsolicited

    The main purpose of this LDP session between the PE routers is to advertise the VC label that is associated with the pseudowire. This label is advertised in a Label Mapping message using the downstream unsolicited label advertisement mode.


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