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  • 1.  Trace Route in MPLS Network

     
    Posted 11-16-2019 19:41

    Can you someone explain the default behavior for the traceroute in MPLS.

     

    MPLS LSP Traceroute in MPLS Network
    ICMP Traceroute in MPLS Network
    ICMP Trace Triggered from PE to Remote PE
    ICMP Trace Triggered from CE to Remote CE

     

    I wanted to understand in which scenarios, traceroute response packet is tunneled back to the egress and then sent back from egress to ingress router?

    I did not configure the no-propagate-ttl command. In such a case,  I should be able to see the core routers hops when I  issue traceroute from CE to CE? 

    I still remember when I wrote this KB. I did not use the command no-propagate-ttl. In such a case, I should be able to see the core router Hops from CE is that right?. But I did not see that in output.

    https://kb.juniper.net/InfoCenter/index?page=content&id=KB33434&cat=MX960_1&actp=LIST

    I understand that MPLS traceroute response will be sent directly to the neighboring router.  Is that right?

     

     

     



  • 2.  RE: Trace Route in MPLS Network
    Best Answer

    Posted 11-16-2019 22:19

    Hello,

     

    @rram wrote:

     

    I wanted to understand in which scenarios, traceroute response packet is tunneled back to the egress and then sent back from egress to ingress router?

     

     

    I assume You are talking about IP traceroute here, no MPLS traceroute. I answered Your other MPLS traceroute question below.

    In the network running JUNOS, IP traceroute response forwarding to egress PE happens only when every P-router in the path is configured with "icmp-tunneling" knob

    https://networkfoo.github.io/juniper/mpls/2016/12/18/juniper-day-one-lab-l3vpn.html

     

    @rram wrote:

     

    I did not configure the no-propagate-ttl command. In such a case,  I should be able to see the core routers hops when I  issue traceroute from CE to CE? 

     

     

    As above, You need "icmp-tunneling" knob enabled in P-routers.

     

    @rram wrote:

    Can you someone explain the default behavior for the traceroute in MPLS.

     

    MPLS LSP Traceroute in MPLS Network

     

     

    Please see RFC 4379 https://tools.ietf.org/html/rfc4379 

     

    @rram wrote:


    ICMP Traceroute in MPLS Network
    ICMP Trace Triggered from PE to Remote PE
    ICMP Trace Triggered from CE to Remote CE

     

     

    The default behaviour is to examine the TTL in the outer label.

    1/ If TTL is less than 2, examine the underlying packet. If the underlying packet is IP, look up the header, if no route found, drop it.

    2/ If underlying packet is MPLS, look up the inner label in mpls.0 table. If no label entry is found, drop the packet.  

    3/ if underlying packet is not IP and not MPLS, drop it.

    [1] and [2] can be changed with "icmp-tunneling" knob, please see  https://www.juniper.net/documentation/en_US/junos/topics/concept/mpls-ttl-processing-incoming-packets.html

     

    @rram wrote:

     

    I understand that MPLS traceroute response will be sent directly to the neighboring router.  Is that right?

     

     

    Please see RFC 4379 section 4.5 https://tools.ietf.org/html/rfc4379#section-4.5

     

    HTH

    Thx

    Alex