Penultimate Hop Popping (PHP) - The last router advertising a connected network will send a special reserved label to the neighboring LSRs, with value 3 (implicit null).See MPLS TTL Behavior for more information how MPLS' TTL field relates to the IP's TTL field.The TTL field can be used for path tracking like MPLS Traceroute.Just like in IP header, the TTL field is used to prevent infinite forwarding loops of MPLS frames.This bit is also referred to as S-bit in some literature.For example with MPLS VPN the VPN label will have BoS set to 1, which tells the MPLS router to process the embedded transport protocol.BoS is a field that is set to 1 for the last MPLS header.Based on RFC5462, the EXP bit has been renamed to Traffic Class field.The Experimental bit is not really experimental, but used for QoS similarly like the Class of Service field.For more reserved labels see Multiprotocol Label Switching Architecture (MPLS) Label Values.This is mainly used for traceroute or other management features.
Mpls fundamentals luc de ghein example 10 4 software#
Label 1 - Router Alert - label that informs the LSR to look at the packet using software instead of forwarding in hardware.Label 2 - Explicit Null (IPv6) - same as with label 0 but for IPv6.The biggest advantage of explicit null is transferring the QoS information. The egress router strips the label, paying attention to the QoS value, looks up the IP forwarding, without doing a lookup on the label. Label 0 - Explicit Null (IPv4) - the egress LSR tells the neighboring LSRs to forward the packet keeping the explicit null label (0).The benefit of doing implicit NULL is that egress LSR does not have to lookup label, strip it and then lookup IP forwarding. This also removes the EXP (QoS) value which may not be ideal when using MPLS DiffServ Tunneling. Label 3 - Implicit NULL (Penultimate Hop Popping) - the egress LSR tells the neighboring LSR to pop the topmost label before forwarding to the egress LSR.Reserved labels 0 - 15 have a special meaning on LSR routers.Juniper uses a different range, please see.Each routers label range can be specified with the command mpls label range. Labels 16 - 100,000 are is the default range used by Cisco.Labels 0 - 15 are reserved by IETF and can not be used for dynamic label binding (see below).MPLS label field has an addres space of 1,048,576 labels.
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