Unlike a GRE tunnel, MPLS does not change the IP header. How much overhead does an MPLS LSP tunnel have?Īn MPLS LSP tunnel has one label (four bytes) or two labels (for example, when using Link Protection Fast reroute) of overhead. Generic Routing Encapsulation (GRE) tunnel has an overhead of 24 bytes. Software Advisor provides a complete list of feature sets supported in the different Cisco IOS versions and on different platforms. Label Distribution Protocol (LDP), Resource Reservation Protocol (RSVP), and Border Gateway Protocol (BGP) information can be found using the Software Advisor ( registered customers only) tool. These platforms support the Cisco Tag Distribution Protocol (TDP) as the label distribution protocol. What platforms and Cisco IOSes support MPLS? Likewise, if an MPLS packet is received on an interface which does not have MPLS enabled, the packet is dropped. For example, if a router receives an Appletalk packet on an interface which does not have Appletalk enabled, it drops the packet. When a Cisco router receives a packet with an Ethertype which is not enabled on the interface, it drops the packet. MPLS has a certain Ethertype code associated with it (just as IP, IPX, and Appletalk have unique Ethertypes). Packets are never transmitted on an interface which is not enabled for that protocol. Can an LSR receive/transmit a labeled packet on a non-MPLS interface? The receiving LSR is aware of the MPLS packet, based on the Layer 2 encoding. MPLS packets have a different Layer 2 encoding. Native packets are received/transmitted as usual. Yes, if the IP is enabled on the interface. In this scenario:Ĭan an LSR transmit/receive a native IP packet (non-MPLS) on an MPLS interface? R3 uses label L when it forwards data to FEC-F (because R2 is its downstream LSR for FEC-F). R2 distributes a label L for FEC F to R3. What do the terms incoming, outgoing, local, and remote mean when you refer to labels?Ĭonsider R2 and R3 in this topology. No, data flows from upstream to downstream. Is R3 a Downstream LSR to R4 for 10.1.1.0/24? The R4 routing table has R1, R2, and R3 as the next-hops to reach 10.1.1.0/24. For FEC 10.2.2.0/24, R2 is the Downstream LSR to R1.ĭata flows from upstream to downstream to reach that network (prefix). These examples further explain this.įor FEC 10.1.1.0/24, R1 is the Downstream LSR to R2.įor FEC 10.1.1.0/24, R2 is the Upstream LSR to R1.įor FEC 10.1.1.0/24, R1 is the Downstream LSR to R2 and R2 is the Downstream LSR to R3.įor FEC 10.1.1.0/24, R1 is the Downstream LSR to R2. They always refer to a prefix (more appropriately, a FEC). Upstream and downstream are relative terms in the MPLS world. What is an upstream label switch router (LSR)? What is a downstream LSR? For example, all traffic with a certain value of IP precedence might constitute a FEC. A FEC might correspond to a destination IP subnet but it also might correspond to any traffic class that the Edge-LSR considers significant. What is a Forwarding Equivalence Class (FEC)?Ī FEC is a group of IP packets which are forwarded in the same manner, over the same path, and with the same forwarding treatment. The network layer packet immediately follows the last label in the label stack. The top of the label stack appears first in the packet, and the bottom appears last. The label is imposed between the data link layer (Layer 2) header and network layer (Layer 3) header. Where will the label be imposed in a packet? Label - Label Value (Unstructured), 20 bitsĮxp - Experimental Use, 3 bits currently used as a Class of Service (CoS) field The label which is put on a particular packet represents the FEC to which that packet is assigned. What is a label? What is the structure of the label?Ī label is a short, four-byte, fixed-length, locally-significant identifier which is used in order to identify a Forwarding Equivalence Class (FEC). MPLS technologies are applicable to any network layer protocol. MPLS provides these beneficial applications:Īdditionally, it decreases the forwarding overhead on the core routers. Label inspection drives subsequent packet forwarding. With MPLS, the Layer 3 header analysis is done just once (when the packet enters the MPLS domain). MPLS is a packet-forwarding technology which uses labels in order to make data forwarding decisions. What is Multi-Protocol Label Switching (MPLS)? This document answers the most frequently asked questions related to Multiprotocol Label Switching (MPLS) from a beginner level.
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