![]() The information in this document is based on this software version:Ĭisco IOS® Software release version 12.0(21)ST2 ![]() There are no specific requirements for this document. However, the steps used to diagnose and correct this problem are applicable to any problem involving label bindings and the LFIB on routers configured for MPLS. The document covers one scenario wherein label bindings received by a router are not used to forward MPLS-switched packets. This document gives an example of this kind of LSP failure and several possible solutions. In a similar manner, if a router advertises labels for a subnet/subnet mask pair, which do not correspond to the routing updates also advertised by this router for the same subnet/subnet mask pair, these labels will not be used by upstream neighbors and the Label Switched Path (LSP) between these devices will fail. If label bindings are received from a downstream neighbor for prefixes (including subnet mask) which do not appear in a router's routing and CEF tables, these bindings will not be used. Meaning the labels used for forwarding are those received from a router's next hop to a destination, according to the router's Cisco Express Forwarding (CEF) and routing tables. If the labels are received from a downstream neighbor for their FEC, then the labels stored in the LIB are used for packet forwarding by the LFIB. All labels received from neighbors are retained in the LIB, whether or not they are used. For Cisco implementation, labels are sent for all routes in a given router's routing table (with the exception of BGP routes), to all LDP or TDP neighbors. The Label Information Base (LIB) is a structure which stores labels received from all Label Distribution Protocol (LDP) or Tag Distribution Protocol (TDP) neighbors. Another example could be all packets with a given IP precedence going to an Interior Gateway Protocol (IGP) next hop associated with a group of Border Gateway Protocol (BGP) routes. The most simple example of a FEC is all packets traveling to a certain subnet. A FEC is a grouping of IP packets which travel over the same path and receive the same forwarding treatment. Labels are associated with destinations in the LFIB according to Forwarding Equivalence Classes (FECs). A packet leaving a router over a label-switched interface will receive labels with values specified by the LFIB. MPLS-switched packets are forwarded based on information contained in the Label Forwarding Information Base (LFIB). In order to remove the loopback interface, use the no interface loopback 1 in config mode.This document assumes you have a prior understanding of basic Multiprotocol Label Switching (MPLS) concepts. When you have a Layer 3 switch, like a Catalyst 4500,6500,3550 or 3750, to configure loopback interface from the config mode, complete these steps: ![]() The management vlan of the switch acts like the loopback interface since they are both logical interfaces. The Layer 2 switches like 3500xl or 2950, the loopback interface is not needed since these switches are not capable of routing between interfaces. Loopback interfaces do not have internal VLAN IDs or MAC addresses. There is no limit on the number of loopback interfaces you can create. This is an issue in networks with multiple equal-cost paths because under normal circumstances the packets that are generated by a networking device use the IP address from the outbound interface as the source address for the packets and because in a network with two or more equal-cost paths from the networking device to the receiving host each packet might use a different outbound interface. ![]() ![]() This address can be configured as the source address when the networking device needs to send data for protocols such as NetFlow,OSPF or Cisco Discovery Protocol (CDP) to another device in your network and you want the receiving device to always see the same source IP address from the networking device. A loopback interface can provide a stable interface on which you can assign a Layer 3 address. ![]()
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