Describe the general function of the label forwarding information base (LFIB).
The LFIB is part of the data plane and provides the database used in forwarding labeled packets.
What are the two ways that label distribution can occur?
– Unsolicited downstream – When an MPLS neighbor receives an update due to a convergence event. – Downstream-on-demand – When a neighbor requests an update from another neighbor.
Describe the process of interim packet propagation.
Interim packet propagation is when a packet arrives at an LSR and it does not have a known label associated with the necessary FEC to get the packet out.
So the packet is forwarded to the next-hop router using information stored in the FIB.
Describe the general term of a label-switched path (LSP).
Label-switched paths (LSPs) are created when MPLS has a path from point A to point B. The LSP is essentially a tunnel between the source and destination for a particular FEC.
What range of label numbers are reserved from being used?
Label values 0-15 are reserved and can not be used.
Describe the concept of penultimate hop pop.
Penultimate hop pop allows the LSP immediately prior to the edge LSR to pop the label. The keeps the edge LSR from having to perform a lookup on both the LFIB and FIB.
When using penultimate hop pop, how does the edge router know that it is the last hop before the destination network?
The downstream router will distribute a label value of 3 to the upstream router. When the upstream router makes the next hop determination and sees that the label value is 3, it will remove the label and forward the packet normally.
Briefly describe the 2 different planes of MPLS.
– Control plane – Exchanges routing information and labels. – Data plane – Forwards packets based on labels
Describe the main purpose of each of the listed tables below: – FIB – LIB – LFIB – Adjacency table
– FIB – Maps destination networks to next-hop address and outbound interface.
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Also forwards unlabeled packets. – LIB – Associates local labels with FECs. Also performs label distribution to adjacent peers. – LFIB – Database used to forward labeled packets to next-hop addresses. – Adacency table – Maintains needed L2 information as well as LDP exchange capabilities.
What are the 2 modes that CEF can run in?
– Central mode – Only one instance of CEF is run on a router. – Distributed mode – Each blade in a router runs its own instance of CEF and maintains its own switching cache.
What 2 tables provide the operational base for CEF?
CEF used the FIB to make IP destination switching decision. The adjacency table keeps a database of L2 information, including L2 next-hop information. CEF used the adjacency table to prepend L2 information to outbound traffic., avoiding the need for ARP requests.
What command is used to enable CEF on a router, using the default mode (central mode)?
What command is used to enable CEF in distributed mode on a router?
ip cef distributed
What command is used to enable CEF operation on a particular interface?
ip route-cache cef
It what order must CEF be enabled on a router for it to work properly?
The ip route-cache cef interface command allows CEF on a particular interface. Then, running the ip cef globalcommand enables CEF on all interfaces that allow CEF.
What command displays CEF statistics on a router?
show ip cef [detail]
What command enables MPLS on a router, and on participating interfaces?
What command enables the label distribution protocol on an MPLS interface?
mpls label protocol [tdp | ldp | both]
What TCP ports are used for Tag Distribution Protocol (TDP) and Label Distribution Protocol (LDP)?
TDP use TCP port 711 and LDP uses TCP port 646.
What command is used to view LDP adjacencies?
show mpls ldp neighbor
What command sets the MPLS MTU size on a particular interface?
mpls mtu size
What command can be used to monitor the label exchange in real time between adjacencies?
debug mpls ldp bindings
What information must be unique on a provider’s edge for each customer going through a P network?
Each customer is assigned its own RD and VRF table dedicated to maintaining routing information within the provider infrastruture.
How are prefixes advertised by customers handled throughout the provider’s network?
BGP neighbor relationships are configured between PE routers directly so that the prefixes can be exchanges for a given customer. The global IP routing table in the P network need not actually carry any of the actual customer routes.
Describe the use of route disinguishers (RD) on a provider’s network.
Route distiguishers (RDs) allow different customer private address prefixes to be kept unique. The RD is a 64-bit identifier that is tacked on to the front of the IPv4 address.
These VPNv4 addresses are advertised between BGP peers on PE routers. Multiprotocol BGP (MPBGP) supports address families other than IPv4 addressing. This creates a 96-bit entry known as a VPNv4 address.
Describe the use of route targets in a MPLS VPN network.
Route targets (RTs) are an additional attibute that are attached to a VPNv4 BGP route to indicate VPN membership. Also called export RTs, they identify the VPNs to which the sites associated with a particular VRF belong. Import RTs specify the routes associated with a particular VRF.
Describe how packet forwarding is handled in an MPLS VPN environment?
PE routers use a two-label stack to label the VPN packets for forwarding across the P network. The label stack is imposed by the ingress PE router.
The top label in the stack will be used by LDP for P network traversal along an LSP that will get it to the egress PE router. The second label will be assigned by the egress PE router.
Describe the use of the S-bit in a MPLS label.
The S-bit is the end-of-stack pointer.
When set to 0, there will be further labels in the stack. The bottom label in the stack will have the S-bit set to 1, indicating its position as the last label.