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Flashcards in MPLS Deck (90):

What are some MPLS applications

Traffic engineering, MPLS VPN, AToM, and VPLS are the MPLS applications mentioned in this chapter.


Name three advantages of running MPLS in a service provider network.

- Better integration of IP over ATM

- A network that is running a BGP-free core

- Easy deployment of a peer-to-peer VPN model (MPLS VPN)

- One unified network infrastructure

- Optimal flow of traffic

- Traffic engineering enables the steering of traffic through the network on a path different than the least cost path computed by the dynamic routing protocol


What are the advantages of the MPLS VPN solution for the service provider over all the other VPN solutions?

MPLS VPN allows for easy provisioning of sites and allows for optimal traffic flow in the backbone network at all times.


Name the four technologies that can be used to carry IP over ATM.

- RFC 1483





Name two pre-MPLS protocols that use label switching.

ATM and Frame Relay are two pre-MPLS protocols that use label switching.


What do the ATM switches need to run so that they can operate MPLS?

The ATM switches need an IP routing protocol and a label distribution protocol to operate MPLS.


How do you ensure optimal traffic flow between all the customer sites in an ATM or Frame Relay overlay network?

The connectivity between the customer sites needs to be a full mesh of virtual circuits.


Name the four fields that are part of a label.

The four fields that are part of a label are a 20-bit label value, 3 experimental bits, 1 Bottom of Stack bit, and an 8-bit TTL field.


How many labels can reside in a label stack?

Any number of labels can reside in a label stack.


In which layer does MPLS fit in the OSI reference model?

MPLS fits in no category of the OSI reference model. The best description for MPLS would be Layer 2.5.


Which table does an LSR use to forward labeled packets?

The LSR uses the LFIB table to forward labeled packets.


What type of interfaces in Cisco IOS use the Downstream-on-Demand label distribution mode and the per-interface label space?

LC-ATM interfaces use the Downstream-on-Demand label distribution mode and the per-interface label space.


Why does the MPLS label have a Time To Live (TTL) field?

As the packets are labeled, the IP TTL can no longer be used. A mechanism is still needed to avoid a packet circulating in a loop forever.


What does the push operation do on a labeled packet?

The push operation replaces the top label with another and then pushes one or more labels onto the label stack.


Which Cisco IOS command do you use to see what the swapped label is and which labels are pushed onto a received packet for a certain prefix?

show mpls forwarding-table [network {mask | length}] [detail]


What does the outgoing label entry of “Aggregate” in the LFIB of a Cisco IOS LSR mean?

The outgoing label entry “Aggregate” means that the LSR removes the label and does an IP lookup to be able to determine where the packet needs to be forwarded.


What label value signals the penultimate LSR to use penultimate hop popping (PHP)?

The label with value 3, known as the implicit NULL label, signals the penultimate LSR to use PHP.


What are the value and the function of the Router Alert label?

The value of the Router Alert label is 1 and its function is to make sure that all LSRs forwarding this packet take a closer look at it.


Why does an LSR forward the ICMP message “time exceeded” along the LSP of the original packet with the TTL expiring instead of returning it directly?

The LSR—or an intermediate LSR—might not have the information it needs to return the ICMP message, so it forwards it along the LSP in the hope that the packet reaches a router that can return the ICMP message to the sender of the original packet.


Is using Path MTU Discovery a guarantee that there will be no MTU problems in the MPLS network?

No, the ICMP messages might not make it back to the originator of the packet for various reasons.


Why is MTU or MRU such an important parameter in MPLS networks?

The MRU is so important in MPLS networks because the addition of a label stack increases the size of a frame slightly. As such, the size of the frame might become more than the maximum allowed size of a frame on the data link. Especially on Ethernet links, care must be taken that such frames can still be forwarded. Such frames are known as baby giant frames.


What is the fundamental purpose of LDP?

The fundamental purpose of LDP is to distribute label bindings.


Name the four main functions that LDP takes care of.

- The discovery of LSRs that are running LDP

- Session establishment and maintenance

- Advertising of label mappings

- Housekeeping by means of notification


How can you reduce the number of label bindings on an LSR?

To reduce the number of label bindings on an LSR, you can either control the advertisement of label bindings via LDP in the outbound direction, or you can filter the incoming label bindings.


What problem does MPLS LDP-IGP synchronization solve?

MPLS LDP-IGP synchronization solves the problem of labeled packets being dropped when the LDP information is out of sync with the IGP.


How many LDP sessions are established between two LSRs that have six links between them, of which two links are LC-ATM links and four are frame links?

Three LDP sessions are established in that case.


What do you need to configure to protect the LDP sessions against attacks?

You need MD5 authentication for the LDP neighbor.


What trick does MPLS LDP-IGP Synchronization employ to ensure that the link is not used to forward traffic while the LDP session is unsynchronized?

MPLS LDP-IGP Synchronization has the IGP advertise the link with the maximum metric.


What does LDP Session Protection use to protect an LDP session?

LDP Session Protection uses a targeted LDP session.


In what ways is an ATM LSR different from a frame-based LSR?

- The label value is encoded in the VPI/VCI fields.

- ATM LSRs forward cells.

- ATM LSRs are not capable of decrementing the TTL.

- LDP runs in Downstream-on-Demand label advertisement mode on ATM LSRs.


What is the default control VC for LDP?

0/32 is the default control VC for LDP.


What is the preferred control mode for LDP on the ATM LSRs?

Ordered Control mode is the preferred mode for LDP on the ATM LSRs.


Name two ways that LDP can detect loops.

- By means of a Hop Count TLV

- By means of a Path Vector TLV


Which two features must an ATM switch have in the control plane to become an ATM LSR?

An ATM LSR must have an IGP and a label distribution protocol.


Which label space is used on an LC-ATM interface?

Per-interface label space is used on an LC-ATM interface


What IP precedence values are by default mapped to the standard LVC type with Multi-VC TBR?

Precedence values 1 and 5 are mapped by default to the standard LVC type with Multi-VC TBR.


What would be the reason to disable the head end VCs on an LSC?

You might disable the head end VCs on an LSC to reduce the number of LVCs through the ATM network.


Why does the Cisco equipment not advertise bound IP addresses of the LSR on LC-ATM interfaces with LDP?

The Cisco equipment does not advertise these addresses because they are not needed when the LSRs are running LDP on LC-ATM links. Mapping the received label to the downstream LDP peer is clear.


What is the advantage and disadvantage of VC-Merge?

The advantage of VC-Merge is the reduction of LVCs; the disadvantage is the buffering of the cells, which implies the need for more memory on the LSR.


Name the two components of CEF.

The two components of CEF are the adjacency table and the FIB or CEF table.


Name the three most common packet switching methods in Cisco IOS.

Process switching, fast switching, and CEF switching are the three most common packet switching methods in Cisco IOS.


Why does MPLS use CEF?

MPLS uses CEF because it is the only switching method in Cisco IOS that implements labeling of incoming IP packets.


What is the adjacency table used for?

The adjacency table takes care of the Layer 2 rewrite of the frames that are switched by the router.


What fields of the IP header does CEF use to load-balance IP packets?

CEF uses the destination IP address and the source IP address to load-balance IP packets.


How does CEF perform equal and unequal cost load balancing?

CEF uses a hashing algorithm that looks at the source and destination IP address of the packet. The hash result points to one or more of the 16 hash buckets, which in turn indicates which adjacency to use to forward the packet.


How does a prefix in the CEF table get the imposed label stack?

Each label in the imposed label stack can be assigned directly via LDP, BGP, or RSVP, or it can be inherited from recursion.


How is load balancing of labeled packets performed in Cisco IOS?

If the MPLS payload is an IPv4 or IPv6 packet, CEF hashing is performed on the source and destination IP address. If the MPLS payload is anything else, the load balancing is determined by the value of the bottom label in the label stack.


Name two reasons not to use CEF per-packet load balancing.

- Receipt of out-of-sequence packets, resulting in reordering of the packets or dropping of the packets

- Jitter (variable delay)


Name two huge differences between the fast switching and the CEF switching methods.

Fast switching and CEF switching are different in the following two ways:
- Fast switching uses an on-demand route cache, whereas the CEF switching table is prebuilt.

- The CEF table holds the MPLS label information to label IP packets before switching them out of the router; fast switching does not.


What is a route distinguisher?

A route distinguisher is a 64-bit value—chosen by the service provider—that is appended to the IPv4 routes of the customer and makes the routes unique.


How is a packet that is coming from the CE router identified as to which VRF it belongs?

The packet is identified as belonging to a VRF by the VRF configuration on the interface.


What is the purpose of RTs?

You use RTs to import vpnv4 routes into the VRF routing table and to export the vpnv4 routes to VRF sites that accept the RT.


What is an RR group?

An RR group can be configured on route reflectors to filter vpnv4 routes.


What is the BGP neighbor command with as-override used for?

The BGP speaker checks the as-path of the BGP route and replaces all occurrences of the autonomous system number of the BGP peer in the as-path with his own autonomous system number.


When would you use different route distinguishers for routes of the same VPN?

You would use different route distinguishers for hub-and-spoke topologies and when you need to make different vpnv4 routes from the same IPv4 route when sites are dual homed to PE routers and the MPLS VPN network has route reflectors.


What command should you configure on a Multi-VRF CE router that is running OSPF?

You should configure capability vrf-lite on a Multi-VRF CE router that is running OSPF.


What three characteristics does an OSPF sham link have?

- Unnumbered

- Point-to-point

- Demand-circuit


Why do MPLS VPN packets have two MPLS labels?

P routers use the IGP label to forward the packet to the correct egress PE router. The egress PE router uses the VPN label to forward the IP packet to the correct CE router.


Which BGP extended community can prevent routing loops from occurring in MPLS VPN networks?

Site-of-Origin (SOO) can prevent routing loops from occurring in MPLS VPN networks.


Define FIB

A neighbor state that signifies the other router has reached neighbor status, having passed the parameter check.


Define LIB



Define LFIB

An MPLS data structure used for forwarding labeled packets. The LFIB lists the incoming label, which is compared to the incoming packet’s label, along with forwarding instructions for the packet.


Define MPLS unicast IP routing

The simplest MPLS application, involving the advertisement of an IGP to learn IP routes, and LDP or TDP to advertise labels.


Define MPLS VPNs

An MPLS application that allows the MPLS network to connect to multiple different IP networks, with overlapping IP addresses, and provide IP connectivity to those multiple networks.


Define LDP

The RFC-standard MPLS protocol used to advertise the binding (mapping) information about each particular IP prefix and associated label. See also TDP.


Define TDP

The original MPLS protocol used to advertise the binding (mapping) information about each particular IP prefix and associated label. It is slightly different from LDP, but functionally equivalent. See also LDP.


Define LSP

The combination of MPLS labels and links over which a packet will be forwarded over an MPLS network, from the point of ingress to the MPLS network to the point of egress.


Define LSP segment

A single label and link that is part of a complete LDP. See also label switched path.


Define MPLS TTL propagation

The MPLS feature by which an ingress E-LSR copies the IP packet’s IP TTL field into the MPLS header’s TTL field.


Define local label

In MPLS, a term used to define a label that an LSR allocates and then advertises to neighboring routers. The label is considered “local” on the router that allocates and advertises the label.


Define remote label

In MPLS, a term used to define a label that an LSR learned from a neighboring LSR.


Define label binding

In MPLS, the mapping of an IP prefix and a label, which is then advertised to neighbors using LDP.


Define VRF

In MPLS VPNs, an entity in a single router that provides a means to separate routes in different VPNs. The VRF includes per-VRF instances of routing protocols, a routing table, and an associated CEF FIB.


Define RD

A 64-bit extension to the BGP NLRI field, used by MPLS for the purpose of making MPLS VPN customer routes unique in spite of the possibility of overlapping IPv4 address spaces in different customer networks.


Define RT

In MPLS VPNs, a 64-bit Extended Community path attribute attached to a BGP route for the purpose of controlling into which VRFs the route is added.


Define overlapping VPN

An MPLS term describing designs in which one or more MPLS customer sites can be reached from multiple other VPNs.


Define inner label

An MPLS term referring to the MPLS label just before the IP header. Also called the VPN label when implementing MPLS VPNs.


Define outer label

An MPLS term referring to the first of several labels when an MPLS-forwarded packet has multiple labels (a label stack).


Define VPN label

The innermost MPLS header in an packet traversing an MPLS VPN, with the label value identifying the forwarding details for the egress PE’s VRF associated with that VPN.


Define PHP

An MPLS VPN term referring to the more efficient choice of popping the outer label at the second-to-last (penultimate) LSR, which then prevents the egress PE from having to perform two LFIB lookups to forward the packet.


Define FEC

A set of packets in an MPLS network for which the MPLS network will apply the exact same forwarding behavior.


Define LSR

An MPLS term referring to any device that can forward packets that have MPLS labels.


Define E-LSR

An MPLS LSR that can forward and receive both labeled and unlabeled packets.


Define PE

An MPLS VPN term referring to any LSR that connects to customers to support the forwarding of unlabeled packets, as well as connecting to the MPLS network to support labeled packets, thereby making the LSR be on the edge between the provider and the customer.


Define CE

An MPLS VPN term referring to a router at a customer site that does not implement MPLS.


Define P

An MPLS VPN term referring to an LSR that has no direct customer connections, meaning that the P router does not need any visibility into the VPN customer’s IP address space.


Define Ingress PE

An E-LSR in an MPLS VPN network whose role in a particular discussion is to receive unlabeled packets over customer links and then forward the packets as labeled packets into the MPLS network.


Define Egress PE

An E-LSR in an MPLS VPN network whose role in a particular discussion is to receive labeled packets from other LSRs and then forward the packets as unlabeled packets to CE routers.


Define VRF Lite

A commonly used name for Multi-VRF CE.


Define Multi-VRF CE

An IOS feature in which multiple routing tables and routing forwarding instances exist in a single router, with interfaces being assigned to one of the several VRFs. This feature allows separating of routing domains inside a single router platform.