CCNP ENARSI 300-410 Exam Cram Notes

2.0 VPN Technologies

2.1 Describe MPLS operations (LSR, LDP, label switching, LSP)

LDP is a protocol that automatically generates and exchanges labels between routers. Each router will locally generate labels for its prefixes and will then advertise the label values to its neighbors. LDP uses the Transmission Control Protocol (TCP) to transmit Session, Advertisement, and Notification messages to ensure reliable message transmission. LDP uses the User Datagram Protocol (UDP) only for transmitting Discovery messages.

Cisco Express Forwarding is an advanced layer 3 switching technology used mainly in large core networks or internet to enhance the overall network performance is mainly used to increase packet switching speed by reducing the overhead and delays introduced by other routing techniques. CEF consists of two key components: The Forwarding Information Base (FIB) and adjacency.

LDP(Label Distribution Protocol):A protocol used between MPLS-enabled routers to generate and exchange labels that will be used to forward packets in the MPLS domain.

LSP(Label Switched Path):The cumulative labeled path (sequence of routers) that a labeled packet takes through the MPLS domain.

LFIB(Label Forwarding Information Base):A data plane table that is used to forward labeled packets.

LIB(Label Information Base): A control plane table that stores label information.

LSR(Label Switching Router):A router in an MPLS domain that forwards packets using label information.

1. MPLS PEs form a backbone area

2. Each PE-CE link can be any area.

3. MPLS uses MPBGP to redistribute routes

4. The Area 0 may exist in both customer premise as well as the Service providers (SP) network.

CE(Customer Edge):CE A CE router ( Customer Edge router ) is a router located on the customer premises that provides an Ethernet interface between the customer's LAN and the provider's core network. Customer Edge normally unaware of mpls labeling. Connects customer network to MPLS network

PE(Provider Edge): Provider edge is the egress and ingress for the mpls domain , it remove labels before sending them to CE and add labels to traffic received from CE

P(Provider): Provider , MPLS devices in the core of the MPLS domain , forward traffic based on labels

LSP (Label Switch path(): LSP is a predefined path that the packet takes during the transmission

LSR( Label Switch Router) : MPLS network contains Label Switch Routers(LSR). These routers are capable of understanding MPLS labels and of receiving and transmitting the labeled packet.

ingress LSR:A router at the edge of the MPLS domain that adds labels to packets that are entering the MPLS domain.

egress LSR: A router at the edge of the MPLS domain that removes labels from packets that are leaving the MPLS domain.

The MPLS header is of 32 bits. It contains the following information:-

ccnp enarsi cram notes mpls header

1. Label: The label field is of 20 bits.

2. Experimental(Exp):The three bits are reserved as experimental bits. They are used for Quality of Service(QoS).

3. Bottom of Stack(BoS): A network packet can have more than one MPLS labels which are stacked one over another. To ensure which MPLS label is at the bottom of stack we have a BoS field which is of 1 bit.

4. Time to Live(TTL): The last 8 bits are used for Time to Live(TTL).

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