frames or, sometimes, giants or baby giants). Some devices support jumbo frames, and some devices need to be configured to support the frames. Label switching increases the maximum MTU requirements on an interface because of the additional label headers. The interface MTU is automatically increased on WAN interfaces, but not on LAN interfaces. Because the MTU is not automatically increased on LAN interfaces, you must increase the MTU manually using the mpls mtu command. To set the per-interface MTU for labeled packets, use the mpls mtu interface configuration command as shown in Figure . The only parameter in the mpls mtu command is bytes. This parameter specifies the MTU in bytes. The minimum MTU is 64 bytes. The maximum MTU depends on the type of interface medium that is being used. Figure shows an example label switching MTU configuration script. The MPLS MTU size has to be increased on all routers that are attached to a LAN segment (in the example, Routers B and C). Additionally, the LAN switch (Cat6000) that is used to implement switched LAN segments needs to be configured to support jumbo frames.
Content 4.4 Describing MPLS VPN Technology 4.4.1 MPLS VPN Architecture VPNs have evolved in a number of virtual networking concepts that are discussed in this lesson: Figure illustrates some key VPN implementation technologies, highlighting the two major VPN models, overlay and peer-to-peer. Overlay VPNs—Layers 1, 2, and 3
Layer 1 overlay VPN: The Layer 1 overlay VPN model is mentioned only for historical reasons. This implementation adopts the traditional time-division multiplexing (TDM). The SP sells Layer 1 circuits (bit pipes) that are implemented with technologies such as ISDN, digital service level zero (DS0), E1/T1, synchronous digital hierarchy (SDH), and SONET. Essentially this means that the SP assigns bit pipes and establishes the physical-layer (Layer-1) connectivity. The customer implements all higher layers, such as PPP, High-Level Data Link Control (HDLC), and IP. Layer 2 overlay VPN: The Layer 2 overlay model is a traditional switched WAN model and is the basis for traditional VPN implementations. A Layer 2 overlay VPN is implemented with technologies including X.25, Frame Relay, ATM, and Switched Multimegabit Data Service (SMDS). Using the Layer 2 overlay model, the SP sells virtual circuits (VCs) between customer sites as a replacement for dedicated point-to-point links. The SP is responsible for transport of Layer 2 frames between customer sites, and the customer is responsible for all higher layers. The figure shows a typical Layer 2 overlay VPN implemented by a Frame Relay network. Figure is an example of a Layer 2 overlay VPN using Frame Relay. In this example, the customer needs to connect three sites to Site A (the central site, or hub) and orders connectivity between Site A (hub) and Site B (spoke), between Site A and Site C (spoke), and between Site A and Site D (spoke). The SP implements this request by providing three permanent virtual circuits (PVCs) across the Frame Relay network. Note
The implementation that appears in this example does not provide full connectivity; data flow between spoke sites is through the hub. Layer 3 Overlay VPN: Figure illustrates a Layer 3 overlay VPN implementation. From the Layer 3 perspective, the SP network is invisible to the customer routers that are linked with emulated point-to-point links. IP tunneling allows a destination to be reached transparently without the source having to know the topology specifics. Therefore, virtual networks can be created by tying otherwise unconnected devices (at least in an indirect sense they are not connected), or hosts together through a tunnel. Tunnels also enable the use of private network addressing across a SP’s backbone without the need for Network Address Translation (NAT). The Layer 3 model establishes tunnels with GRE or IPsec. The routing protocol runs directly between customer routers that establish routing adjacencies and exchange routing information. The SP is not aware of customer routing and has no information about customer routes. The responsibility of the SP is simply to provide point-to-point data transport between customer sites. Peer-to-Peer VPNs
The most significant drawback of the Layer 2 overlay VPN model is the need for customers to establish point-to-point links or VCs between sites. The peer-to-peer model adopts a simple routing scheme for the customer. Both the SP and the customer networks use the same network protocol, and the SP’s core carries all the customer routes. The provider edge (PE) routers exchange routing information with the customer edge (CE) routers, and CE and PE routers at each site establish Layer 3 routing adjacencies between themselves. Because of this implementation, peer-to-peer routing between sites is now optimal. Fully meshed deployment of point-to-point links or VCs over the SP backbone is no longer required for attaining optimal routing. Since there is no overlay mesh to contend with, it is easy to add new sites and circuit capacity sizing does not create problems. Because the SP now participates in customer routing, provider-assigned or public address space needs to be deployed at the customer’s network, so private addressing is no longer an option. Figure shows an example of peer-to-peer routing. The formula to calculate how many point-to-point links or VCs are needed is ([n]*[n-1])/2, where n is the number of sites that must be connected. For example, if you need to have full mesh connectivity among four sites, you will need a total of six (4*[4-1]/2) point-to-point links or VCs. To overcome this drawback and provide the customer with optimum data transport across the SP backbone, Cisco introduced the peer-to-peer VPN concept. In this concept, the SP actively participates in customer routing. The SP accepts customer routes, transports those customer routes across the SP backbone, and finally propagates the routes to other customer sites.
Content 4.4 Describing MPLS VPN Technology 4.4.2 Benefits and Drawbacks of Each VPN Implementation Model Each VPN model has benefits and disadvantages. Figure shows the benefits and disadvantages of overlay VPNs: Figure shows the benefits and disadvantages of peer-to-peer VPNs: