When both attachment circuits (AC) of a pseudowire (PW) belong to the same service type, L2VPN thus formed is referred to as a like to like. In this case, because the data plane is still homogeneous (like to like), both end points are of the same L2 encapsulation. Even though there is no data plane interworking, it might involve some kind of network interworking at the control plane during PW setup. Control planes interworking might be required when one attachment circuit is connected to a legacy ATM network (non-IP network) and the other end of the pseudowire (AC) might or might not be connected to the legacy network. The following are the types of like-to-like L2VPN over MPLS:
Any to AnyAny-to-any L2VPN is formed when both end points of the AC are heterogeneous, meaning they belong to different services. In this case, both end points of an AC belong to different Layer 2 services, such as ATM, FR, Ethernet, and so on. Service interworking (conversion from one type to another) can be performed at either of the end points and can be either signaled or provisioned at the corresponding end points. Common any-to-any scenarios are as follows:
Local SwitchingWhen both ACs of the L2VPN connection are terminated on the same node, this is called local switching. Encapsulation Types and ModesATM over MPLSIn this mode, one ATM connection is mapped to a pair of PWs (one for each direction) between the PE nodes. Two types of modes are used for carrying cells over the MPLS/IP networknamely, cell mode and packet mode.
Both of these modes support cell packing for transport efficiency. Similarly, two packet mode encapsulations are defined:
ATM N:1 ModeThis encapsulation supports the binding of multiple VCCs/VPCs to a single pseudowire. N:1 mode can also be used to carry VCs, VPs, or an entire port. In all these cases, the encapsulation remains the same but the content differs. If used in the context of VC/VP mode, it degenerates into one VC for one PW. To clarify, N:1 here indicates the number of vicks mapped to a single PW and not the number of cells packed in a frame. ATM 1:1 ModeIn this mode, one VC is mapped to one PW. Because only one ATM VCC or VPC is carried on a PW, the VCI and/or VPI of the ATM VCC or VPC can be derived from the context of the PW using the PW label. AAL5 SDU ModeThe AAL5 SDU encapsulation is more efficient for small AAL5 SDUs than the VCC cell encapsulations because it does not carry the AAL5 trailer, such as length, CRC, and padding. AAL5 PDU ModeThe primary application supported by AAL5 PDU frame encapsulation over PSN is the transparent carriage of ATM layer services that use AAL5 to carry higher layer frames. The main advantage of this AAL5 mode is that it is transparent to ATM OAM and ATM security applications. Frame Relay over MPLSCurrently, two modes of operation are defined for transporting Frame Relay over MPLS:
A PW can potentially fail for many reasons, and we can't list all the failure scenarios here. However, this section delves into some of the most commonly occurring failures and highlights the various scenarios that can cause these failures to occur. In general, the failures can be broadly classified into line/hardware-related, control plane-related, or OAM- or time-based. Even though, strictly speaking, OAM is not a failure and indicates a failure condition only in the upstream or downstream node, we loosely accept that as one of the failure reasons. |