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Cisco 10000 Series Router Software Configuration Guide
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Chapter 20 Configuring L2 Virtual Private Networks
L2VPN Local Switching—HDLC/PPP
L2VPN Local Switching—HDLC/PPP
The L2VPN Local Switching - HDLC/PPP feature enables service providers to support different
encapsulations over HDLC local switched circuits that function as back-to-back circuits. The
provisioned HDLC Local Switched circuits can also be backed by using PWRED.
Prerequisites of L2VPN Local Switching—HDLC/PPP
In Cisco IOS Release 12.2(33)SB, the L2VPN Local Switching - HDLC/PPP, you must ensure that
interfaces must be HDLC encapsulated on the PE router. The CE routers can choose any HDLC-based
encapsulation, including Frame Relay and PPP.
Restrictions of L2VPN Local Switching—HDLC/PPP
In Cisco IOS Release 12.2(33)SB, the L2VPN Local Switching - HDLC/PPP feature has the following
restrictions:
• On the PE HDLC interface, the IP address cannot be configured because it conflicts with the
connect command.
• Interworking is not supported on HDLC/PPP interfaces.
• Only same-speed interfaces should be connected, to avoid arbitrary packet drops due to a higher
speed interface overrunning a lower speed one.
• For some HDLC/PPP applications which are sensitive to time delay, the PE may introduce some
network delay, enough to prevent the HDLC/PPP link from coming up because of a protocol timeout
(an ISDN Q921 link).
PPP Like-to-Like Local Switching
Some applications, such as transport of compressed voice between the two CEs, require a setup of an
end-to-end PPP session between two CE routers that are connected to the same PE router. In such cases,
HDLC pass-through mechanism is proposed and the interworking scenario is simplified to PPP transport
for like-to-like services. PPP local switching functionality on the PE router provides simple HDLC
connectivity between two end-users found on different CE routers as shown in
Figure 20-1.
Figure 20-1 PPP Local Switching
The interfaces are HDLC encapsulated on the PE router. The CE routers may use PPP-based
encapsulation.
RFC 1661 (Protocol)
Data
FCS
PE
PPP
CE
PPP
CE