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Cisco 10000 Series Router Software Configuration Guide
OL-2226-23
Chapter 20 Configuring L2 Virtual Private Networks
Configuration Tasks for L2VPN
Estimating the Size of Packets Traveling Through the Core Network
The following calculation helps you determine the size of the packets traveling through the core network.
You set the maximum transmission unit (MTU) on the core-facing interfaces of the P and PE routers to
accommodate packets of this size.
The MTU should be greater than or equal to the total bytes of the items in the following equation:
Core MTU >= (Edge MTU + Transport header + AToM header + (MPLS label stack size* MPLS
label size))
The following sections describe the variables used in the equation.
Edge MTU
The edge MTU is the MTU for the customer-facing interfaces.
Transport Header
The transport header depends on the transport type. Table 20-6 lists the specific sizes of the headers.
AToM Header
The AToM header is 4 bytes (control word). The Cisco 10000 series router adds the control word for all
supported transport types by default.
MPLS Label Stack
The MPLS label stack size depends on the configuration of the core MPLS network.
• AToM uses one MPLS label to identify the AToM VCs (VC label). Therefore, the minimum MPLS
label stack is 1 for directly connected AToM PE routers, which are PE routers that do not have a
P
router between them.
• If LDP is used in the MPLS network, the label stack size is 2 (the LDP label and the VC label).
• If a TE tunnel instead of LDP is used between PE routers in the MPLS network, the label stack size
is 2 (the TE label and the VC label).
• If a TE tunnel and LDP are used in the MPLS network (for example, a TE tunnel between P routers
or between P and PE routers, with LDP on the tunnel), the label stack is 3 (TE label, LDP label,
VC
label).
• If you use MPLS Fast Reroute in the MPLS network, you add a label to the stack. The maximum
MPLS label stack in this case is 4 (FRR label, TE label, LDP label, VC label).
Table 20-6 Header Size of Packets
Transport Type Header Size (bytes)
ATM AAL5 SDU support over MPLS 12
Ethernet over MPLS in VLAN mode 18
Ethernet over MPLS in port mode 14
Frame Relay over MPLS with
DLCI-to-DLCI connections
4
HDLC over MPLS 4
PPP over MPLS 4