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20-38
Cisco 10000 Series Router Software Configuration Guide
OL-2226-23
Chapter 20 Configuring L2 Virtual Private Networks
Configuration Tasks for L2VPN
• If AToM is used by the customer carrier in the MPLS-VPN Carrier Supporting Carrier environment,
you add a label to the stack. The maximum MPLS label stack in the provider carrier network is 5
(FRR label, TE label, LDP label, VPN label, VC label).
• If an AToM tunnel spans different service providers that exchange MPLS labels using IPv4 BGP
(RFC 3107), you add a label to the stack. The maximum MPLS label stack is 5 (FRR label, TE label,
BGP label, LDP label, VC label).
Other circumstances can increase the MPLS label stack size. Therefore, analyze the complete data path
between the AToM tunnel endpoints and determine the maximum MPLS label stack size for your
network. Then multiply the label stack size by the size of the MPLS label.
Estimating Packet Size—Example
Example 20-28 shows how to estimate the size of packets. The example uses the following assumptions:
• The edge MTU is 1500 bytes.
• The transport type is Ethernet VLAN, which designates 18 bytes for the transport header.
• The AToM header is 4 bytes, because the control word is always used.
• The MPLS label stack size is 2, because LDP is used. The MPLS label size is 4 bytes.
Example 20-28 Estimating the MTU for Packets
Core MTU >= (Edge MTU + Transport header + AToM header + (MPLS label stack size* MPLS
label size))
1500 + 18 + 0 + (2 * 4 ) = 1526
You must configure the P and PE routers in the core to accept packets of 1526 bytes. See the following
section for setting the MTU size on the P and PE routers.
Changing the MTU Size on P and PE Routers
After you determine the MTU size to set on your P and PE routers, you can issue the mtu command on
the routers to set the MTU size. The following example specifies an MTU size of 1526 bytes.
Router(config-if)# mtu 1526
Setting Experimental Bits with AToM
MPLS AToM uses the three experimental (EXP) bits in a label to determine the queue of packets.The
EXP bits are set to 0 (zero) by default.
Table 20-7 summarizes the commands you can use to override
the default values.