Jumbo frames fail across an operational VPLS pseudowire
VPLS 200 connects PE-A and PE-B over an LDP-signaled MPLS SDP. PE-A’s access port 1/1/1 has an Ethernet MTU of 9212 and SAP 1/1/1:200; PE-B’s access port 1/2/1 is 1518 and SAP 1/2/1:200. The customer has begun sending 2,000-byte frames between two servers. ICMP pings of 100 bytes pass, and MAC addresses learn in both directions. Pings of 1,800 bytes fail. On PE-B, show port 1/2/1 shows egress discards rising; show service sdp 10 reports the SDP operational, and both PEs advertise adv-mtu 9000 on SDP 10. The core path between the PEs supports 9212 bytes. The customer insists on jumbo frames end to end and says the application worked over a direct fiber link. The provider’s standard access MTU is 1518, and changing it requires a maintenance window.
1. What is the most likely reason large frames fail, and which checks confirm it?
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The SDP adv-mtu 9000 only advertises the pseudowire’s capability; it does not raise the PE-B access port or SAP MTU. PE-B’s port is 1518, so a 2,000-byte frame is discarded on egress toward the SAP, which matches the egress discard counter. Confirm with show port 1/2/1 detail for the Ethernet MTU, show service id 200 sap 1/2/1 detail for the SAP MTU, and show service sdp 10 detail for the advertised MTU. A large ping failing while a small ping succeeds is the classic MTU symptom; MAC learning can still work because small control frames pass. Also verify the core path MTU and any intermediate LSP MTU constraints.
2. Recommend a configuration plan to support 2,000-byte frames, including risks and verification.
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Jumbo is end-to-end or not at all; VPLS does not fragment. Set PE-B’s port Ethernet MTU and SAP MTU to at least 2020 (2,000-byte payload plus headers), set PE-A’s SAP MTU to the same, and ensure SDP adv-mtu is at least the service MTU on both ends and no larger than the core path MTU. Changing the access port MTU can affect other services on that port, so schedule a maintenance window and verify with a 2,000-byte ping with DF set. Also check that the customer’s servers and switches are configured for jumbo. If PE-B hardware or the access provider cannot support 2020, the correct answer is to keep 1518 and state that jumbo is not supported on that path, not to raise only the SDP MTU.