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Cisco 10000 Series Router Software Configuration Guide
OL-2226-23
Chapter 6 Configuring PPPoE over Ethernet and IEEE 802.1Q VLAN
TCP MSS Adjust
Clearing PPPoE Sessions
To clear PPPoE sessions, enter the following commands in privileged EXEC mode:
TCP MSS Adjust
The TCP MSS Adjustment feature enables the configuration of the maximum packet segment size
(MSS).
When a host (usually a PC) initiates a TCP session with a server, it negotiates the IP segment size by
using the MSS option field in the TCP SYN packet. The value of the MSS field is determined by the
maximum transmission unit (MTU) configuration on the host. The default MSS value for a PC is 1500
bytes.
The PPP over Ethernet (PPPoE) standard supports a MTU of only 1492 bytes. The disparity between the
host and PPPoE MTU size can cause the router in between the host and the server to drop 1500-byte
packets and terminate TCP sessions over the PPPoE network. Even if the path MTU (which detects the
correct MTU across the path) is enabled on the host, sessions may be dropped because system
administrators sometimes disable the ICMP error messages that must be relayed from the host in order
for path MTU to work.
In most cases, the optimum value for the max-segment-size argument is 1452 bytes. This value plus the
20-byte IP header, the 20-byte TCP header, and the 8-byte PPPoE header add up to a 1500-byte packet
that matches the MTU size for the Ethernet link.
Feature History for TCP MSS Adjust
Information about TCP MSS Adjust
• This feature works for both PTA and LNS sessions.
• The MSS value is configured globally, so every packet transiting through the router are subject to a
rewrite.
• The per interface command is only applicable to packets that get punted to the RP, so it is not
recommended to use this command.
Command Purpose
Router# clear pppoe all
Clears all PPPoE sessions.
Router# clear pppoe interface
Clears all PPPoE sessions on a physical interface or subinterface.
Router# clear pppoe rmac
Clears PPPoE sessions from a client host MAC address.
Cisco IOS Release Description Required PRE
12.2(31)SB3 This feature was introduced on the Cisco 10000 series
router.
PRE2 or PRE3