Lucent Technologies 6000 Manuale Utente

11-4

MAX 6000/3000 Network Configuration Guide

Setting Up Virtual Private Networks
Configuring ATMP tunnels

uses MTU discovery mechanisms to determine the maximum packet size, and fragments 
packets before sending them.

If any kind of compression is on (such as VJ header or link compression), the connection can 
transfer larger packets without exceeding a link’s Maximum Receive Units (MRU). If 
compressing a packet makes it smaller than the MRU, it can be sent across the connection, 
whereas the same packet without compression could not.

To transmit packets through an ATMP tunnel, the MAX adds an 8-byte GRE header and a 
20-byte IP header to the frames it receives. The addition of these packet headers can make the 
packet larger than the MTU of the tunneled link, in which case the MAX must either fragment 
the packet after encapsulating it or reject the packet.

Fragmenting packets after encapsulating them has several disadvantages for the Foreign Agent 
and Home Agent. For example, it degrades performance because both agents have extra 
overhead. It also means that the Home Agent device cannot be a GRF switch. (To maintain its 
very high aggregate throughput, Lucent’s GRF switch does not perform reassembly.)

To avoid the extra overhead incurred when ATMP agents perform fragmentation, you can 
either set up a link between the two units that has an MTU greater than 1528 (which means it 
cannot include Ethernet segments), or you can set the Ethernet > Mod Config > ATMP > GRE 
MTU parameter to a value that is 28 bytes less than the path MTU.

If you set GRE MTU to zero (the default), the MAX might fragment encapsulated packets 
before transmission. The other ATMP agent must then reassemble the packets.

If you set GRE MTU to a nonzero value, the MAX reports that value to the client software as 
the path MTU, causing the client to send packets of the specified size. This pushes the task of 
fragmentation and reassembly out to the connection end-points, lowering the overhead on the 
ATMP agents.

For example, if the MAX is communicating with another ATMP agent across an Ethernet 
segment, you can set the GRE MTU parameter to a value 28 bytes smaller than 1500 bytes, as 
shown in the following example, to enable the unit to send full-size packets that include the 
8-byte GRE header and a 20-byte IP header without fragmenting the packets:

GRE MTU=1472

With this setting, the connection end-point sends packets with a maximum size of 1472 bytes. 
When the MAX encapsulates them, adding 28 bytes to the size, the packets still do not violate 
the 1500-byte Ethernet MTU.

To discover the path MTU, some clients normally send packets that are larger than the 
negotiated Maximum Receive Unit (MRU) and that have the Don’t Fragment (DF) bit set. 
Such packets are returned to the client with an ICMP message informing the client that the host