MTU Question [7:20096]
Hi Groups, I am a little confused about how the MTU size configured on an interface affects the transmission of packets through that interface. My question is does it affects packets received on the interface or packets transmitted out of the interface? Thanks Lw __ Terrorist Attacks on U.S. - How can you help? Donate cash, emergency relief information http://dailynews.yahoo.com/fc/US/Emergency_Information/ Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=20096&t=20096 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: MTU Question [7:20096]
> I am a little confused about how the MTU size > configured on an interface affects the transmission of > packets through that interface. My question is does it > affects packets received on the interface or packets > transmitted out of the interface? Great question. It definitely affects packets transmitted OUT of the interface - if the packet is larger than the interface ip mtu, the router fragments the packet using the configured interface MTU value. A demonstration of this is in GRE tunnel applications, which are by default 1478 bytes (as opposed to 1500), so that a 1500 byte packet gets fragmented when traversing the tunnel. On an INBOUND packet, the MTU is ignored. Just to verify this, I ran a bunch of debugs that show outbound fragmentation, but inbound the packets are not fragmented, just forwarded to the next interface. _ Do You Yahoo!? Get your free @yahoo.com address at http://mail.yahoo.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=20106&t=20096 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: MTU Question [7:20096]
I believe the correct way to answer this question is, the MTU effects any interface to interface communication where a TCP handshake takes place. That would mean incoming or outgoing. The window of information must match what I expect to receive. Have I come close ??? Phil - Original Message - From: "Lists Wizard" To: Sent: Sunday, September 16, 2001 2:19 AM Subject: MTU Question [7:20096] > Hi Groups, > > I am a little confused about how the MTU size > configured on an interface affects the transmission of > packets through that interface. My question is does it > affects packets received on the interface or packets > transmitted out of the interface? > > > Thanks > > Lw > > __ > Terrorist Attacks on U.S. - How can you help? > Donate cash, emergency relief information > http://dailynews.yahoo.com/fc/US/Emergency_Information/ Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=20108&t=20096 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: MTU Question [7:20096]
Ignoring Inbound I think depends on what type of packet it is. A while back while experimenting with RFC 1483, one end of a circuit had a MTU of 4470 (default ATM if memory serves...) and the other had 1500. OSPF was configured across this circuit but an adjacency would not form. When OSPF debugging was turned on it was cool that we actually got an English type of answer on the lines of "MTU mismatch". When the ATM interface was changed to 1500 the adjacency formed and routes were propagated. So another case of "it depends". Kevin Wigle - Original Message - From: "EA Louie" To: Sent: Sunday, 16 September, 2001 08:26 Subject: Re: MTU Question [7:20096] > > I am a little confused about how the MTU size > > configured on an interface affects the transmission of > > packets through that interface. My question is does it > > affects packets received on the interface or packets > > transmitted out of the interface? > > Great question. It definitely affects packets transmitted OUT of the > interface - if the packet is larger than the interface ip mtu, the router > fragments the packet using the configured interface MTU value. A > demonstration of this is in GRE tunnel applications, which are by default > 1478 bytes (as opposed to 1500), so that a 1500 byte packet gets fragmented > when traversing the tunnel. > > On an INBOUND packet, the MTU is ignored. > > Just to verify this, I ran a bunch of debugs that show outbound > fragmentation, but inbound the packets are not fragmented, just forwarded to > the next interface. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=20114&t=20096 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: MTU Question [7:20096]
At 08:50 AM 9/16/01, Circusnuts wrote: >I believe the correct way to answer this question is, the MTU effects any >interface to interface communication where a TCP handshake takes place. >That would mean incoming or outgoing. The window of information must match >what I expect to receive. > >Have I come close ??? Nope. Sorry. ;-) TCP is end-to-end, so "interface-to-interface" has no meaning at the TCP layer. You may be thinking about the TCP segment size. Each side of a TCP session specifies the size of the largest TCP segment that it can handle receiving. This info is carried in the TCP Options field in a SYN packet during the 3-way handshake. The two sides do not need to agree. It is not a negotiated value. The segment size is the size of each message. This is not the same as the window size which is much larger. The window size is how much data the host is ready to receive before the other side should stop and wait for an ACK. Regardless, the original question is down a layer and not specific to TCP. When IP goes to send a datagram, if the datagram is larger than the MTU of the output data-link-layer interface, IP fragments the datagram. The end recipient reassembles it. How does the TCP segment size relate to MTU? It usually defaults to something that matches the local interface. For example on a PC that is on Ethernet, it defaults to 1460 (1500 minus the 20-byte IP header and 20-byte TCP header). Cisco lets you set both the interface MTU and an IP MTU. They can actually differ, but there's generally no need for them to be different. As far as incoming frames, I doubt you could affect this by setting the interface MTU. My guess is that checking the size of any incoming frame is done at the chip level. An Ethernet chip would trash a frame bigger than 1522 (counting header, CRC, and any tagging) and report a giant. Priscilla >Phil > >- Original Message - >From: "Lists Wizard" >To: >Sent: Sunday, September 16, 2001 2:19 AM >Subject: MTU Question [7:20096] > > > > Hi Groups, > > > > I am a little confused about how the MTU size > > configured on an interface affects the transmission of > > packets through that interface. My question is does it > > affects packets received on the interface or packets > > transmitted out of the interface? > > > > > > Thanks > > > > Lw > > > > __ > > Terrorist Attacks on U.S. - How can you help? > > Donate cash, emergency relief information > > http://dailynews.yahoo.com/fc/US/Emergency_Information/ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=20129&t=20096 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
