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Serial and Fastethernet

Zayar Win
Level 1
Level 1

Dear ,

           I would like to know following - 

MTU size of Serial interface  =   ?

MTU size of Fast Ethernet    =  ?

How ISP handle MTU size and How packet can pass through ISP network ?

I mean Original Packet may be lager than 1500 Bytes. In this case, ISP is only allowed MTU 1500. If so, Orignial Packet can pass through on this ISP link without fragmentation or ISP devices will reply to make fragmentation to original sender.

Thanks

2 Accepted Solutions

Accepted Solutions

Mark Malone
VIP Alumni
VIP Alumni

Hi

depends on ipv4 or ipv6

ipv4 the router will fragment a packet if its is too big on ISP side or client side

ipv6 doesn't do fragmentation so it requests the source host do it first rather than the router

The mtu size of the interface can be irrelevant it can be more to do with the underlying technology , you could have a 1500 mtu standard Ethernet but its running pppoe so the mtu will most likely be set to work with that say 1452 , you can test what your max mtu is by sending a ping across the circuit and constantly increasing mtu size until you hit maximum , you will see on wan links where this starts to become fixed by ISPs , atm circuits/dsl circuits etc can limit mtu in real world , that's why when packets come in fragmented we use the ip virtual reassembly , this has 2 sides one its a security mechanism that prevents attacks and 2 it can set a limit on the amount of packets it reassembles , if the limit is reached packets can be dropped but its best to avoid fragmentation with the tcp mss syntax in the first place

I would read this to understand fully, good doc with explanations on how/why fragmentation and what to do

http://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/25885-pmtud-ipfrag.html

View solution in original post

Joseph W. Doherty
Hall of Fame
Hall of Fame

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

The max MTU of a serial interface depends on the serial interface technology.

The max MTU of FastEthernet, is 1500 by standard.  Some vendors support larger "jumbo" MTU, but that's non-standard.  Vendor jumbos often are supported up to around 9K.

If so, Orignial Packet can pass through on this ISP link without fragmentation or ISP devices will reply to make fragmentation to original sender.

If IPv4 packet cannot be contained within an interface's max MTU, the interface cannot transmit it.  If interface is L2, frame will dropped.  If interface is L3, if DF set, drop packet and notify source; or if DF not set, fragment packet and transmit.

As you didn't specify what technology the ISP is using, cannot say exactly what might happen, but likely ISP will support MTU up to 1500, and follow L3 rules for entry into their network.

View solution in original post

4 Replies 4

Mark Malone
VIP Alumni
VIP Alumni

Hi

depends on ipv4 or ipv6

ipv4 the router will fragment a packet if its is too big on ISP side or client side

ipv6 doesn't do fragmentation so it requests the source host do it first rather than the router

The mtu size of the interface can be irrelevant it can be more to do with the underlying technology , you could have a 1500 mtu standard Ethernet but its running pppoe so the mtu will most likely be set to work with that say 1452 , you can test what your max mtu is by sending a ping across the circuit and constantly increasing mtu size until you hit maximum , you will see on wan links where this starts to become fixed by ISPs , atm circuits/dsl circuits etc can limit mtu in real world , that's why when packets come in fragmented we use the ip virtual reassembly , this has 2 sides one its a security mechanism that prevents attacks and 2 it can set a limit on the amount of packets it reassembles , if the limit is reached packets can be dropped but its best to avoid fragmentation with the tcp mss syntax in the first place

I would read this to understand fully, good doc with explanations on how/why fragmentation and what to do

http://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/25885-pmtud-ipfrag.html

Hello

Extended ping to check Df bit


R1#ping
Protocol [ip]:
Target IP address: 10.1.12.2
Repeat count [5]: 2 <-------------------------------------
Datagram size [100]:
Timeout in seconds [2]:
Extended commands [n]: y
Source address or interface: 10.1.12.1
Type of service [0]:
Set DF bit in IP header? [no]: y
Validate reply data? [no]:
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Sweep range of sizes [n]: y
Sweep min size [36]: 1450 <------------------------------------start of range
Sweep max size [18024]: 1490<----------------------------------end of range
Sweep interval [1]: 5 <----------------------------------------increment of each ping
Type escape sequence to abort.
Sending 18, [1450..1490]-byte ICMP Echos to 10.1.12.2, timeout is 2 seconds: <-----------------------Sweep count =See below
Packet sent with a source address of 10.1.12.1
Packet sent with the DF bit set
!!!!!!!..!!!!!!!..
Success rate is 77 percent (14/18), round-trip min/avg/max = 12/19/28 ms


Sweep max - Sweep min  / sweep interval +1 * repeat count
1490-
1450 = 40

40/5 +1 = 9

9*2 = 18


18 packets will be sent at beginning at 1450 with increments of 5

res
Paul


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This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

Joseph W. Doherty
Hall of Fame
Hall of Fame

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

The max MTU of a serial interface depends on the serial interface technology.

The max MTU of FastEthernet, is 1500 by standard.  Some vendors support larger "jumbo" MTU, but that's non-standard.  Vendor jumbos often are supported up to around 9K.

If so, Orignial Packet can pass through on this ISP link without fragmentation or ISP devices will reply to make fragmentation to original sender.

If IPv4 packet cannot be contained within an interface's max MTU, the interface cannot transmit it.  If interface is L2, frame will dropped.  If interface is L3, if DF set, drop packet and notify source; or if DF not set, fragment packet and transmit.

As you didn't specify what technology the ISP is using, cannot say exactly what might happen, but likely ISP will support MTU up to 1500, and follow L3 rules for entry into their network.

Hi Mr. Joseph,

          Thanks for your comment. What I have a doubt with ISP Link . I will explain in here,

Currently I am using serial link that we called as IPLC which is L3 MPLS VPN link. I am using both data and VOIP traffic on this link. So far ok but Link bandwidth is only 2 Mbps.

So we had hire 10 Mbps link from other ISP. That link technology is totally changed , they called as IEPL L2 VPN link. On this L2 VPN link , I also use data and  VOIP traffic . It is ok but I have encountered the problem with one CUCM can't call to international call. In the meanwhile , some voip pattern is still ok. But only that CUCM can't call to international call. In our Voice gateway have only one Public IP address. In the mean while , I can call international call with other VOIP Pattern. So I don't suspect to Routing issue.

When I roll back to serial link call is back to normal. I have tried several time change to IEPL link and Serial link , it is still same error.

So I had capture the packet on both serial and IEPL link .  In the capture file, I saw  ipv4 fragmentation on IEPL link . And I saw the fact SIP-UDP can't accept UDP fragmentation . I am not sure but I found on this blog.

http://blog.csrpswitch.com/sip-udp-fragmentation-and-kamailio-the-sip-header-diet/

Thus, I suspect this IEPL link is drop my VOIP packet.

Please let me have your opinion what should I check for my problem.

Thanks you

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