Tech Tip
BayRS Routers
MTU size when connecting a Nortel Networks router to a Cisco router
Overview of possible problem
Setting MTU size of a Nortel Networks* router’s serial interface to the default value may result in
packet drop at Cisco router.
The cause
There are two caveats to this.
1. If the Cisco* router sends two fragmented frames to the Nortel Networks router and the total of
the two fragmented frames after reassembly at Nortel Networks serial interface is under 1600
bytes (which is the default MTU at Nortel Networks), then the Nortel Networks router will not
fragment this frame but will send it back to Cisco in one frame and Cisco will drop the frame if the
frame size is over 1500 bytes (the Cisco default MTU). This only applies if the packet is destined
to this serial interface whereby a reassemble of the fragmented frames is required.
2. If the Cisco router sends two fragmented frames to Nortel Networks, and the total of the two
frames after reassembly at Nortel Networks is over 1600 bytes which is bigger than Nortel
Networks MTU, then Nortel Networks will fragment the frame before sending it back to Cisco.
However, some of the fragmented frames may also be dropped at Cisco.
The details that follow elaborate on these.
Cause details
1. In the first scenario, the cause for the packet drop at Cisco is strictly because of the MTU
size (1600 bytes) configured at Nortel Networks. This size is big enough for the Nortel
Networks router to transmit this packet without any fragmentation.
2. In the second scenario, the cause of the packet drop is because of the criteria used in
choosing the offset boundary. These criteria are as follows:
a) Each frame must contain units (or multiple) of 8 octets (or 64 bytes) excluding
the IP header.
b) The interface MTU.
Based on these criteria, it is possible that some of the fragmented frames will be bigger than the
Cisco MTU and Cisco will subsequently drop these frames, which will result in the entire packet
being dropped since re-assembly at Cisco will be impossible.
Testing shows that for Nortel Networks to fragment frames that Cisco will accept, the interface
MTU at Nortel Networks needs to be reduced. The range that works is from 1506 to 1587.
TTR040624 2.00 October 2004 ©2005 Nortel Networks. All Rights Reserved Page: 1 of 3
Tech Tip
BayRS Routers
MTU size when connecting a Nortel Networks router to a Cisco router
Examples
1. Nortel Networks MTU = 1600 and Cisco MTU = 1500
Packet size of 1493 sent from a PC hanging off Cisco to Nortel Networks serial interface.
The total packet size will be 1493 + 8 (packet + ICMP header) = 1501 bytes excluding the
IP header. And since this is bigger than Cisco MTU, the packet will be fragmented into
two frames before sending it to Norte Networks. The re-assembled frame at Nortel
Networks will be under its MTU size even when IP and DLCI headers are added. As
such, Nortel Networks will send it back to Cisco in one frame; again 1501 bytes is bigger
than Cisco MTU - this will cause Cisco to drop the packet.
2. Nortel Networks MTU = 1506 and Cisco MTU = 1500 and same packet flow.
Here, even though actual frame count at Nortel Networks is 1501, when adding IP header
and DLCI, the frame will be above this MTU and Nortel Networks will have to fragment
the frame and each fragment frame will be acceptable at Cisco.
3. Nortel Networks MTU = 1587 and Cisco MTU = 1500 and same packet flow.
Now, because of the 8 octet boundary per unit in the fragment frame stipulation and the
MTU size configured, Nortel Networks will fragment the frame as follows:
First fragment boundary will be 187 units multiply by 8 octets or 1496 bytes. The total
frame size for the first fragment will then be 1496 + 20 + 4 (1496 + IP header + DLCI
header) = 1520. Note that Nortel Networks includes the data link header in its MTU
calculation whereas Cisco does not. Since Cisco only cares about the 1496 bytes as far
as MTU/MRU is concerned, then this packet will NOT be dropped. The second fragment
frame will be 1501 - 1496 = 5 bytes. An the total frame for the second fragment frame will
then be 5 + 20 + 4 = 29 bytes
4. Nortel Networks MTU = 1588 and Cisco MTU = 1500 and same packet flow.
At this MTU, Nortel Networks calculates the total packet as 1501 + 20 + 4 = 1525 bytes
and sends it to Cisco without fragmentation. Again, this will get dropped at Cisco.
5. If the actual packet size is changed to a bigger value, and MTU at Nortel Networks is set
at any value from 1588 to 1600, then Nortel Networks will fragment this packet. The
fragmented frame size will increase (in multiples of 8 octets) with an upper boundary
being the MTU size configured. This is when the fragment frame(s) becomes too large for
Cisco MTU.
It is important to note that this problem only exists if the packet’s destination is Nortel Networks
serial interface where re-assembly might be required. Packets traversing across Nortel Networks
serial interface are not affected.
TTR040624 2.00 October 2004 ©2005 Nortel Networks. All Rights Reserved Page: 2 of 3
Tech Tip
BayRS Routers
MTU size when connecting a Nortel Networks router to a Cisco router
Copyright © 2005 Nortel Networks Limited - All Rights Reserved. Nortel, Nortel Networks, the Nortel logo, Globemark, and
BayRS are trademarks of Nortel Networks Limited.
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Author: Iboro Ukpong
TTR040624 2.00 October 2004 ©2005 Nortel Networks. All Rights Reserved Page: 3 of 3
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