Avaya BayRS Routers User Manual

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.

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 149 6 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.

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