The 7705 SAR-8 Shelf V2 and the 7705 SAR-18 do not support ingress fragmentation, and this is true for jumbo frames. Therefore, any jumbo frame packet arriving on one of these routers that gets routed to an adapter card that does not support jumbo frame MTU (for example, a 16-port T1/E1 ASAP Adapter card or a 4-port OC3/STM1 / 1-port OC12/STM4 Adapter card) is discarded if the packet size is greater than the TDM port’s maximum supported MTU. If the maximum supported MTU is exceeded, the following occurs.
An appropriate ICMP reply message (Destination Unreachable) is generated by the 7705 SAR. The router ensures that the ICMP-generated message cannot be used as a DOS attack (that is, the router paces the ICMP message).
The port statistics show IP or MPLS Interface MTU discards, for IP or MPLS traffic, respectively. MTU Exceeded Packets and Bytes counters exist separately for IPv4/6 and MPLS under the IP interface hierarchy for all discarded packets where ICMP Error messages are not generated.
For example, if a packet arrives on an 8-port Gigabit Ethernet Adapter card and is to be forwarded to a 16-port T1/E1 ASAP Adapter card with a maximum port MTU of 2090 bytes and a channel group configured for PPP with the port MTU of 1000 bytes, the following may occur.
If the arriving packet is 800 bytes, then forward the packet.
If the arriving packet is 1400 bytes, then forward the packet, which will be fragmented by the egress adapter card.
If the arriving packet is fragmented and the fragments are 800 bytes, then forward the packet.
If the arriving packet is 2500 bytes, then send an ICMP error message (because the egress adapter card has a maximum port MTU of 2090 bytes).
If the arriving packet is fragmented and the fragment size is 2500 bytes, then there is an ICMP error.