Interface Configuration Guide 7705 SAR Interfaces
Edition: 01 3HE 11011 AAAC TQZZA 99
3.2.12.3.1 Behavior of Adapter Cards Not Supporting Jumbo Frames on
7705 SAR-8 and 7705 SAR-18 only
The 7705 SAR-8 (with CSMv2) 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.
3.2.12.3.2 Jumbo Frame Behavior on the Fixed Platforms
The 7705 SAR-A, 7705 SAR-Ax, 7705 SAR-H, 7705 SAR-Hc, 7705 SAR-M,
7705 SAR-W, 7705 SAR-Wx, and 7705 SAR-X are able to fragment packets
between Ethernet ports (which support jumbo frames) and TDM ports (which do not
support jumbo frames). In this case, when a packet arrives from a port that supports
jumbo frames and is routed to a port that does not support jumbo frames (that is, a
TDM port) the packet will get fragmented to the port MTU of the TDM port.
To Next Page
Loading...
