Cisco Cisco Packet Data Gateway (PDG) Troubleshooting Guide
APN Configuration Mode Commands
access-link ▀
Cisco ASR 5000 Series Command Line Interface Reference ▄
OL-22947-02
access-link
Configures IP fragmentation processing over the Access-link (PPP, GTP etc).
Product
GGSN, P-GW
Privilege
Security Administrator, Administrator
Syntax
Default: Disabled
Ignore the DF bit setting. Fragment and forward the packet over the access link.
Ignore the DF bit setting. Fragment and forward the packet over the access link.
Default: Disabled
Partially ignore the DF bit. Fragment and forward the packet, but also return an ICMP error message to the
source of the packet. The number of ICMP errors sent like this is rate-limited to 1 ICMP error packet per
second per session.
Partially ignore the DF bit. Fragment and forward the packet, but also return an ICMP error message to the
source of the packet. The number of ICMP errors sent like this is rate-limited to 1 ICMP error packet per
second per session.
Default: Enabled
Normal processing. Drop the packet and send an ICMP unreachable message to the source of packet. This is
the default behavior.
Normal processing. Drop the packet and send an ICMP unreachable message to the source of packet. This is
the default behavior.
Usage
If the IP packet to be forwarded is larger than the access-link MTU and if the DF (Don't Fragment) bit is set
for the packet, then the fragmentation behavior configured by this command is applied. Use this command to
fragment packets even if they are larger than the access-link MTU.
Note that regardless of whether or not fragmentation is performed because of one of the above reasons,
fragmentation may also occur for other reasons.
Payloads are encapsulated within IP/UDP/GTP before being sent to the SGSN. If that encapsulation causes
the packet to exceed 1500 bytes, the inner IP payload is fragmented (even if it's not considered too-large by
the above tests) into two payloads (if the DF bit is not set). If the DF bit is set (and access-link ip-
fragmentation normal is configured), the system performs IP fragmentation of the entire packet (i.e., IP
fragmentation in the outer IP header) rather than fragmenting the inner IP payload. Either way, the result is
two packets, but in one case the MS would have to perform IP reassembly while in the other case the SGSN
would have to perform reassembly.
for the packet, then the fragmentation behavior configured by this command is applied. Use this command to
fragment packets even if they are larger than the access-link MTU.
Note that regardless of whether or not fragmentation is performed because of one of the above reasons,
fragmentation may also occur for other reasons.
Payloads are encapsulated within IP/UDP/GTP before being sent to the SGSN. If that encapsulation causes
the packet to exceed 1500 bytes, the inner IP payload is fragmented (even if it's not considered too-large by
the above tests) into two payloads (if the DF bit is not set). If the DF bit is set (and access-link ip-
fragmentation normal is configured), the system performs IP fragmentation of the entire packet (i.e., IP
fragmentation in the outer IP header) rather than fragmenting the inner IP payload. Either way, the result is
two packets, but in one case the MS would have to perform IP reassembly while in the other case the SGSN
would have to perform reassembly.
Example
Set fragmentation so that the DF bit is ignored and the packet is forwarded anyway by entering the following command:
Set fragmentation so that the DF bit is ignored and the packet is forwarded anyway by entering the following command: