Cisco ASR 9000 Series Configuration Manual page 183

Establishing Subscriber Sessions
• Option Handling
BNG supports handling IP options; this is consistent with non-BNG interface types. These are punted
from the NPU to the CPU. These go through the SINT interface and are handled by the appropriate
application.
• Support for traceroute, PMTU discovery, ICMP unreachable
• Fragmentation
BNG does not support fragmentation of packets destined to the PPPoE or IP subscriber interfaces.
Restrictions
These restrictions apply to implementing subscriber interfaces:
• During an ACL logging, packets are punted to CPU, and BNG interfaces are directed to the SINT
interface. The SINT interface drops these log packets because the system does not support ACL Logging
on BNG interfaces.
• IPv6 Ping and traceroute functions should use both the CPE and BNG routers global addresses. IPv6
Ping and traceroute functions using link local address does not work in all cases.
• Logging on subscriber ACLs is not supported.
OL-28375-03
Excessive Punt Flow Trap feature should be disabled when sending a high rate of pings
Note
to, or from subscriber interfaces.
• BNG supports sending ICMP for packets that are received from or destined to a PPPoE or IP
subscriber interface that cannot be forwarded. This functionality is similar to other non-BNG
subscriber interfaces.
• BNG supports PMTU, in which BNG sends ICMPs, when a packet is destined to a subscriber
interface, but the packet exceeds the subscriber MTU and the DF bit is set.
• BNG supports sending ICMPs when packets to (egress ACL) or from (ingress ACL) the subscriber
interface are denied due to the ACL. If the ACL is configured do both deny and log, then the
packets get dropped, but no ICMP is generated.
• BNG supports traceroute functionality that enables sending an ICMP when the time to live (TTL)
of the packet is exceeded.
• BNG supports traceroute functionality for both IPv4 and IPv6 subscribers.
Caution In Cisco IOS XR, fragmentation is handled by linecard (LC) CPU or route processor
(RP) CPU. All packets requiring fragmentation are policed by local packet transport
service (LPTS), to a maximum of 2500 packets per second (pps) for each network
processing unit (NPU).
The fragmentation path is supported only in software, and fragmented packets skip all
features, including subscriber features, QoS, ACL and so on. Therefore, irrespective of
BNG, it should not be used as a general forwarding path.
Cisco ASR 9000 Series Aggregation Services Router Broadband Network Gateway Configuration Guide, Release
Packet Handling on Subscriber Interfaces
4.3.x
171