Cisco Ip Accounting - Cisco ASR 1000 Series Configuration Manual

Configuring IP Services
If the "don't fragment ? bit of the datagram is set, the datagram would be dropped because the 512-byte router
is unable to forward it. All packets larger than 512 bytes are dropped in this case. The second router returns
an ICMP destination unreachable message to the source of the datagram with its Code field indicating
"Fragmentation needed and DF set. ? To support IP Path MTU Discovery, it would also include the MTU
of the next hop network link in the low-order bits of an unused header field.
IP Path MTU Discovery is also useful when a connection is being established and the sender has no information
at all about the intervening links. It is always advisable to use the largest MTU that the links will bear; the
larger the MTU, the fewer packets the host must send.
IP Path MTU Discovery is a process initiated by end hosts. If an end host does not support IP Path MTU
Note
Discovery, the receiving device will have no mechanism to avoid fragmenting datagrams generated by
the end host.
If a router that is configured with a small MTU on an outbound interface receives packets from a host that is
configured with a large MTU (for example, receiving packets from a Token Ring interface and forwarding
them to an outbound Ethernet interface), the router fragments received packets that are larger than the MTU
of the outbound interface. Fragmenting packets slows the performance of the router. To keep routers in your
network from fragmenting received packets, run IP Path MTU Discovery on all hosts and routers in your
network, and always configure the largest possible MTU for each router interface type.

Cisco IP Accounting

Cisco IP accounting support provides basic IP accounting functions. By enabling IP accounting, users can
see the number of bytes and packets switched through the software on a source and destination IP address
basis. Only transit IP traffic is measured and only on an outbound basis; traffic generated by the software or
terminating in the software is not included in the accounting statistics. To maintain accurate accounting totals,
the software maintains two accounting databases: an active and a checkpointed database.
Cisco IP accounting support also provides information identifying IP traffic that fails IP access lists. Identifying
IP source addresses that violate IP access lists alerts you to possible attempts to breach security. The data also
indicates that you should verify IP access list configurations. To make this functionality available to users,
you must enable IP accounting of access list violations using the ip accounting access-violations interface
configuration command. Users can then display the number of bytes and packets from a single source that
attempted to breach security against the access list for the source destination pair. By default, IP accounting
displays the number of packets that have passed access lists and were routed.
Show and Clear Commands for IOS Sockets
The Show and Clear Commands for IOS Sockets feature introduces the show udp, show sockets, and clear
socketscommands. These new commands are useful for monitoring and managing the Cisco IOS Socket
library.
In Cisco IOS software, sockets are a per process entity. This means that the maximum number of sockets is
per process and all sockets are managed on a per process basis. For example, each Cisco IOS process could
have a socket with file descriptor number 1. This is unlike UNIX or other operating systems that have per
system file descriptor allocations.
IP Application Services Configuration Guide, Cisco IOS XE Release 3S (Cisco ASR 1000)
Cisco IP Accounting
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