Usage Guidelines

The ip accounting command records the number of bytes (IP header and data) and packets switched through the system on a source and destination IP address basis. Only transit IP traffic is measured and only on an outbound basis; traffic generated by the router access server or terminating in this device is not included in the accounting statistics.

If you specify the access-violations keyword, the ip accounting command 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 might also indicate that you should verify IP access list configurations.

To receive a logging message on the console when an extended access list entry denies a packet access (to log violations), you must include the log keyword in the access-list (IP extended) or access-list (IP standard) command.

Statistics are accurate even if IP fast switching or IP access lists are being used on the interface. If the access-violations keyword is specified and any IP access list is being used on an interface, then only process switching can generate accurate statistics (IP fast switching or CEF cannot).

IP accounting disables autonomous switching, SSE switching, and distributed switching (dCEF) on the interface. IP accounting will cause packets to be switched on the Route Switch Processor (RSP) instead of the Versatile Interface Processor (VIP), which can cause performance degradation.

Usage Guidelines

The wildcard argument is a 32-bit quantity written in dotted-decimal format. Address bits corresponding to wildcard bits set to 1 are ignored in comparisons; address bits corresponding to wildcard bits set to zero are used in comparisons.

Usage Guidelines

This feature is supported on Ethernet, Fast Ethernet, and FDDI interfaces.

To display the MAC accounting information, use the show interface mac EXEC command.

MAC address accounting provides accounting information for IP traffic based on the source and destination MAC address on LAN interfaces. This calculates the total packet and byte counts for a LAN interface that receives or sends IP packets to or from a unique MAC address. It also records a timestamp for the last packet received or sent. With MAC address accounting, you can determine how much traffic is being sent to and/or received from various peers at NAPS/peering points.

Usage Guidelines

To display IP precedence accounting information, use the show interface precedence EXEC command.

The precedence accounting feature provides accounting information for IP traffic, summarized by IP precedence values. This feature calculates the total packet and byte counts for an interface that receives or sends IP packets and sorts the results based on IP precedence. This feature is supported on all interfaces and subinterfaces and supports Cisco Express Forwarding (CEF), dCEF, flow, and optimum switching.

Usage Guidelines

The accounting threshold defines the maximum number of entries (source and destination address pairs) that the software accumulates, preventing IP accounting from possibly consuming all available free memory. This level of memory consumption could occur in a router that is switching traffic for many hosts. Overflows will be recorded; see the monitoring commands for display formats.

The default accounting threshold of 512 entries results in a maximum table size of 12,928 bytes. Active and checkpointed tables can reach this size independently.

Usage Guidelines

Transit entries are those that do not match any of the filters specified by ip accounting-list global configuration commands. If no filters are defined, no transit entries are possible.

To maintain accurate accounting totals, the Cisco IOS software maintains two accounting databases: an active and a checkpointed database.

Usage Guidelines

If more than the maximum udp-limit value arrives in a burst, the Cisco Appliance Services Architecture (CASA) wildcard updates from the service manager might get dropped.

The control-address value is unique for each forwarding agent.

Usage Guidelines

The ip nhrp trigger-svc command sets the threshold by which NHRP sets up and tears down a connection. The threshold is the Cisco Express Forwarding traffic load statistics. The thresholds in the ip nhrp trigger-svc command are measured during a sampling interval of 30 seconds, by default. To change that interval over which that threshold is determined, use the load-interval seconds option of the ip cef traffic-statistics command.

When NHRP is configured on a Cisco Express Forwarding switching node with a Versatile Interface Processor (VIP2) adapter, you must make sure the update-rate keyword is set to 5 seconds.

Other Cisco IOS features could also use the ip cef traffic-statistics command; this NHRP feature relies on it.

Usage Guidelines

An IP directed broadcast is an IP packet whose destination address is a valid broadcast address for some IP subnet, but which originates from a node that is not itself part of that destination subnet.

A router that is not directly connected to its destination subnet forwards an IP directed broadcast in the same way it would forward unicast IP packets destined to a host on that subnet. When a directed broadcast packet reaches a router that is directly connected to its destination subnet, that packet is “exploded” as a broadcast on the destination subnet. The destination address in the IP header of the packet is rewritten to the configured IP broadcast address for the subnet, and the packet is sent as a link-layer broadcast.

The ip directed-broadcast command controls the explosion of directed broadcasts when they reach their target subnets. The command affects only the final transmission of the directed broadcast on its ultimate destination subnet. It does not affect the transit unicast routing of IP directed broadcasts.

If directed broadcast is enabled for an interface, incoming IP packets whose addresses identify them as directed broadcasts intended for the subnet to which that interface is attached will be exploded as broadcasts on that subnet. If an access list has been configured with the ip directed-broadcast command, only directed broadcasts that are permitted by the access list in question will be forwarded; all other directed broadcasts destined for the interface subnet will be dropped.

If the no ip directed-broadcast command has been configured for an interface, directed broadcasts destined for the subnet to which that interface is attached will be dropped, rather than being broadcast.

Note

Because directed broadcasts, and particularly Internet Control Message Protocol (ICMP) directed broadcasts, have been abused by malicious persons, we recommend that security-conscious users disable the ip directed-broadcast command on any interface where directed broadcasts are not needed and that they use access lists to limit the number of exploded packets.

Usage Guidelines

Enabling a helper address or UDP flooding on an interface causes the Cisco IOS software to forward particular broadcast packets. You can use the ip forward-protocol command to specify exactly which types of broadcast packets you would like to have forwarded. A number of commonly forwarded applications are enabled by default. Enabling forwarding for some ports [for example, Routing Information Protocol (RIP)] may be hazardous to your network.

If you use the ip forward-protocol command, specifying only UDP without the port enables forwarding and flooding on the default ports.

One common application that requires helper addresses is Dynamic Host Configuration Protocol (DHCP). DHCP is defined in RFC 1531. DHCP protocol information is carried inside of BOOTP packets. To enable BOOTP broadcast forwarding for a set of clients, configure a helper address on the router interface closest to the client. The helper address should specify the address of the DHCP server. If you have multiple servers, you can configure one helper address for each server. Because BOOTP packets are forwarded by default, DHCP information can now be forwarded by the software. The DHCP server now receives broadcasts from the DHCP clients.

If an IP helper address is defined, UDP forwarding is enabled on default ports. If UDP flooding is configured, UDP flooding is enabled on the default ports.

If a helper address is specified and UDP forwarding is enabled, broadcast packets destined to the following port numbers are forwarded by default:

• Trivial File Transfer Protocol (TFTP) (port 69)

• Domain Naming System (port 53)

• Time service (port 37)

• NetBIOS Name Server (port 137)

• NetBIOS Datagram Server (port 138)

• Boot Protocol (BOOTP) client and server packets (ports 67 and 68)

• TACACS service (port 49)

• IEN-116 Name Service (port 42)

Note	If UDP port 68 is used as the destination port number, it is not forwarded by default.

Usage Guidelines

A packet must meet the following criteria to be considered for flooding:

• The MAC address of the received frame must be all-ones broadcast address (ffff.ffff.ffff).

• The IP destination address must be one of the following: all-ones broadcast (255.255.255.255), subnet broadcast for the receiving interface; major-net broadcast for the receiving interface if the no ip classless command is also configured; or any local IP broadcast address if the ip forward-protocol spanning-tree any-local-broadcast command is configured.

• The IP time-to-live (TTL) value must be at least 2.

• The IP protocol must be User Datagram Protocol (UDP) (17).

• The UDP destination port must be TFTP, Domain Name System (DNS), Time, NetBIOS, ND, or BOOTP packet, or a UDP port specified by the ip forward-protocol udp command.

A flooded UDP datagram is given the destination address specified by the ip broadcast-address command on the output interface. The destination address can be set to any desired address. Thus, the destination address may change as the datagram propagates through the network. The source address is never changed. The TTL value is decremented.

After a decision has been made to send the datagram out on an interface (and the destination address possibly changed), the datagram is handed to the normal IP output routines and is therefore subject to access lists, if they are present on the output interface.

The ip forward-protocol spanning-tree command uses the database created by the bridging Spanning-Tree Protocol. Therefore, the transparent bridging option must be in the routing software, and bridging must be configured on each interface that is to participate in the flooding in order to support this capability.

If an interface does not have bridging configured, it still will be able to receive broadcasts, but it will never forward broadcasts received on that interface. Also, it will never use that interface to send broadcasts received on a different interface.

If no actual bridging is desired, you can configure a type-code bridging filter that will deny all packet types from being bridged. Refer to the Cisco IOS Bridging and IBM Networking Configuration Guide for more information about using access lists to filter bridged traffic. The spanning-tree database is still available to the IP forwarding code to use for the flooding.

The spanning-tree-based flooding mechanism forwards packets whose contents are all ones (255.255.255.255), all zeros (0.0.0.0), and, if subnetting is enabled, all networks (10.108.255.255 as an example in the network number 10.108.0.0). This mechanism also forward packets whose contents are the zeros version of the all-networks broadcast when subnetting is enabled (for example, 10.108.0.0).

This command is an extension of the ip helper-address command, in that the same packets that may be subject to the helper address and forwarded to a single network can now be flooded. Only one copy of the packet will be put on each network segment.

Usage Guidelines

Used in conjunction with the ip forward-protocol spanning-tree command, this command is supported over Advanced Research Projects Agency (ARPA)-encapsulated Ethernets, FDDI, and High-Level Data Link Control (HDLC) encapsulated serials, but is not supported on Token Rings. As long as the Token Rings and the non-HDLC serials are not part of the bridge group being used for UDP flooding, turbo flooding will behave normally.

When you enter the ip forward-protocol turbo-flood command, the outgoing UDP packets have a NULL checksum. If you want to have UDP checksums on all outgoing packets, you must enter the ip forward-protocol turbo-flood udp-checksum command.

Usage Guidelines

Use the ip tcp header-compression command to enable the default VJ format of TCP header compression. Then use the ip header-compression special-vj command to enable the special VJ format of TCP header compression.

To enable the special VJ format of TCP header compression so that context IDs are included in compressed packets, use the special-vj command in IPHC profile configuration mode.

Usage Guidelines

The ip forward-protocol command along with the ip helper-address command allows you to control broadcast packets and protocols that are forwarded.

One common application that requires helper addresses is DHCP, which is defined in RFC 1531. To enable BOOTP or DHCP broadcast forwarding for a set of clients, configure a helper address on the router interface connected to the client. The helper address must specify the address of the BOOTP or DHCP server. If you have multiple servers, configure one helper address for each server.

The following conditions must be met for a UDP or IP packet to be able to use the ip helper-address command:

• The MAC address of the received frame must be all-ones broadcast address (ffff.ffff.ffff).

• The IP destination address must be one of the following: all-ones broadcast (255.255.255.255), subnet broadcast for the receiving interface, or major-net broadcast for the receiving interface if the no ip classless command is also configured.

• The IP time-to-live (TTL) value must be at least 2.

• The IP protocol must be UDP (17).

• The UDP destination port must be for TFTP, Domain Name System (DNS), Time, NetBIOS, ND, BOOTP or DHCP packet, or a UDP port specified by the ip forward-protocol udp command in global configuration mode.

If the DHCP server resides in a VPN or global space that is different from the interface VPN, then the vrf name or the global option allows you to specify the name of the VRF or global space in which the DHCP server resides.

The ip helper-address vrf name address option uses the address associated with the VRF name regardless of the VRF of the incoming interface. If the ip helper-address vrf name address command is configured and later the VRF is deleted from the configuration, then all IP helper addresses associated with that VRF name will be removed from the interface configuration.

If the ip helper-address address command is already configured on an interface with no VRF name configured, and later the interface is configured with the ip helper-address vrf name address command, then the previously configured ip helper-address address command is considered to be global.

Note	The ip helper-address command does not work on an X.25 interface on a destination router because the router cannot determine if the packet was intended as a physical broadcast.

The service dhcp command must be configured on the router to enable IP helper statements to work with DHCP. If the command is not configured, the DHCP packets will not be relayed through the IP helper statements. The service dhcp command is configured by default.

Usage Guidelines

Counting of packets begins when the command is configured and a packet threshold is specified.

The no ip icmp rate-limit unreachable command turns off the previously configured rate limit. To reset the rate limit to its default value, use the ip icmp rate-limit unreachable command default.

Cisco IOS software maintains two timers: one for general destination unreachable messages and one for DF destination unreachable messages. Both share the same time limits and defaults. If the df option is not configured, the ip icmp rate-limit unreachable command sets the time values in ms for DF destination unreachable messages.

Usage Guidelines

An ICMP redirect message can be generated by a router when a packet is received and transmitted on the same interface. In this situation, the router will forward the original packet and send a ICMP redirect message back to the sender of the original packet. This behavior allows the sender to bypass the router and forward future packets directly to the destination (or a router closer to the destination).

There are two types of ICMP redirect messages: redirect for a host address or redirect for an entire subnet.

The ip icmp redirect command determines the type of ICMP redirects sent by the system and is configured on a per system basis. Some hosts do not understand ICMP subnet redirects and need the router to send out ICMP host redirects. Use the ip icmp redirect host command to have the router send out ICMP host redirects. Use the ip icmp redirect subnet command to set the value back to the default, which is to send subnet redirects.

To prevent the router from sending ICMP redirects, use the no ip redirects interface configuration command.

Usage Guidelines

The ability for the Cisco IOS software to respond to ICMP information request messages with an ICMP information reply message is disabled by default. Use this command to allow the software to send ICMP information reply messages.

Usage Guidelines

If an IP packet exceeds the MTU size that is set for the interface, the Cisco software fragments the IP packet.

When an IPsec MTU is less than 256 bytes, the crypto engine MTU is set to 256 bytes and packets greater than 256 bytes are fragmented.

For VASI interfaces that involve Ethernet type interfaces (Ethernet, Fast Ethernet, or Gigabit Ethernet), the IP MTU size of a VASI interface must be set to the same value as the lower default setting of the Ethernet type interface of 1500 bytes. If this adjustment is not made, OSPF reconvergence on the VASI interface requires a long time.

Note

Changing the MTU value (by using the mtu interface configuration command) can affect the IP MTU value. If the current IP MTU value is the same as the MTU value and you change the MTU value, then the IP MTU value is modified automatically to match the new MTU value. However, the reverse is not true; changing the IP MTU value has no effect on the MTU value.

If a dynamic virtual tunnel interface (VTI) configured with an IP MTU causes encapsulating security payload (ESP) fragmentation, clear and re-establish the encryption session.

When a loopback interface is used as the VTI tunnel source, you must manually configure the ip mtu command. This is because the IPsec encapsulation bytes are calculated based on the outgoing physical interface.

MTU Size in an IPsec Configuration

In an IPsec configuration, such as in a crypto environment, an MTU value that is less than 256 bytes is not accepted. If you configure an MTU value less than 256 bytes, the MTU value is automatically overwritten and given a value of 256 bytes.

MTU Size in Cisco ME 3600X Series Ethernet Access Switches

In Cisco ME 3600X Series Ethernet Access Switches, you can configure seven unique MTU sizes on router and switchport interfaces and eight unique sizes on VLAN interfaces. This does not include the default size of 1500.

Usage Guidelines

Previously, if the Hot Standby Router Protocol (HSRP) was configured on an interface, ICMP redirect messages were disabled by default for the interface. With Cisco IOS Release 12.1(3)T, ICMP redirect messages are enabled by default if HSRP is configured.

Usage Guidelines

The ASCONF chunk format requires the receiving SCTP to not report to the sender if it does not understand the ASCONF chunk. This command enables you to configure sending the ASCONF chunk automatically in response to an IP address change in an SCTP stream, or to authenticate the endpoint before sending the ASCONF chunk.

The ASCONF chunk is used to communicate to the endpoint of an SCTP stream that at least one of the configuration change requests in the stream must be acknowledged.

Usage Guidelines

SCTP Authentication procedures use either Message Digest 5 (MD5) or Secure Hash Algorithm 1 (SHA-1), which can be memory and CPU intensive. Enabling SCTP Authentication on data chunks could impact CPU utilization when a large number of authenticated chunks are sent.

You cannot disable the authentication of the ASCONF or ASCONF-ACK chunks.

Enabling the authentication of a chunk type applies only to new endpoints and associations.

The table below provides a list of SCTP chunk types and SCTP chunk numbers.