Router Discovery
Router Discovery Overview
The Router Discovery Protocol is an IETF standard protocol (RFC 1256) used to inform hosts of the existence of routers. It is intended to be used instead of having hosts wiretap routing protocols, such as RIP. It is used in place of, or in addition to, statically-configured default routes in hosts. RFC 1256 can be found at: http://ietf.org/rfc/rfc1256.txt. The protocol is split into two portions: the server portion, which runs on routers, and the client portion, which runs on hosts. Advanced Routing Suite currently supports only the server configuration.
The router discovery server runs on routers and announces their existence to hosts by periodically multicasting or broadcasting a router advertisement to each interface on which it is enabled. These router advertisements contain a list of all the routers' addresses on a given interface and the preference of each address for use as the default router on that interface.
Initially, these router advertisements occur every few seconds, then fall back to every few minutes. In addition, a host can send a router solicitation to which the router will respond with a unicast router advertisement (unless a multicast or broadcast advertisement is due momentarily).
Each router advertisement contains an advertisement "lifetime", indicating for how long the advertised addresses are valid. This lifetime is configured such that another router advertisement will be sent before the lifetime has expired. A lifetime of zero is used to indicate that one or more addresses are no longer valid.
On systems supporting IP multicasting, the router advertisements are, by default, sent to the all-hosts multicast address, 224.0.0.1. However, an advertisement method of broadcast can be specified. When router advertisements are being sent to the all-hosts multicast address, or an interface is configured for the limited-broadcast address, 255.255.255.255, all IP addresses configured on the physical interface are included in the router advertisement. When the router advertisements are being sent to a net or subnet broadcast, only the address associated with that net or subnet is included.
A host listens for router advertisements via the all-hosts multicast address (224.0.0.1) if IP multicasting is available and enabled, or on the interface's broadcast address. When starting up, or when reconfigured, a host can send a few router solicitations to the all-routers multicast address, 224.0.0.2, or the interface's broadcast address.
When a router advertisement with non-zero lifetime is received, the host installs a default route to each of the advertised addresses. If the preference is ineligible, or the address is not on an attached interface, the route is marked unusable but is retained. If the preference is usable, the metric is set as a function of the preference such that the route with the best preference is used. If more than one address with the same preference is received, the one with the lowest IP address will be used. These default routes are not exportable to other protocols.
When a router advertisement with a zero lifetime is received, the host deletes all routes with next-hop addresses learned from that router. In addition, any routes learned from ICMP redirects pointing to these addresses will be deleted. The same will happen when a router advertisement is not received to refresh these routes before the lifetime expires.
ip router-discovery address-policy
Name
ip router-discovery address-policy - specifies policy-related parameters when configuring logical interface addresses in router discovery.
Syntax
ip router-discovery address-policy ipv4_address
[ (method (broadcast | ignore | multicast)) |
(preference pref) | ineligible]
no ip router-discovery address-policy ipv4_address
Mode
Global Configuration
Parameters
ipv4_address - a valid IPv4 address in dotted-quad notation
method (multicast | broadcast | ignore) - optionally specify whether the associated IPv4 address should be included in router advertisements and whether the router advertisements should be unicast or multicast. If ignore is specified, then the associated IPv4 address will not be included in router advertisements. Note: If an interface does not support multicast, but multicast is specified for that interface, then that interface will not be advertised.
preference pref - configures the preferability of the address as a default router address. Otherwise, you can specify a value for pref between 1 and 255. The default preference value is 55.
ineligible - optionally specify that the router should be advertised with the worst possible preference, making it ineligible to become the default router
Description
Use the ip router-discovery address-policy command to specify the logical interface or interfaces that should be advertised in router advertisements. This command can be given multiple times, creating multiple interfaces to be advertised in router advertisements.
Default
The ip router-discovery address-policy command is not explicitly configured by default.
Command History
NGC 2.2 - This command was introduced.
Examples
Example 1
The following example configures the logical interface 223.1.5.8 with an advertisement method of broadcast and a preference of 10.
(config)# ip router-discovery address-policy 223.1.5.8
method broadcast preference 10
(config)#
Example 2
The following example configures the logical interface 224.5.5.5 with an advertisement method of multicast. This interface is then configured such that it is ineligible to become the default router.
(config)# ip router-discovery address-policy 224.5.5.5
method multicast ineligible
(config)#
ip router-discovery enable
Name
ip router-discovery enable - enables the Router Discovery server
Syntax
ip router-discovery enable
no ip router-discovery enable
Mode
Global Configuration
Parameters
none
Description
Use the ip router-discovery enable command to enable Router Discovery on the server. The negative form of this command disables Router Discover on the server.
Default
If ip router-discovery enable is not specified, it is the same as if the user had specified the following:
(config)# ip router-discovery enable
Command History
NGC 2.2 - This command was introduced.
Examples
The following example turns Router Discovery off on the server
(config)# no ip router-discovery enable
(config)#
ip router-discovery trace file
Name
ip router-discovery trace file - specifies the file to receive tracing information, the size of the file, whether to overwrite existing files, and the maximum number of files allowed
Syntax
ip router-discovery trace file file_name [no-timestamp || overwrite]?
no ip router-discovery trace file file_name [ no-timestamp ||overwrite]?
Mode
Global Configuration
Parameters
file_name - specifies the name of the file to receive the tracing information. Note that the file name is not specified in quotes.
no-timestamp - specifies that a timestamp should not be prepended to all trace lines
overwrite - specifies to begin tracing by appending or truncating an existing file
Description
The trace file command is associated with each protocol, so that information pertaining to a single protocol can be written to its own file. For Router Discovery, the ip router-discovery trace file command specifies a file for tracing of all Router Discovery events. The negative form of this command disables this tracing. The specific events that are traced are controlled by the ip router-discovery trace flag command.
The no-timestamp option disables the pre-pending of a timestamp to all lines written to the trace file. The default is to prepend a timestamp to all lines written to a trace file.
The overwrite option specifies whether to start tracing by truncating or appending to an existing file.
These options are not cumulative across multiple commands.
Default
Router Discovery tracing is turned off by default.
Command History
NGC 2.2 - This command was introduced.
Examples
In the following example, Router Discovery tracing is written to the file "rdisc.log". No timestamp will display at the beginning of the trace lines.
(config)# ip router-discovery trace file rdisc.log no-timestamp
ip router-discovery trace flag
Name
ip router-discovery trace flag - specifies tracing options that are common across all protocols
Syntax
ip router-discovery trace flag ( [ route | normal |
state | policy | task | timer | all ] )
no ip router-discovery trace flag ( [ route | normal |
state | policy | task | timer | all ] )
Mode
Global Configuration
Parameters
Flags common to all protocols:
[ route | normal | state | policy | task | timer | all ] - These tracing flags are common to all protocols. They cannot be associated with a send, receive, or send-receive action item. Similarly, you cannot specify to show detailed information when tracing these flags. These flags are defined as follows:
route - trace routing table changes for routes installed by this protocol or peernormal - trace normal protocol occurrences. Note: Abnormal protocol occurrences are always traced.state - trace state machine transition in the protocolpolicy - trace the application of protocol and user-specified policy to routes being imported or exportedtask - trace system interface and processing associated with this protocoltimer - trace timer usage by this protocolall - turns on all trace flags
Description
Use the ip router-discovery trace flag command to specify tracing flags for Router Discovery tracing. Each flag must reside on its own configuration line. For example, you cannot specify to trace both task and policy packets in the same command.
Default
The default is for no flags to be explicitly configured.
Command History
NGC 2.2 - This command was introduced.
Examples
In the following example, flags specify to trace state machine transitions. This tracing information will be written to the file rdisc.log.
(config)# ip router-discovery trace file rdisc.log
(config)# ip router-discovery trace flag state
router-discovery lifetime
Name
router-discovery lifetime - specifies the length of time that the addresses in a given router advertisement are valid
Syntax
router-discovery lifetime time-seconds
no router-discovery lifetime time-seconds?
Mode
Interface Configuration
Parameters
time-seconds - specifies the length of time, in seconds, with a value between the Maximum Advertisement Interval value (which has a minimum value of 4) and 9,000, inclusive
Description
The router-discovery lifetime command specifies how long the addresses in a given router advertisement are valid. Lifetime must be no less than "router-discovery maximum-interval" and no more than 2 hours and 30 minutes. If this value is set to less than the Maximum Advertisement Interval, then Advanced Routing Suite will reconfigure router-discovery lifetime to be equal to the Maximum Advertisement Interval.
The negative form of this command, no router-discovery lifetime, removes the configured time-seconds value and returns this to its default value. : Specifying a value for time-seconds in the no form has no effect on the configuration. Thus, it is displayed above as optional.
Default
If router-discovery lifetime is not specified, it will default to 3 times the value of the Maximum Advertisement Interval. Therefore, if neither value is configured, then the router-discovery lifetime will default to:
(config-if)# router-discovery lifetime 1800
Command History
NGC 2.2 - This command was introduced.
Examples
The following example runs the router discovery server on interface fxp0. All routers that it advertises out of interface fxp0 will be advertised with a lifetime of 10 minutes.
(config)# interface fxp0
(config-if)# router-discovery lifetime 1800
(config-if)# exit
(config)#
See Also
"router-discovery maximum-interval"
"router-discovery minimum-interval"
router-discovery maximum-interval
Name
router-discovery maximum-interval - specifies the maximum time allowed between sending unsolicited router advertisements from the interface
Syntax
router-discovery maximum-interval time-seconds
no router-discovery maximum-interval time-seconds?
Mode
Interface Configuration
Parameters
time-seconds - specifies the length of time, in seconds, with a value between 4 and 1800, inclusive
Description
The router-discovery maximum-interval command specifies the maximum time allowed between sending unsolicited router advertisements from the interface. This must be no less than 4 seconds and no more than 30 minutes (1800 seconds).
The negative form of this command, no router-discovery maximum-interval, removes the configured time-seconds value and returns this to its default value of 600 seconds. : Specifying a value for time-seconds in the no form has no effect on the configuration. Thus, it is displayed above as optional.
Default
If router-discovery maximum-interval is not specified, it is the same as if the user had specified the following:
(config-if)# router-discovery maximum-interval 600
Command History
NGC 2.2 - This command was introduced.
Examples
The following example runs the router discovery server on interface fxp0, sending advertisements no more often than once every 6 minutes.
(config)# interface fxp0
(config-if)# router-discovery maximum-interval 360
(config-if)# exit
(config)#
See Also
"router-discovery lifetime"
"router-discovery minimum-interval"
router-discovery minimum-interval
Name
router-discovery minimum-interval - specifies the minimum time allowed between sending unsolicited router advertisements from the interface
Syntax
router-discovery minimum-interval time-seconds
no router-discovery minimum-interval time-seconds?
Mode
Interface Configuration
Parameters
time-seconds - specifies the length of time, in seconds, with a value between 3 and "router-discovery maximum-interval" (which has a maximum value of 1800), inclusive
Description
The router-discovery minimum-interval command specifies the minimum time allowed between sending unsolicited router advertisements from the interface. This must be no less than 3 seconds and no more than the "router-discovery maximum-interval" value. If this value is set to greater than the Maximum Advertisement Interval, Advanced Routing Suite will reconfigure the value of the Minimum Advertisement Interval to be equal to the value of the Maximum Advertisement Interval log the change.
The negative form of this command, no router-discovery minimum-interval, removes the configured time-seconds value and returns this to its default value. Note: Specifying a value for time-seconds in the no form has no effect on the configuration. Thus, it is displayed above as optional.
Default
If router-discovery minimum-interval is not specified, it will default to 3/4 the value of the Maximum Advertisement Interval. Therefore, if neither value is configured, then the router-discovery minimum-interval will default to:
(config-if)# router-discovery minimum-interval 450
Command History
NGC 2.2 - This command was introduced.
Examples
The following example runs the router discovery server on interface fxp0, sending advertisements no less often than once every minute.
(config)# interface fxp0
(config-if)# router-discovery minimum-interval 60
(config-if)# exit
(config)#
See Also
"router-discovery lifetime"
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