6.2.4 Other Routing Protocols
6.2.4.1 The Router Discovery Protocol
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.
The protocol is split into two portions, the server portion which runs on routers, and the
clientportion which runs on hosts. GateD treats these much like two separate protocols, only one
of which may be enabled at a time.
6.2.4.2 The Router Discovery Server
The Router Discovery Server runs on routers and announces their existence to hosts. It does this
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 router's 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 may 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 Lifetimefield 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that one or more addresses are no longer valid.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, the use of broadcast may 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.
The Router Discovery Server Statement
routerdiscovery server yes | no | on | off [ {
traceoptions trace_options ;
interface interface_list
[ minadvinterval time ] |
[ maxadvinterval time ] |
[ lifetime time ]
;
address interface_list
[ advertise ] | [ ignore ] |
[ broadcast ] | [ multicast ] |
[ ineligible ] | [ preference preference ]
;
} ] ;
- traceoptions trace_options
- Specifies the Router Discovery tracing options. (See Trace Statements and the Router
Discovery specific tracing options below.)
- interface interface_list
- Specifies the parameters that apply to physical interfaces. Note a slight difference in
convention from the rest of GateD; interfacespecifies a list of physical interfaces (such
as le0, ef0and en1), while address specifies a list of IP addresses.
One or more of the following parameters must be provided for the interface:
- maxadvinterval time
- The maximum time allowed between sending broadcast or multicast Router
Advertisements from the interface. Must be no less than 4 and no more than
30:00 (30 minutes or 1800 seconds). The default is 10:00(10 minutes or 600
seconds).
- minadvinterval time
- The minimum time allowed between sending unsolicited broadcast or multicast
Router Advertisements from the interface. Must be no less than 3seconds and no
greater than maxadvinterval. The default is 0.75 * maxadvinterval.
- lifetime time
- The lifetime of addresses in a Router Advertisement. Must be no less than
maxadvinterval and no greater than 2:30:00 (two hours, thirty minutes or 9000
seconds). The default is 3 * maxadvinterval.
- address interface_list
- Specifies the parameters that apply to the specified set of addresses on this physical
interfaces. Note a slight difference in convention from the rest of GateD;
interfacespecifies a list of physical interfaces (such as le0, ef0 and en1), while
addressspecifies a list of IP addresses.
- One or more of the following parameters must be provided for the address:
- advertise
- Specifies that the specified address(es) should be included in Router
Advertisements. This is the default.
- ignore
- Specifies that the specified address(es) should not be included in Router
Advertisements.
- broadcast
- Specifies that the given address(es) should be included in a broadcast Router
Advertisement because this system does not support IP multicasting, or some
hosts on attached network do not support IP multicasting. It is possible to mix
addresses on a physical interface such that some are included in a broadcast
Router Advertisement and some are included in a multicast Router
Advertisement. This is the default if the router does not support IP multicasting.
- multicast
- Specifies that the given address(es) should only be included in a multicast
Router Advertisement. If the system does not support IP multicasting the
address(es) will not be included. If the system supports IP multicasting, the
default is to include the address(es) in a multicast Router Advertisement if the
given interface supports IP multicasting, if not, the address(es) will be included
in a broadcast Router Advertisement.
- preference preference
- The degree of preference of the address(es) as a default router address, relative
to other router addresses on the same subnet. This is a 32-bit, signed,
two's-complement integer, with higher values meaning more preferable. Note
that hex 80000000 may only be specified as ineligible. The default is 0.
- ineligible
- Specifies that the given address(es) will be assigned a preference of hex
80000000 which means that it is not eligible to be the default route for any
hosts.
This is useful when the address(es) should not be used as a default route, but are given as the
next hop in an ICMP redirect. This allows the hosts to verify that the given addresses are up and
available.
6.2.4.3 The Router Discovery Client
A host listens for Router Advertisements via the all-hosts multicast address (224.0.0.2), if IP
multicasting is available and enabled, or on the interface's broadcast address. When starting up,
or when reconfigured, a host may 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 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 routers 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.
The Router Discovery Client Statement
routerdiscovery client yes | no | on | off [ {
traceoptions trace_options ;
preference preference ;
interface interface_list
[ enable ] | [ disable ]
[ multicast ]
[ quiet ] | [ solicit ]
;
} ] ;
- traceoptions trace_options
- Specifies the tracing options for OSPF. (See Trace Statements and the OSPF specific
tracing options below.)
- preference preference ;
- Specifies the preference of all Router Discovery default routes. The default is 55.
- interface interface_list
- Specifies the parameters that apply to physical interfaces. Note a slight difference in
convention from the rest of GateD; interface specifies just physical interfaces (such as
le0, ef0and en1). The Router Discovery Client has no parameters that apply only to
interface addresses.
- enable
- Specifies that Router Discovery should be performed on the specified
interface(s). This is the default.
- disable
- Specifies that Router Discovery should not be performed on the specified
interface(s).
- multicast
- Specifies that Router Solicitations should be multicast on the specified
interface(s). If IP multicast is not available on this host and interface, no
solicitation will be performed. The default is to multicast Router Solicitations if
the host and interface support it, otherwise Router Solicitations are broadcast.
- quiet
- Specifies that no Router Solicitations will be sent on this interface, even though
Router Discovery will be performed.
- solicit
- Specifies that initial Router Solicitations will be sent on this interface. This is
the default.
6.2.4.4 Tracing options
The Router Discovery Client and Server support the state trace flag which traces various
protocol occurrences.
- state
- State transitions
The Router Discovery Client and Server do not directly support any packet tracing options.
Tracing of router discovery packets is enabled via the ICMP Statement.
Last updated November 22, 1997
gated@gated.merit.edu