10.1.3 Advertisement of routing information

<Structure of this section>

(1) Advertised routes

(a) Learning protocol

If filtering of advertised routes has not been specified, learned RIP routes and directly connected routes within the RIP network are advertised. If filtering is specified, the advertising behavior is governed by the filter conditions. The following table describes the learning protocol for route advertising via RIP.

Table 10-3: Destination learning protocols
Learning protocol		Advertising behavior without route filtering	Order in which advertisement metrics are applied #5
Directly connected route #1	Route within the RIP network	Advertised	Setting for advertised route filtering Default (metric: 1)
Directly connected route #1	Route outside the RIP network	Not advertised
Summarized route		Not advertised
Static route		Not advertised	Setting for advertised route filtering Setting by default-metric Default (metric: 1)
RIP^#2		Advertised	Setting for advertised route filtering Metric in the routing table
OSPF		Not advertised	Setting for advertised route filtering If inherit-metric is set, the value in the routing table #3 Default-metric specification #4
BGP		Not advertised
Route imported from another VRF or the global network		Not advertised

Note #1: The secondary address is also advertised.
Note #2: Split horizon is applied.
Note #3: The route is not advertised if the metric in the routing table is 16 or higher.
Note #4: The route is not advertised if advertised route filtering is unspecified or if no metric is specified by the inherit-metric or default-metric command.
Note #5: If set, the metric-offset out setting is added to the selected metric. If this results in a metric of 16 or higher, the route is not advertised.

(b) Address type

The following table describes the types of addresses that can be advertised via RIP.

Table 10-4: Address types advertised
Address types	Definition	Example	Advertised
Address types	Definition	Example	RIP-1	RIP-2
Default routing information	Routing information about all destination networks	0.0.0.0/0	OK	OK
Natural mask routing information	Information about the network mask for the class of IP address (Class A: 8 bits) (Class B: 16 bits) (Class C: 24 bits)	172.16.0.0/16 Class B Netmask: 16 bits (255.255.0.0)	OK	OK
Subnet routing information	Routing information about a specific subnet destination	172.16.10.0/24 Class B Netmask: 24 bits (255.255.255.0)	#^{#1 #2}	OK^#2
Supernet routing information	Routing information that encompasses multiple networks	172.0.0.0/8 Class B Netmask: 8 bits (255.0.0.0)	NG	OK
Host routing information	Routing information about a specific destination host	172.16.10.1/32 Network mask: 32 bits (255.255.255.255)	OK	OK

Legend: OK: Can be advertised, NG: Cannot be advertised, #: Some information can be advertised

#1: In RIP-1 there are limitations on the subnet routes that can be advertised. For more information, see "10.1.5 RIP-1 (1) Advertising routing information in RIP-1".

#2: When the auto-summary configuration command is set, subnet routing information is automatically summarized and advertised as natural mask routing information [E] ?. For more information, see (4) Automatically summarizing RIP advertised routes.

(2) Route information advertisement destination

Route advertisements are sent to all neighboring routers on the network specified by the network configuration command. Alternatively, you can restrict the destination to a specific neighboring router by setting the neighbor configuration command. The following table describes the destination of route advertisements via RIP.

Table 10-5: Route information advertisement destinations
Destination device	Destination address
Networks that RIP operates on #1#2	Multicast address (RIP-2) or subnet broadcast address (RIP-1)
Specific neighboring router #3	Unicast addresses

#1: The distribution of advertisements to interfaces specified as passive-interface is suppressed.

#2: Also distributed to secondary addresses.

#3: The neighboring router must be included in the RIP network.

(3) Timing of advertisement of routing information

The following table describes the functionality related to the timing of route advertisements distributed via RIP.

Table 10-6: Route advertisement timing
Functionality	Description
Periodic route advertisement	Neighboring routers are reported periodically about routing information held by the switch.
Triggered update	Any change in the routing information held by the switch is reported immediately without waiting for a periodic advertisement.
Response to a request from a neighboring router	The neighboring router that sent the request packet is notified.
Route poisoning	Neighboring routers are notified for a set duration of any deleted routing information.

(a) Periodic advertisement of routing information

RIP periodically advertises routing information held by the local router to neighboring routers. The following figure shows an example of a periodic route advertisement.

Figure 10-1: Advertising periodic routing information

(b) triggered update

Changes in the routing information held by the switch are distributed immediately without waiting for the periodic distribution cycle. The following figure shows a route advertisement distributed as a triggered update.

Figure 10-2: Advertising routing information using triggered update

(c) Response to a request packet

On receipt of a request packet, the Switch sends the requested information to the neighboring router that sent the packet. The following figure shows how routing information is advertised when a request packet is received.

Figure 10-3: Advertising route information when a request packet is received

(d) Root poisoning

When a route changes from reachable to unreachable status (on receipt of a metric-16 route advertisement or on deletion of a route learned from an interface that has since failed), the router advertises the metric-16 (unreachable) route to its neighboring routers for a set period (60 seconds: garbage collection timer). The following figure shows route poisoning.

Figure 10-4: Route poisoning

If a new route to the affected destination is learned while route poisoning is in effect, the Switch advertises the new routing information. This is illustrated in the following figure.

Figure 10-5: Relearning during route poisoning

(4) Auto-summarization of RIP advertised routes

By setting the auto-summary configuration command, you can advertise multiple subnet routing entries to neighboring routers, automatically summarizing the entries as a single natural mask route. This configuration command is supported in both RIP-1 and RIP-2.

The following table describes the types of addresses for which automatic route summarization is supported.

Table 10-7: Address types subject to automatic summarization of advertised routes
Address types	Summarization
Address types	RIP-1	RIP-2
Default routing information	NG	NG
Natural mask routing information	NG	NG
Subnet routing information	OK^#1	OK^#2
Supernet routing information	NG	NG
Host routing information	NG	NG

Legend: OK: Can be summarized, NG: Cannot be summarized

#1: In RIP-1, when the advertised routing information and the interface of the advertisement destination are in the same natural network and have the same mask length, the information is advertised to neighboring routers as subnet routing information without automatic summarization. For more information, see "Auto-summarization of advertised routes when using graphical 10-6 RIP-1".

#2: In RIP-2, when the advertised routing information and the interface of the advertisement destination are in the same natural network, the information is advertised to neighboring routers as subnet routing information without automatic summarization. For more information, see "Auto-summarization of advertised routes when using graphical 10-7 RIP-2".

The following figure shows automatic summarization of subnet routes when RIP-1 is used.

Figure 10-6: Auto-summarization of advertised routes when using RIP-1

The following figure shows automatic summarization of subnet routes when RIP-2 is used.

Figure 10-7: Auto-summarization of advertised routes when using RIP-2

(a) Advertising metrics during automatic aggregation

The summarized information is advertised with the smallest metric in the original subnet routing information.

(b) Advertised route tagging during auto-aggregation (only when RIP-2 is used)

The route tag in a summarized advertisement is 0.

(c) Advertisement next-hop during auto-aggregation (only when RIP-2 is used)

The next-hop field in a summarized advertisement is 0.