10.1.5 RIP-1

<Structure of this section>

(1) Advertising routing information in RIP-1

In RIP-1, depending on the value of the subnet mask of the port that sent the RIP message, restrictions apply to the routing entries that can be advertised. There is no problem when all entries have the same network address and use the same subnet mask. Difficulties arise, however, when two or more subnet masks, such as Variable Length Subnet Mask (VLSM) are in use. In a VLSM network, RIP-2 (compliant with RFC 2453) must be used as the routing protocol. If RIP-1 is also used, note that advertising of routing information in RIP-1 is subject to the following conditions:

Table 10-9 RIP-1 route advertisement conditions
Routing information to be advertised	Conditions
Default routing information	Advertised unconditionally. However, advertised route filtering must be enabled to advertise default routing information learned via a non-RIP protocol.
Natural mask routing information	Advertised when the natural mask routing information held by the Switch has a different network address (address class) from the interface.
Subnet routing information^#	Advertised when the subnet routing information held by the Switch has the same network address (address class) and the same subnet length as the interface.
Host routing information	Advertised unconditionally.

#: When the auto-summary configuration command is set, subnet routing information is automatically summarized and advertised as natural mask routing information.

(a) Advertising Natural Mask and Subnet Mask Route Information

The following figure shows the natural mask and subnet mask routing information that is advertised in RIP.

Figure 10-11: Natural mask routes and subnet mask routes advertised by RIP

The advertising conditions in the above figure are described in the following table.

Table 10-10: Advertising conditions for natural mask routes and subnet mask routes
Type of routing information	Routing information in the routing table	Conditions		Advertised
Type of routing information	Routing information in the routing table	Comparison with the network address of interface A	Comparison with the subnet length of interface A	Advertised
Natural mask route	172.16.0.0/16 (No.1)	Match	-	NG
Natural mask route	172.17.0.0/16 (No.4)	Mismatch	-	OK
Subnet route	172.17.1.0/24 (No.5)	Mismatch	Match	NG
	172.16.2.0/24 (No.2)	Match	Match	OK
	172.16.3.0/28 (No.3)	Match	Mismatch	NG

(Legend) OK: advertised NG: not advertised-: not applicable

(b) Notes on Advertising Subnet Routing Information

In the Switch, if you have not set automatic route summarization using the auto-summary configuration command, only subnet routing information will be generated for the IP address of each interface, and the routes will not be automatically summarized as natural mask routing information. Note that when the route extends beyond the address boundary, subnet routing information is not advertised in RIP-1. The following figure shows the configuration example.

Figure 10-12: Example configuration in which directly connected routes are not advertised

Key points in the above configuration

The routing protocol is RIP-1.
Automatic route summarization has not been set by the auto-summary configuration command.
Address boundaries are aligned on the Switch.
The interface's subnet mask is not a natural mask.

Action 1

Set the configuration command to enable automatic route summarization.

Solution 2

Set the configuration command to enable automatic route summarization (summarize subnet routing information and host routing information into a natural mask route).
Set the configuration command to enable advertised route filtering (redistribute summarized routes via RIP).

Countermeasure 3

Enable generation of directly connected routes that have a natural mask for the subnetworked interface (ip auto-class-route configuration command).
Because the above routes are handled as directly connected routes, they are advertised by default (the redistribution filter is disabled).

(2) Difference from RFC

With some exceptions, RIP-1 as implemented in the Switch complies with RFC 1058. The following table describes the differences with the RFC.

Table 10-11: Difference from RFC
RFC			Switch
RFC 1058	Subnet advertisements	A border gateway connected to a subnetted network advertises only a single entry for the entire network to neighboring gateways.	RIP automatic route summarization must be used to generate a network route from subnet routes.
		The metric for the network as a whole is normally the smallest metric among the subnets.	RIP automatic route summarization must be used to generate a network route from subnet routes.
		Border gateways must not advertise host routes within a directly connected network to other networks.	Host routes within directly connected networks are added to the routing table and are advertised.
	Receipt of response messages	Host routes included in an existing network route or subnet route should not be added to the routing tables.	Host routes received in responses are added to the routing tables.