Unicast IP Routing
Unicast IP Routing |
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RIP version 2 (v2) as defined in RFC 1723 seeks to address some of the problems associated with RIP v1. The decision to refine RIP was controversial in the context of newer, smarter routing protocols such as OSPF. However, RIP has the following advantages over OSPF:
To help today's IP internetworks minimize broadcast traffic, use variable length subnetting to conserve IP addresses, and secure their routing environment from misconfigured or malicious routers, several key features were added to RIP v2.
Rather than broadcasting RIP announcements, RIP v2 supports sending RIP announcements to the IP multicast address of 224.0.0.9. Non-RIP nodes are not disturbed by RIP router announcement traffic.
The disadvantage of this new feature is that Silent RIP nodes must also be listening for multicast traffic sent to 224.0.0.9. If you are using Silent RIP, verify that your Silent RIP nodes can listen for multicasted RIP v2 announcements before deploying multicasted RIP v2.
The use of multicasted announcements is optional. The broadcasting of RIP v2 announcements is also supported.
RIP v2 announcements send the subnet mask (also known as a network mask) along with the network ID. RIP v2 can be used in subnetted, supernetted, and variable-length subnet mask environments. Subnets of a network ID do not have to be contiguous (they can be disjointed subnets).
RIP v2 supports the use of authentication mechanisms to verify the origin of incoming RIP announcements. Simple password authentication was defined in RFC 1723, but newer authentication mechanisms such as Message Digest 5 (MD5) are available.
Note
Windows 2000 supports only simple password authentication.
RIP v1 was designed with forward compatibility in mind. If a RIP v1 router receives a message and the RIP version in the RIP header is not 0x01, it does not discard the RIP announcement but processes only the RIP v1 defined fields.
Also, RIP v2 routers send a RIP v1 response to a RIP v1 request except when configured to send only RIP v2 announcements.
To ensure that RIP v1 routers can process RIP v2 announcements, RIP v2 does not modify the structure of the RIP message format. RIP v2 makes use of fields that were defined in RIP v1 as Must be Zero.
The use of the Command, Family Identifier, IP Address, and Metric fields is the same as previously defined for RIP v1. The Version field is set to 0x02 to indicate a RIP v2 message. Figure 3.9 illustrates the RIP v2 message format.
Figure 3.9 RIP Version 2 Message Format
The Route Tag field is used as a method of marking specific routes for administrative purposes. Its original use as defined by RFC 1723 was to distinguish routes that were RIP-based routes (internal to the RIP environment) from non-RIP routes (external to the RIP environment). The Route Tag is configurable on routers that can support multiple routing protocols.
Note
Windows 2000 supports the configuration of the Route Tag for RIP v2 interfaces.
The 4-byte Subnet Mask field contains the subnet mask (also known as a network mask) of the network ID in the IP Address field.
The 4-byte Next Hop field contains the forwarding IP address (also known as the gateway address) for the network ID in the IP Address field. If the next hop is set to 0.0.0.0, the forwarding IP address (the next hop) for the route is assumed to be the source IP address of the route announcement.
The Next Hop field is used to prevent non-optimal routing situations. For example, if a router announces a host route for a host that resides on the same network as the router interface advertising the route and the Next Hop field is not used, the forwarding IP address for the host route is the IP address of the router's interface, not the IP address of the host. Other routers that receive the announcement on that network forward packets destined for the host's IP address to the announcing router's IP address rather than to the host. This creates a non-optimal routing situation.
Using the Next Hop field, the router announces the host route with the host's IP address in the Next Hop field. Other routers receiving the announcement on that network forward packets destined for the host's IP address to the host's IP address rather than forwarding them to the announcing router.
Because the Next Hop field becomes the Gateway Address field in the IP routing table, the IP address in the Next Hop field should be directly reachable using a router interface.
The authentication process for RIP v2 announcements uses the first route entry in the RIP message to store authentication information. The first route entry must be used, leaving a maximum of 24 routes in a RIP v2 authenticated announcement. To indicate authentication, the Family Identifier field is set to 0xFF-FF. The Authentication Type field, normally used as the Route Tag field for a route, indicates the type of authentication being used. Simple password authentication uses the Authentication Type value of 0x00-01.
The 16 bytes after the Authentication Type are used to store the authentication value. For simple password authentication, the 16-byte Authentication Value field stores the left-justified, null-padded, case-sensitive, clear-text password. Figure 3.10 illustrates the RIP v2 authentication message.
Figure 3.10 RIP v2 Message Format Using Authentication
RIP v1 routers disregard the first route in a RIP v2 authenticated announcement because the Family Identifier for the route is unknown.
Note
Simple password authentication for RIP v2 prevents unauthorized or misconfigured RIP routers from being placed on the network. The simple password is not secure, however, because it is sent on the network in clear text. Anyone with a protocol analyzer such as Microsoft Network Monitor can capture the RIP v2 packets and view the authentication password.
RIP v2 routers and RIP v1 routers should be used together with caution. Because RIP v1 routers do not interpret the Subnet Mask field in the route, RIP v2 routers must not announce routes which can be misinterpreted by a RIP v1 router. Variable length subnet masks (VLSM) and disjointed subnets cannot be used in mixed environments.
For an interface using RIP v2 to make announcements such that RIP v1 routers can process the announced routes, the RIP v2 routers must summarize subnet routes when announcing outside a subnetted environment. A specific subnet route announced to a RIP v1 router can be misinterpreted as a host route. Also, the RIP v2 routers cannot announce supernet routes. A RIP v1 router would misinterpret the route as a single network, rather than as a range of networks.
If RIP v2 routers are on the same network as RIP v1 routers, the RIP v2 router interface must be configured to broadcast its announcements. Multicasted RIP v2 announcements are not processed by the RIP v1 routers.
