Features

Some of the major RIP features supported by the router include:

  • authentication
  • RIP version 1
  • BFD liveness detection
  • RIP version 2
  • equal-cost multipath
  • route summarization
  • multicast addressing
  • route tags
  • next hop
  • split horizon
  • poison reverse
  • subnet masks
  • remote neighbors

 

Route Tags

A route tag is a field in a RIP message that allows boundary routers in an autonomous system (AS) to exchange information about external routes. Route tags provide a method of separating internal RIP routes (routes within the RIP routing domain) from external RIP routes, which may have been imported from an EGP (exterior gateway protocol) or another IGP (interior gateway protocol).

Routers supporting protocols other than RIP should be configurable to allow the route tags to be configured for routes imported from different sources. For example, routes imported from BGP should be able to have their route tags set to the number of the ASs from which the routes were learned.

Authentication

RIPv1 does not support authentication. If you are sending and receiving RIPv2 packets, you can enable RIP authentication on an interface.

The router provides the simple authentication scheme for RIPv2. Because authentication is a per message function and only one 2-octet field is available in the RIP message header, authentication uses the space of an entire RIP message.

The first 20-byte entry in a RIP authentication message contains an address family identifier value of 0xffff and a route tag value of 2. If the 0xffff address family is present in the RIP message, the remaining 16 octets of the entry contain a plain text password. If the password is fewer than 16 octets, it must be left-justified and padded to the right with nulls (0x00).

Authentication is applied per RIP interface. You can specify either text or MD5 authentication. Text authentication uses a simple password that must be shared by the neighbors receiving updates or requests. If they do not have this password, the neighbors reject all updates or requests from the router. MD5 authentication uses a shared key to encrypt the RIP message. The neighbors must have the MD5 key to decrypt the message and encrypt a response.

Note: Do not use text authentication when security is important, because the router sends the unencrypted password in every RIP packet it sends.

Example 1

The following example shows how to use password authentication:

host1(config)#interface fastEthernet 0/0 host1(config-if)#ip rip send version 2 host1(config-if)#ip rip authentication mode text host1(config-if)#ip rip authentication key ke6G72mV

Example 2

The following example shows how to use MD5 authentication:

host1(config)#interface fastEthernet 0/0 host1(config-if)#ip rip send version 2 host1(config-if)#ip rip authentication mode md5 8 host1(config-if)#ip rip authentication key sf43nBScE9

Subnet Masks

The Subnet Mask field of a RIP message contains the subnet mask that is applied to the IP address to set the nonhost portion of the address. If the subnet mask field in a RIP message contains a zero, then no subnet mask was included for the entry.

On an interface where a RIPv1 router may hear and operate on information in a RIPv2 routing entry, the following rules apply:

Next Hop

The Next Hop field in a RIP message contains the next IP address where a packet is sent. A value of zero in this field indicates that the next address the packet should be sent to is the router that originally sent the RIP message.

Multicasting

To reduce unnecessary load on hosts that are not listening to RIPv2 messages, an IP multicast address is used for periodic broadcast messages. The IP multicast address is 224.0.0.9.

Route Summaries

You can summarize routes reported by RIP to reduce the size of the routing table and the amount of traffic resulting from RIP updates. Configuring a RIP summary will cause that prefix to be advertised with the associated metric regardless of the presence of more-specific prefixes. Any more-specific prefixes will not be advertised when they are covered by the summary. You can choose the degree of summarization by using a prefix tree to specify the number of bits to report for routes matching a route map. Alternatively, you can explicitly specify routes for RIP to summarize.

Prefix Tree Example

The following example shows how to configure a 16-bit route summary:

  1. Specify a route map for RIP in Router Configuration mode.
    host1#configure t Enter configuration commands, one per line. End with CNTL/Z.host1(config)#router rip host1(config-router)#route-map 1 host1(config-router)#exit
  2. Define a route map associated with a prefix tree.
    host1(config)#host1(config)#route-map 1 host1(config-route-map)#match-set host1(config-route-map)#match-set summary prefix-tree boston host1(config-route-map)#exit host1(config)#
  3. Set the conditions for summarization in the prefix tree, including which routes are summarized and how many bits of the network addresses are preserved as the network prefix.
    host1(config)#ip prefix-tree boston permit 2.1.0.0/16

This example summarizes routes for networks addressed by 2.1.x.x. The first 16 bits of the network address are preserved in the summary. For example, routes 2.1.3.0, 2.1.2.0, and 2.1.1.0 would all be summarized as 2.1.0.0.

Static Summary Example

You can use the ip summary-address command to specify routes that RIP will summarize.

host1(config-router)#ip summary-address 4.4.0.0 255.255.0.0 5 host1(config-router)#ip summary-address 4.3.0.0 255.255.0.0 6

Split Horizon

Split horizon is a mechanism to aid in preventing routing loops when distance-vector routing protocols such as RIP are employed in broadcast networks. When split horizon is enabled, the router cannot advertise information about routes on an interface from which the information originates. Split horizon is enabled by default on the router.

You can disable split horizon and enable poison reverse routing updates that advertise routes originating on the interface, but for each of these routes the metric is set to infinity to explicitly advertise that these networks are not reachable.

Equal-Cost Multipath

RIP supports equal-cost multipath (ECMP) and installs into the routing table multiple entries for paths to the same destination. Each of these multiple paths to a given destination must have the same cost as the others, but a different next hop.

Applying Route Maps

You can apply a policy to redistributed routes with the route-map command. See JunosE IP Services Configuration Guide, for more information about route maps. You can use the table-map command to apply a route map to RIP routes that are about to be added to the IP routing table.