Ftl Png

RIPng (ng stands for "next generation") is based on RIP version 2 (RIP-2). None of the operational procedures, timers, or stability functions have been changed. RIPng is RIP-2, modified to support the larger IP addresses and multiple addresses on each interface of IPv6. The UDP port number for RIPng is 521. RIPng does not support both IPv4 and IPv6 and is therefore not backward-compatible with RIP-2.

NOTE Chapter 7, "Routing Information Protocol Version 2," of Routing TCP/IP, Volume /, discusses RIP version 2.

Figure 8-15 shows the RIPng message format. The basic structure is very similar to RIP-2.

Figure 8-15 RIPng Message Format

Route Table Entry 1 (20)

Route Table Entry N (20)

The RIPng message fields are defined as follows (with lengths shown in bytes):

• Command is set to either 1, signifying a request, or 2, signifying a response.

• Version is currently 1.

The rest of the message contains the list of route table entries (RTEs). Figure 8-16 shows the format of the RTEs.

Figure 8-16 RIPng Route Table Entry Format

IPv6 Prefix (16)

The fields in the RTE format are defined as follows:

• IPv6 Prefix is the 128-bit IPv6 address prefix.

• Route Tag is identical to RIP-2, which provides a field for tagging external routes or routes that have been redistributed into the RIPng process.

• Prefix Length specifies the significant part of the address prefix.

• Metric is the same as in RIP-2, a hop count value between 1 and 15, inclusive.

The number of routes that a RIPng update can contain depends on the link MTU, the number of octets of header information preceding the RIPng message, the size of the RlPnr header, and the size of a route table entry (RTE). The formula for determining the numbci of RTEs in a single update is as follows:

#RTE = Integer

MTU - sizeoff(ipheaders) - UDP_header_len - RIPng_header_len

RTE size

The number of RTEs directly relates to the link MTU and the length of the IP headers, UDP header, and RIPng header.

Each RIP-2 RTE contains a Next-Hop field associated with it, specifying a better next-hop address than the address of the advertising router. IPv6 addresses are so large that this would almost double the size of the RTE. RIPng specifies a single next-hop RTE that applies to all the following RTEs until the end of the message or until the existence of another next-hop RTE. The next-hop RTE in Figure 8-17 shows that the Route-Tag field and prefix field must contain all zeros. The metric value will be OxFF. A value of 0:0:0:0:0:0:0:0 in the Address field indicates the next-hop is the originator of the RIPng advertisement.

Figure 8-17 Next-Hop RTE

IPv6 Next-Hop Address (16)

The next-hop address must be the link-local address of the next-hop router. If the address is not a link-local address, the receiver of the advertisement treats the packet as if the address prefix value is 0:0:0:0:0:0:0:0.

Periodic and triggered RIPng responses must remain local to a link—they must not traverse a router. Both periodic updates and triggered updates must have the router's link-local address as the source of the advertisement and the IPv6 hop limit equal to 255. The hop limit of 255 ensures that the advertisement has not traversed a router, because a router decrements the hop limit of every packet. The destination multicast address is the all-rip-routers multicast address FF02::9.

The Cisco router is capable of running multiple RIPng processes. The routing process is enabled as an interface subcommand:

ipv6 rip tag enable

The command must be enabled on any interface addressed with a prefix that needs to be advertised in the RIPng update. Multiple processes are distinguished by the tag. Currently, up to four processes are supported. Each process must use a unique UDP port number. A single process can use the default value, 521. The port number must be modified for subsequent processes; otherwise, the new process will not start up. The global command to modify the UDP port number and the multicast address used by RIPng is as follows:

ipv6 rip tag port udp-port multicast-group multicast-address

More than one process can use the same multicast address. If this command is not given, the default port number, 521, and the default multicast address, FF02::9, are used.

Unlike RIP-2, for which the global command router rip is required to enable the routing protocol, no global commands are required to enable RIPng.

Optional global commands control the entire RIPng process, affecting all configured interfaces. Global commands are available to disable or enable split-horizon and poison reverse, modify UDP port numbers and RIPng multicast addresses, change default timers, change the administrative distance, and redistribute static routes. Most of these functions are also available with RIP-2. *

Table 8-10 lists the available global commands.

Table 8-10 RIPng Global Commands

Table 8-10 lists the available global commands.

Table 8-10 RIPng Global Commands

Command

Description

[no] ipv6 rip tag port udp-port multicast-

Configures the RIP routing process to use the

group multicast-address

specified UDP port and multicast address.

[no] ipv6 rip tag table table-number

Assigns the specified routing table to the RIP

process. Default is table 0. Note that only table

0 will be used for IPv6 unicast packet

forwarding.

[no] ipv6 rip tag distance distance-value

Sets the administrative distance for this

process. Default is 120.

[no] ipv6 rip tag timers update expire

Modifies the RIPng timers for this process. The

holddown garbage-collect

values indicate seconds. Default values are 30

180 180 120.

[no] ipv6 rip tag redistribute static

Advertises static routes into IPv6 as if they

were directly connected.

[no] ipv6 rip tag split-horizon

Performs split-horizon processing of updates.

This is on by default.

[no] ipv6 rip tag poison-reverse

Performs poison-reverse processing of updates.

This is off by default.

Additional RIPng interface subcommands are also available. There are interface subcommands to initiate the advertisement of default routes on updates out the specific interface, to summarize routes advertised out the interface, to apply input and output filters to updates received or sent from the interface, and to change the metric-offset for routes received on the interface. All these functions are available with RIP-2. Table 8-11 lists the interface subcommands.

Table 8-11 RIPng Interface Subcommands

Command

Description

[no] ipv6 rip tag enable

Configures RIPng routing on an interface.

[no] ipv6 rip tag default-information Originates the default route (0::0/0) and originate includes it in updates sent from this interface.

[no] ipv6 rip tag default-information Originates the default route (0::0/0) and originate includes it in updates sent from this interface.

Table 8-11 RIPng Interface Subcommands (Continued)

[no] ipv6 rip tag default-information only Originates the default route (0::0/0). Suppresses sending any routes except the default route on this interface.

Summarizes routing information. If the first length bits of a route match the given prefix, the prefix will be advertised instead. Multiple routes are thus replaced by a single route whose metric is the lowest metric of the multiple routes. You may use this command multiple times.

[no] ipv6 rip tag input-filter name Applies a simple access list to RIP routing updates received on the interface.

[no] ipv6 rip tag output-filter name Applies a simple access list to RIP routing updates generated on the interface.

[no] ipv6 rip tag metric-offset number Changes the metric-offset of a route entering the routing table. Default is 1. Value may be between 1 and 16.

A simple network diagram along with the routers' configurations helps illustrate the minimal router configurations needed to run RIPng (see Figure 8-18).

Figure 8-18 Simple RIPng Network

RIPng is configured on both routers, on the Ethernet link and the serial link. Example 8-6 shows the router configurations.

Example 8-6 Configuring RIPng on Routers Falcon and Eagle Falcon ipv6 unicast-routing no ipv6 rip birdbath split-horizon

[no] ipv6 rip tag summary-address prefix/length continues

Example 8-6 Configuring RIPng on Routers Falcon and Eagle (Continued)

interface Ethernet© no ip address no ip directed-broadcast ipv6 enable ipv6 address FEC0::/64 eui-64 ipv6 address FEC0:: 1:0:0:0:0/64 eui-64^ ipv6 address FEC0::2:0:0:0:0/64 eui-64 ipv6 rip birdbath enable

Eagle ipv6 unicast-routing no ipv6 rip birdbath split-horizon interface Ethernet© no ip address no ip directed-broadcast ipv6 address FEC0::/64 eui-64 ipv6 address FEC0::2:0:0:0:0/64 eui-64 ipv6 address FEC0:-.3:0:0:0:0/64 eui-64 ipv6 rip birdbath enable i interface Seriall ipv6 address FEC0::A:0:0:0:1/126 ipv6 rip birdbath enable

The two routers share two common prefixes: FEC0::/64 and FEC0::2:0:0:0:0/64. Each also is configured with a third prefix. To enable the routers to advertise their noncommon prefix to each other, split-horizon has been disabled. RIPng is enabled on the Ethernet ports and on Eagle's seriall. The process name is birdbath.

Example 8-7 shows Falcon's routing table. Example 8-7 IPv6 Routing Table Showing RIPng-Learned Routes

Falcon#show ipv6 route

IPv6 Routing Table - 9 entries

Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP Timers: Uptime/Expires

via Null0, 01:37:41/never L FEC0::200:CFF:FE0A:2C51/128 [0/0]

via FEC0::200:CFF:FE0A:2C51, Ethernet©, 01:20:58/never C FEC0::/64 [0/0]

via FEC0::200:CFF:FE0A:2C51, Ethernet©, 01:20:58/never

Example 8-7 IPv6 Routing Table Showing RIPng-Learned Routes (Continued)

via FEC0::1:200:CFF:FE0A:2C51, Ethernet©, 01:01:36/never C FEC0::1:0:0:0:0/64 [0/0]

via FEC0::1:200:CFF:FE0A:2C51, Ethernet©, 01:01:36/never L FEC0::2:200:CFF:FE0A:2C51/128 [0/0]

via FEC0::2:200:CFF:FE0A:2C51, Ethernet©, 01:00:21/never C FEC0::2:0:0:0:0/64 [0/0]

via FEC0::2:200:CFF:FE0A:2C51, Ethernet©, 01:00:21/never R FEC0::3:0:0:0:0/64 [120/2]

via FE80::200:CFF:FE76:5B7C, Ethernet©, 00:00:08/00:02:51 R FEC0::A:0:0:0:0/126 [120/2]

via FE80::200:CFF:FE76:5B7C, Ethernet©, 00:00:08/00:02:51

The routing table in Example 8-7 shows that the prefixes configured on Falcon's Ethernet port are connected. Eagle's Ethernet prefix FEC0::3:0:0:0:0/64 and serial prefix FEC0::A:0:0:0:0/126 are learned via the RIPng process.

RIPng is still a very easy protocol to implement, and the introduction of multiple processes adds a little more flexibility over RIP-2; however, the drawbacks still exist, as detailed in Chapter 7 of Volume L For instance, it still has a small maximum hop count, limiting the size of network that can run the protocol.

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