introduction to classless routing concepts and ripv2

2© 2002, Cisco Systems, Inc. All rights reserved.

Session NumberPresentation_ID

Introduction to Classless Routing Concepts and RIPv2

(Extra: The Routing Table)Rick Graziani

[email protected]

http://www.cabrillo.cc.ca.us/~rgraziani

Updated:2/18/03


Note to Instructors

This presentation will introduce you to the following:

• Classless Routing Protocols

• RIPv2

• The Routing Table

– The route lookup process

– Classful versus Classless routing behavior

• Please email any questions or issues you might find in this presentation to Rick Graziani at [email protected]


Classless Routing Protocols

“The true characteristic of a classless routing protocol is the ability to carry subnet masks in their route advertisements.” Jeff Doyle, Routing TCP/IP

Benefits:

• All-zeros and all-ones subnets - Although some vendors, like Cisco, can also handle this with classful routing protocols.

• VLSM

– Can have discontiguous subnets

– Better IP addressing allocation

• CIDR

– More control over route summarization


Classless Routing Protocols

Classless Routing Protocols:

• RIPv2

• EIGRP

• OSPF

• IS-IS

• BGPv4


Few RIP facts

• RIP still working on routers and hosts today.

• IP RIP derived from RIP by Xerox for its XNS protocol stack.

• Initially implemented in Berkeley UNIX routed program.

• RIPv1 – Charles Hedrick, RFC 1058, 1988

• RIPv2 – Gary Malkin, RFC 1723, 1994

• RIPng for IPv6 – Gary Malkin, RFC 2080, 1997 (proposed standard), extension to RIPv2 message format.

The Grim Router


RIP version 1

0 1 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | command (1) | version (1) | must be zero (2) | +---------------+---------------+-------------------------------+ | address family identifier (2) | must be zero (2) | +-------------------------------+-------------------------------+ | IP address (4) | +---------------------------------------------------------------+ | must be zero (4) | +---------------------------------------------------------------+ | must be zero (4) | +---------------------------------------------------------------+ | metric (4) | +---------------------------------------------------------------+

• Classful Routing Protocol, sent over UDP port 520

• Does not include the subnet mask in the routing updates.

• Automatic summarization done at major network boundaries.

• Updates sent as broadcasts unless the neighbor command is uses which sends them as unicasts.


RIP version 2

0 1 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | command (1) | version (1) | must be zero (2) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address Family Identifier (2) | Route Tag (2) | +-------------------------------+-------------------------------+ | IP Address (4) | +---------------------------------------------------------------+ | Subnet Mask (4) | +---------------------------------------------------------------+ | Next Hop (4) | +---------------------------------------------------------------+ | Metric (4) | +---------------------------------------------------------------+

• Classless Routing Protocol, sent over UDP port 520

• Includes the subnet mask in the routing updates.

• Automatic summarization at major network boundaries can be disabled.

• Updates sent as multicasts unless the neighbor command is uses which sends them as unicasts.


Configuring RIPv2

RIP defaults to RIPv1. To modify RIP to use version 2:

Router(config)# router rip

Router(config-router)# version 2

To go back to RIP version 1:


Router(config-router)# version 1

To turn off automatic summarization at major network boundaries:

Router(config-router)# no auto-summary


Default Routes and RIPv2

Configuring a default route using a “quad-zero” static route:

Router(config)# ip route 0.0.0.0 0.0.0.0 serial0


Router(config-router)# default-information originate

• Required starting with IOS 12.1 (always sends?)

Configuring a default route using the ip default-network command:

Router(config)# ip route 0.0.0.0 0.0.0.0 serial0

or route to the specific network


Router(config-router)# ip default-network 192.168.1.0

Router(config-router)# network 192.168.1.0


RIPv2 Example

Scenario:

• Discontiguous subnets

• VLSM

• CIDR

e0

ISP

SantaCruz2SantaCruz1

192.168.4.20/30

172.30.1.0/24

Internet

s0

s0 s0

s1

172.30.100.0/24

e0

192.168.4.24/30

.21

.22

.25

.26

10.0.0.0/8

e0.1

.1.1

Lo0Lo0

172.30.110.0/24172.30.2.0/24 .1

.1

.1

static route to207.0.0.0/8

207.0.0.0/16207.1.0.0/16207.2.0.0/16207.3.0.0/16

etc.

` 172.30.200.16/28

172.30.200.32/28

Lo1

Lo2

With autosummarization ISP will load balance for all packets destined for 172.30.0.0/16


RIPv2 ExampleSantaCruz1router rip network 172.30.0.0 network 192.168.4.0 version 2 no auto-summary SantaCruz2router rip network 172.30.0.0 network 192.168.4.0 version 2 no auto-summary ISProuter rip redistribute static network 10.0.0.0 network 192.168.4.0 version 2 no auto-summary

ip route 207.0.0.0 255.0.0.0 null0

e0

ISP


192.168.4.20/30

172.30.1.0/24

Internet

s0

s0 s0

s1

172.30.100.0/24

e0

192.168.4.24/30

.21

.22

.25

.26

10.0.0.0/8

e0.1

.1.1

Lo0Lo0

172.30.110.0/24172.30.2.0/24 .1

.1

.1


207.0.0.0/16207.1.0.0/16207.2.0.0/16207.3.0.0/16

etc.

` 172.30.200.16/28

172.30.200.32/28

Lo1

Lo2


e0

ISP


192.168.4.20/30

172.30.1.0/24

Internet

s0

s0 s0

s1

172.30.100.0/24

e0

192.168.4.24/30

.21

.22

.25

.26

10.0.0.0/8

e0.1

.1.1

Lo0Lo0

172.30.110.0/24172.30.2.0/24 .1

.1

.1


207.0.0.0/16207.1.0.0/16207.2.0.0/16207.3.0.0/16

etc.

` 172.30.200.16/28

172.30.200.32/28

Lo1

Lo2

SantaCruz2#show ip route

172.30.0.0/16 is variably subnetted, 6 subnets, 2 masks

C 172.30.200.32/28 is directly connected, Loopback2


R 172.30.2.0/24 [120/2] via 192.168.4.21, 00:00:21, Serial0

R 172.30.1.0/24 [120/2] via 192.168.4.21, 00:00:21, Serial0

C 172.30.100.0/24 is directly connected, Ethernet0


192.168.4.0/30 is subnetted, 2 subnets

R 192.168.4.24 [120/1] via 192.168.4.21, 00:00:21, Serial0

C 192.168.4.20 is directly connected, Serial0

R 10.0.0.0/8 [120/1] via 192.168.4.21, 00:00:21, Serial0

R 207.0.0.0/8 [120/1] via 192.168.4.21, 00:00:21, Serial0

Examining a Routing Table

Supernet, classless routing protcols will route supernets (CIDR)


RIPv2: Sending and Receiving Updates

ISP#debug ip ripRIP protocol debugging is onISP#01:23:34: RIP: received v2 update from 192.168.4.22 on Serial101:23:34: 172.30.100.0/24 -> 0.0.0.0 in 1 hops01:23:34: 172.30.110.0/24 -> 0.0.0.0 in 1 hopsISP#01:23:38: RIP: received v2 update from 192.168.4.26 on Serial001:23:38: 172.30.2.0/24 -> 0.0.0.0 in 1 hops01:23:38: 172.30.1.0/24 -> 0.0.0.0 in 1 hopsISP#01:24:31: RIP: sending v2 update to 224.0.0.9 via Ethernet0 (10.0.0.1)01:24:31: 172.30.2.0/24 -> 0.0.0.0, metric 2, tag 001:24:31: 172.30.1.0/24 -> 0.0.0.0, metric 2, tag 001:24:31: 172.30.100.0/24 -> 0.0.0.0, metric 2, tag 001:24:31: 172.30.110.0/24 -> 0.0.0.0, metric 2, tag 001:24:31: 192.168.4.24/30 -> 0.0.0.0, metric 1, tag 001:24:31: 192.168.4.20/30 -> 0.0.0.0, metric 1, tag 0<text omitted>

ISP(config)# line console 0

ISP(config-line)# logging synchronous

multicast

Includes mask


Extra: Other RIPv2 Commands

Router(config-router)# neighbor ip-address

Defines a neighboring router with which to exchange unicast routing information. (RIPv1 or RIPv2)

Router(config-if)# ip rip send|receive version 1 | 2 | 1 2

Configures an interface to send/receive RIP Version 1 and/or Version 2 packets

Router(config-if)# ip summary-address rip ip_address ip_network_mask

Specifies the IP address and network mask that identify the routes to be summarized.

Authentication and other nice configuration commands and examples:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_configuration_guide_chapter09186a00800d97f7.html

16© 2002, Cisco Systems, Inc. All rights reserved.

Session NumberPresentation_ID

EXTRA:The Routing Table

The Lookup Processes, Classless and Classful Routing Behaviors

Rick Graziani

[email protected]


The IP Routing Table

• This information is included as two PDF documents.

• Can also be downloaded from my web site:The Routing Table: Part 1 or 2 - The Routing Table Structure (PDF)

The Routing Table: Part 2 or 2 - The Routing Table Lookup Process (PDF)

http://www.cabrillo.cc.ca.us/ciscoacad/curriculum/graziani/presentations/semester5/PartI-RoutingTableStructure.pdf

http://www.cabrillo.cc.ca.us/ciscoacad/curriculum/graziani/presentations/semester5/PartII-RoutingTableLookup.pdf


Parent and Child Routes

RouterB#show ip route

172.16.0.0/24 is subnetted, 3 subnetsR 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0C 172.16.2.0 is directly connected, Serial0C 172.16.3.0 is directly connected, FastEthernet0C 192.168.1.0/24 is directly connected, Serial1S 172.0.0.0/8 is directly connected, Serial1S 160.0.0.0/4 is directly connected, Serial1S* 0.0.0.0/0 is directly connected, Serial1

Parent Route

• Created automatically whenever there is a route with a mask greater than the classful mask.

• For non-VLSM routes, contains the mask of the child routes.

Child Routes

• Routes with masks greater than the default classful mask.


Lookup what?



Routing Table process matches:

• The routing table process compares the left-most bits in the packet’s destination IP address with the left-most bits in the route in the routing table, looking for a longest-bit-match.

• The subnet mask of the route in the routing table specifies the minimum number of left-most bits that must match.

• Before checking child routes, the classful mask of the parent route is used.

• For child routes the parent route’s mask is used.

• For VLSM routes, the mask is contained with the child route.





DA = 192.168.1.10

• 16 bits of 172.16.0.0 do not match, so child routes are not checked.

• 24 bits of 192.168.1.0/24 do match, so this route is used.

DA = 172.16.2.1

• 16 bits of 172.16.0.0 do match, so child routes are checked.

• 24 bits of 172.16.1.0 do not match, so continue to next child route.

• 24 bits of 172.16.2.0 do match, so this route is used!





DA = 32.1.1.10

• 16 bits of 172.16.0.0 do not match, so child routes are not checked.

• 24 bits of 192.168.1.0/24 do not match, so this route is not used.



• 0 bits of 0.0.0.0/0 does match, so this route is used!





DA = 172.16.4.1

• 16 bits of 172.16.0.0 do match, so child routes are checked.



• 24 bits of 172.16.3.0 do not match, no more child routes.

Now what??? It depends!


Classful Routing Behavior



DA = 172.16.4.1

Router(config)# no ip classless

• With classful routing behavior, if the child routes are checked but there are no matches, the routing lookup process ends and the Packet is dropped. (The packets get in, but they can’t get out!)

• Supernet and default routes are not checked.

• Default with IOS 11.2 and prior


Classless Routing Behavior



DA = 172.16.4.1

Router(config)# ip classless

• With classless routing behavior, if the child routes are checked but there are no matches, the routing lookup process continues with other routes in the routing table, including supernet and default routes.

• 8 bits of 172.0.0.0/8 do match, so this route is used!

• Default with IOS 11.3 and later


VLSM and Routing Tables

RouterX#show ip route

172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks

C 172.16.1.4/30 is directly connected, Serial0

C 172.16.1.8/30 is directly connected, Serial1

C 172.16.3.0/24 is directly connected, FastEthernet0

RouterX#

Parent Route

• Created automatically whenever there is a route with a mask greater than the classful mask.

• For VLSM routes, the mask is the default classful mask.

Child Routes

• Routes with masks greater than the default classful mask.

• For VLSM routes, each child route has its own mask.


Classful and Classless: Routing Behavior versus Routing Protocol

Notes:

• Classful or classless routing behavior has nothing to do with classful or classless routing protocols.

• Classful/Classless routing protocols are only concerned about how the routes get into the table.

• Classful/Classless routing behavior has to do with the lookup process within the routing table AFTER the routes have entered the routing table.

introduction to classless routing concepts and ripv2

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