Ch. 1 – Introduction to Classless Routing

Ch. 1 – Introduction to Classless Routing

CCNA 3 version 3.0

Rick Graziani

Cabrillo College

Rick Graziani [email protected] 2

Note to instructors

• If you have downloaded this presentation from the Cisco Networking Academy Community FTP Center, this may not be my latest version of this PowerPoint.

• For the latest PowerPoints for all my CCNA, CCNP, and Wireless classes, please go to my web site:

http://www.cabrillo.cc.ca.us/~rgraziani/• The username is cisco and the password is perlman for all of

my materials.

• If you have any questions on any of my materials or the curriculum, please feel free to email me at [email protected] (I really don’t mind helping.) Also, if you run across any typos or errors in my presentations, please let me know.

• I will add “(Updated – date)” next to each presentation on my web site that has been updated since these have been uploaded to the FTP center.

Thanks! Rick


Overview of Information in Module 1

• Define VLSM and briefly describe the reasons for its use • Divide a major network into subnets of different sizes using VLSM • Define route aggregation and summarization as they relate to VLSM • Configure a router using VLSM • Identify the key features of RIP v1 and RIP v2 • Identify the important differences between RIP v1 and RIP v2 • Configure RIP v2 • Verify and troubleshoot RIP v2 operation • Configure default routes using the ip route and ip default-

network commands


Note

• Much of the information in this module is in addition to the online curriculum.

• The additional information was included it add clarity and make the topics more understandable.– Advanced IP Management

• Subnetting • Classless interdomain routing (CIDR) • Variable length subnet masking (VLSM) • Route summarization• Network Address Translation (NAT)

– Classless Routing Protocols• RIPv2

Advanced IP Management


IPv4 Address Classes



• No medium size host networks

• In the early days of the Internet, IP addresses were allocated to organizations based on request rather than actual need.



Class D Addresses

• A Class D address begins with binary 1110 in the first octet.

• First octet range 224 to 239.

• Class D address can be used to represent a group of hosts called a host group, or multicast group.

Class E AddressesFirst octet of an IP address begins with 1111

• Class E addresses are reserved for experimental purposes and should not be used for addressing hosts or multicast groups.


IP addressing crisis

• Address Depletion• Internet Routing Table Explosion


IPv4 Addressing

Subnet Mask

• One solution to the IP address shortage was thought to be the subnet mask.

• Formalized in 1985 (RFC 950), the subnet mask breaks a single class A, B or C network in to smaller pieces.


Using /24 subnet...

190.52.1.2190.52.2.2190.52.3.2

Network Network Subnet Host

But internal routers think all these addresses are on different networks, called subnetworks

Internet routers still “see” this net as 190.52.0.0

Class B Network Network Host Host

Given the Class B address 190.52.0.0

Subnet Example


Using the 3rd octet, 190.52.0.0 was divided into:190.52.1.0 190.52.2.0 190.52.3.0 190.52.4.0

190.52.5.0 190.52.6.0 190.52.7.0 190.52.8.0

190.52.9.0 190.52.10.0 190.52.11.0 190.52.12.0

190.52.13.0 190.52.14.0 190.52.15.0 190.52.16.0

190.52.17.0 190.52.18.0 190.52.19.0 and so on ...


Subnet Example


Subnet Example


Network address 190.52.0.0 with /16 network mask

190 52 0 Host

190 52 1 Host

190 52 2 Host

Using Subnets: subnet mask 255.255.255.0 or /24

190 52 3 Host

190 52 Etc. Host

190 52 254 Host

190 52 255 Host

255 Subnets

28 - 1

Cannot use last subnet as it contains broadcast address

Subnets


Subnet Example


Subnet 0 (all 0’s subnet) issue: The address of the subnet, 190.52.0.0/24 is the same address as the major network, 190.52.0.0/16.

190 52 0 Host

190 52 1 Host

190 52 Etc. Host

190 52 254 Host

190 52 255 Host

255 Subnets

28 - 1

Subnets

Last subnet (all 1’s subnet) issue: The broadcast address for the subnet, 190.52.255.255 is the same as the broadcast address as the major network, 190.52.255.255.


All Zeros and All Ones Subnets

Using the All Ones and All Zeroes Subnet

• There is no command to enable or disable the use of the all-ones subnet, it is enabled by default.

Router(config)#ip subnet-zero

• The use of the all-ones subnet has always been explicitly allowed and the use of subnet zero is explicitly allowed since Cisco IOS version 12.0.

RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is obsolete! Modern software will be able to utilize all definable networks." Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use, though, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems.

CCO: Subnet Zero and the All-Ones Subnet http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml


• If you need a Review of Subnets, please review the following links on my web site:– Subnet Review (PowerPoint)– Subnets Explained (Word Doc)

Need a Subnet Review?


Long Term Solution: IPv6 (coming)

• IPv6, or IPng (IP – the Next Generation) uses a 128-bit address space, yielding

340,282,366,920,938,463,463,374,607,431,768,211,456

possible addresses.

• IPv6 has been slow to arrive

• IPv4 revitalized by new features, making IPv6 a luxury, and not a desperately needed fix

• IPv6 requires new software; IT staffs must be retrained

• IPv6 will most likely coexist with IPv4 for years to come.

• Some experts believe IPv4 will remain for more than 10 years.


Short Term Solutions: IPv4 Enhancements

• CIDR (Classless Inter-Domain Routing) – RFCs 1517, 1518, 1519, 1520

• VLSM (Variable Length Subnet Mask) – RFC 1009

• Private Addressing - RFC 1918

• NAT/PAT (Network Address Translation / Port Address Translation) – RFC


• By 1992, members of the IETF were having serious concerns about the exponential growth of the Internet and the scalability of Internet routing tables.

• The IETF was also concerned with the eventual exhaustion of 32-bit IPv4 address space.

• Projections were that this problem would reach its critical state by 1994 or 1995.

• IETF’s response was the concept of Supernetting or CIDR, “cider”.• To CIDR-compliant routers, address class is meaningless.

– The network portion of the address is determined by the network subnet mask or prefix-length (/8, /19, etc.)

– The first octet (first two bits) of the network address (or network-prefix) is NOT used to determine the network and host portion of the network address.

• CIDR helped reduced the Internet routing table explosion with supernetting and reallocation of IPv4 address space.

CIDR (Classless Inter-Domain Routing)


Active BGP entries

http://bgp.potaroo.net/

Report last updated at Thu, 16 Jan 2003


• First deployed in 1994, CIDR dramatically improves IPv4’s scalability and efficiency by providing the following:– Eliminates traditional Class A, B, C addresses allowing for more

efficient allocation of IPv4 address space.– Supporting route aggregation (summarization), also known as

supernetting, where thousands of routes could be represented by a single route in the routing table.

• Route aggregation also helps prevent route flapping on Internet routers using BGP. Flapping routes can be a serious concern with Internet core routers.

• CIDR allows routers to aggregate, or summarize, routing information and thus shrink the size of their routing tables. – Just one address and mask combination can represent the routes to

multiple networks.– Used by IGP routers within an AS and EGP routers between AS.



Without CIDR, a router must maintain individual routing table entries for these class B networks.

With CIDR, a router can summarize these routes using a single network address by using a 13-bit prefix: 172.24.0.0 /13

1. Count the number of left-most matching bits, /13 (255.248.0.0)

2. Add all zeros after the last matching bit:

172.24.0.0 = 10101100 00011000 00000000 00000000

Steps:



• By using a prefix address to summarizes routes, administrators can keep routing table entries manageable, which means the following

– More efficient routing– A reduced number of CPU cycles when recalculating a routing table, or when sorting through the routing table entries to find a match– Reduced router memory requirements

• Route summarization is also known as:– Route aggregation– Supernetting

• Supernetting is essentially the inverse of subnetting.

• CIDR moves the responsibility of allocation addresses away from a centralized authority (InterNIC).

• Instead, ISPs can be assigned blocks of address space, which they can then parcel out to customers.


S ubscribers S ubscribers S ubscribers S ubscribers S ubscribers S ubscribers S ubscribers S ubscribers

IS P IS P IS P IS P IS P IS P IS P IS P

R egiona lS erviceP rovider




N etworkS erviceP rovider

N etworkS erviceP rovider

N A P (N etwork A ccess P o in t)

ISP/NAP Hierarchy - “The Internet: Still hierarchical after all these years.” Jeff Doyle (Tries to be anyways!)


• Company XYZ needs to address 400 hosts. • Its ISP gives them two contiguous Class C addresses:

– 207.21.54.0/24– 207.21.55.0/24

• Company XYZ can use a prefix of 207.21.54.0 /23 to supernet these two contiguous networks. (Yielding 510 hosts)

• 207.21.54.0 /23– 207.21.54.0/24– 207.21.55.0/24

23 bits in common

Supernetting Example


• With the ISP acting as the addressing authority for a CIDR block of addresses, the ISP’s customer networks, which include XYZ, can be advertised among Internet routers as a single supernet.

Supernetting Example


Another example of route aggregation.

CIDR and the Provider


Even Better:200.199.48.32/27 11001000 11000111 00110000 0 0100000200.199.48.64/27 11001000 11000111 00110000 0 1000000200.199.48.96/27 11001000 11000111 00110000 0 1100000200.199.48.0/25 11001000 11000111 00110000 0 0000000 (As long as there are no other routes elsewhere within this range, well…)

200.199.56.0/24 11001000 11000111 0011100 0 00000000200.199.57.0/24 11001000 11000111 0011100 1 00000000200.199.56.0/23 11001000 11000111 0011100 0 00000000

CIDR and the provider

200.199.56.0/23

200.199.48.0/25

Summarization from the customer networks to their provider.


CIDR and the provider200.199.48.0/25

200.199.56.0/23

200.199.48.0/25 11001000 11000111 0011 0000 00000000

200.199.49.0/25 11001000 11000111 0011 0001 00000000

200.199.56.0/23 11001000 11000111 0011 1000 00000000

200.199.48.0/20 11001000 11000111 0011 0000 00000000

20 bits in common

Further summarization happens with the next upstream provider.


• Dynamic routing protocols must send network address and mask (prefix-length) information in their routing updates.

• In other words, CIDR requires classless routing protocols for dynamic routing.

• However, you can still configure summarized static routes, after all, that is what a 0.0.0.0/0 route is.

CIDR Restrictions


172.16.2.0/24

• Merida receives a summarized /16 update from Quito and a more specific /24 update from Cartago.

• Merida will include both routes in the routing table.• Merida will forward all packets matching at least the first 24 bits of

172.16.5.0 to Cartago (172/16/5/0/24), longest-bit match.• Merida will forward all other packets matching at least the first 16 bits

to Quito (172.16.0.0/16).

Summarized and Specific Routes: Longest-bit Match (more later)

172.16.10.0/24

172.16.1.0/24 172.16.5.0/24

172.16.0.0/16 172.16.5.0/24Summarized Update Specific Route Update

Merida

Quito Cartago


Example from online curriculum


Another example from online curriculum








VLSM (Variable Length Subnet Mask)

• Limitation of using only a single subnet mask across a given network-prefix (network address, the number of bits in the mask) was that an organization is locked into a fixed-number of of fixed-sized subnets.

• 1987, RFC 1009 specified how a subnetted network could use more than one subnet mask.

• VLSM = Subnetting a Subnet– “If you know how to subnet, you can do VLSM!”


VLSM – Simple Example

• Subnetting a /8 subnet using a /16 mask gives us 256 subnets with 65,536 hosts per subnet.

• Let’s take the 10.2.0.0/16 subnet and subnet it further…

10.0.0.0/8

10.0.0.0/16

10 Host Host Host

10 Subnet Host Host

1st octet 2nd octet 3rd octet 4th octet

10.0.0.0/16 10 0 Host Host

10.1.0.0/16 10 1 Host Host

10.2.0.0/16 10 2 Host Host

10.n.0.0/16 10 … Host Host

10.255.0.0/16 10 255 Host Host



• Note: 10.2.0.0/16 is now a summary of all of the 10.2.0.0/24 subnets.

• Summarization coming soon!

10.2.0.0/16 10 2 Host Host

Network Subnet HostHost

10.2.0.0/24 10 2 Subnet Host

10.2.0.0/24 10 2 0 Host

10.2.1.0/24 10 2 1 Host

10.2.n.0/24 10 2 … Host

10.2.255.0/24 10 2 255 Host



10.0.0.0/8 “subnetted using /16”

Subnet 1st host Last host Broadcast10.0.0.0/16 10.0.0.1 10.0.255.254 10.0.255.25510.1.0.0/16 10.1.0.1 10.1.255.254 10.1.255.255

10.2.0.0/16 “sub-subnetted using /24”–Subnet 1st host Last host Broadcast–10.2.0.0/24 10.2.0.1 10.2.0.254 10.2.0.255–10.2.1.0/24 10.2.1.1 10.2.1.254 10.2.1.255–10.2.2.0/24 10.2.2.1 10.2.2.254 10.2.2.255– Etc.–10.2.255.0/24 10.2.255.1 10.2.255.254 10.2.255.255

10.3.0.0/16 10.3.0.1 10.3.255.254 10.0.255.255 Etc.10.255.0.0/16 10.255.0.1 10.255.255.254 10.255.255.255



• Your network can now have 255 /16 subnets with 65,534 hosts each AND 256 /24 subnets with 254 hosts each.

• All you need to make it work is a classless routing protocol that passes the subnet mask with the network address in the routing updates.

• Classless routing protocols: RIPv2, EIGRP, OSPF, IS-IS, BGPv4 (coming)

Subnets10.0.0.0/16 10.1.0.0/1610.2.0.0/16

10.2.0.0/24 10.2.1.0/2410.2.2.0/24 Etc.10.2.255.0/24

10.3.0.0/16 Etc.10.255.0.0/16

10.1.0.0/16

An example of VLSM, NOT of good network design.

10.3.0.0/16

10.4.0.0/16 10.5.0.0/16

10.6.0.0/16

10.7.0.0/1610.2.0.0/24

10.2.3.0/24 10.2.4.0/2410.2.5.0/24

10.2.8.0/2410.8.0.0/16

10.2.6.0/24

10.2.1.0/24


Another VLSM Example using /30 subnets

207.21.24.0/24 network subnetted into eight /27 (255.255.255.224) subnets

• This network has seven /27 subnets with 30 hosts each AND eight /30 subnets with 2 hosts each.

• /30 subnets are very useful for serial networks.

207.21.24.192/27 subnet, subnetted into eight /30 (255.255.255.252) subnets


207.21.24.192/27 207.21.24. 11000000

/30 Hosts Bcast 2 Hosts

0 207.21.24.192/30 207.21.24. 110 00000 01 10 11 .193 & .194

1 207.21.24.196/30 207.21.24. 110 00100 01 10 11 .197 & .198

2 207.21.24.200/30 207.21.24. 110 01000 01 10 11 .201 & .202

3 207.21.24.204/30 207.21.24. 110 01100 01 10 11 .205 & .206

4 207.21.24.208/30 207.21.24. 110 10000 01 10 11 .209 & .210

5 207.21.24.212/30 207.21.24. 110 10100 01 10 11 .213 & .214

6 207.21.24.216/30 207.21.24. 110 11000 01 10 11 .217 & .218

7 207.21.24.220/30 207.21.24. 110 11100 01 10 11 .221 & .222


207.21.24.192/30

207.21.24.196/30

207.21.24.200/30

207.21.24.204/30

207.21.24.208/30

207.21.24.212/30

207.21.24.32/27

207.21.24.64/27207.21.24.96/27 207.21.24.128/27

207.21.24.160/27 207.21.24.224/27 207.21.24.0/27

207.21.24.216/30

• This network has seven /27 subnets with 30 hosts each AND seven /30 subnets with 2 hosts each (one left over).

• /30 subnets with 2 hosts per subnet do not waste host addresses on serial networks .


VLSM and the Routing Table

Routing Table without VLSMRouterX#show ip route 207.21.24.0/27 is subnetted, 4 subnetsC 207.21.24.192 is directly connected, Serial0 C 207.21.24.196 is directly connected, Serial1C 207.21.24.200 is directly connected, Serial2C 207.21.24.204 is directly connected, FastEthernet0

Routing Table with VLSMRouterX#show ip route 207.21.24.0/24 is variably subnetted, 4 subnets, 2 masksC 207.21.24.192 /30 is directly connected, Serial0 C 207.21.24.196 /30 is directly connected, Serial1C 207.21.24.200 /30 is directly connected, Serial2C 207.21.24.96 /27 is directly connected, FastEthernet0

• Parent Route shows classful mask instead of subnet mask of the child routes.

• Each Child Routes includes its subnet mask.

Displays one subnet mask for all child routes. Classful mask is assumed for the parent route.

Each child routes displays its own subnet mask. Classful mask is included for the parent route.


Final Notes on VLSM

• Whenever possible it is best to group contiguous routes together so they can be summarized (aggregated) by upstream routers. (coming soon!)

– Even if not all of the contiguous routes are together, routing tables use the longest-bit match which allows the router to choose the more specific route over a summarized route.– Coming soon!

• You can keep on sub-subnetting as many times and as “deep” as you want to go.

• You can have various sizes of subnets with VLSM.


Route flapping

• Route flapping occurs when a router interface alternates rapidly between the up and down states.

• Route flapping can cripple a router with excessive updates and recalculations.• However, the summarization configuration prevents the RTC route flapping

from affecting any other routers.• The loss of one network does not invalidate the route to the supernet. • While RTC may be kept busy dealing with its own route flap, RTZ, and all

upstream routers, are unaware of any downstream problem. • Summarization effectively insulates the other routers from the problem of route

flapping.








Private IP addresses (RFC 1918)

If addressing any of the following, these private addresses can be used instead of globally unique addresses:

• A non-public intranet • A test lab • A home network Global addresses must be obtained from a provider or a registry at some expense.


Discontiguous subnets

• “Mixing private addresses with globally unique addresses can create discontiguous subnets.” – Not the main cause however…

• Discontiguous subnets, are subnets from the same major network that are separated by a completely different major network or subnet.

• Question: If a classful routing protocol like RIPv1 or IGRP is being used, what do the routing updates look like between Site A router and Site B router?



• Classful routing protocols, notably RIPv1 and IGRP, can’t support discontiguous subnets, because the subnet mask is not included in routing updates.

• RIPv1 and IGRP automatically summarize on classful boundaries.• Site A and Site B are all sending each other the classful address of

207.21.24.0/24.• A classless routing protocol (RIPv2, EIGRP, OSPF) would be needed:

– to not summarize the classful network address and – to include the subnet mask in the routing updates.



• RIPv2 and EIGRP automatically summarize on classful boundaries.

• When using RIPv2 and EIGRP, to disable automatic summarization (on both routers):

Router(config-router)#no auto-summary

• SiteB now receives 207.21.24.0/27

• SiteB now receives 207.21.24.32/27








Network Address Translation (NAT)

NAT: Network Address Translatation

• NAT, as defined by RFC 1631, is the process of swapping one address for another in the IP packet header.

• In practice, NAT is used to allow hosts that are privately addressed to access the Internet.


Network Address Translation (NAT)

• NAT translations can occur dynamically or statically. • The most powerful feature of NAT routers is their capability to use port address

translation (PAT), which allows multiple inside addresses to map to the same global address.

• This is sometimes called a many-to-one NAT. • With PAT, or address overloading, literally hundreds of privately addressed nodes can

access the Internet using only one global address. • The NAT router keeps track of the different conversations by mapping TCP and UDP

port numbers.

2.2.2.2 TCP Source Port 1923

2.2.2.2 TCP Source Port 1924

TCP Source Port 1026

TCP Source Port 1026

Classless Routing ProtocolsRIPv2


Classless routing protocols

• The true defining characteristic of classless routing protocols is the capability to carry subnet masks in their route advertisements.

• “One benefit of having a mask associated with each route is that the all-zeros and all-ones subnets are now available for use.” – Cisco allows the all-zeros and all-ones subnets to be used with

classful routing protocols.

Rick Graziani [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

Note: Remember classful/classless routing protocols is different than classful/classless routing behavior. Classlful/classless routing protocols (RIPv1, RIPv2, IGRP, EIGRP, OSPF, etc.) has to do with how routes get into the routing table; how the routing table gets built. Classful/classless routing behavior (no ip classless or ip classless) has to do with the lookup process of routes in the routing table (after the routing table has been built). It is possible to have a classful routing protocol and classless routing behavior or visa versa. It is also possible to have both a classful routing protocol and classful routing behavior; or both a classless routing protocol and classless routing behavior.


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.


RIP v2 operation

• All of the operational procedures, timers, and stability functions of RIP v1 remain the same in RIP v2, with the exception of the broadcast updates.

• RIP v2 updates use reserved Class D address 224.0.0.9.


Issues addressed by RIP v2

The following four features are the most significant new features added to RIP v2:• Authentication of the transmitting RIP v2 node to other RIP v2 nodes • Subnet Masks – RIP v2 allocates a 4-octet field to associate a subnet mask to

a destination IP address. • Next Hop IP addresses – A better next-hop address, that the advertising

router, if one exists. – It indicates a next-hop address, on the same subnet, that is metrically

closer to the destination than the advertising router.– If this router’s interface is closest, then it is set to 0.0.0.0– See Doyle, Routing TCP/IP for an example

• Multicasting RIP v2 messages – Multicasting is a technique for simultaneously advertising routing information to multiple RIP or RIP v2 devices.


RIP v2 message format

• All the extensions to the original protocol are carried in the unused fields.

• The Address Family Identifier (AFI) field is set to two for IP. The only exception is a request for a full routing table of a router or host, in which case it will be set to zero.


RIP v2 message format

• The Route Tag field provides a way to differentiate between internal and external routes. (RIP itself does not use this field.)– External routes are those that have been redistributed into the RIP v2.

• The Next Hop field contains the IP address of the next hop listed in the IP Address field.

• Metric indicates how many internetwork hops, between 1 and 15 for a valid route, or 16 for an unreachable route.


Compatibility with RIP v1

RFC 1723 defines a compatibility with four settings, which allows versions 1 and 2 to interoperate:

1. RIP v1, in which only RIP v1 messages are transmitted 2. RIP v1 Compatibility, which causes RIP v2 to broadcast its messages

instead of multicast them so that RIP v1 may receive them 3. RIP v2, in which RIP v2 messages are multicast to destination

address 224.0.0.9 4. None, in which no updates are sent

• RFC 1723 recommends that routers be configurable on a per-interface basis. (coming soon)


Authentication

• A security concern with any routing protocol is the possibility of a router accepting invalid routing updates.

• The Authentication Type for simple password authentication is two, 0x0002,

• The remaining 16 octets carry an alphanumeric password of up to 16 characters.

• Configuration is coming!

Authentication is supported by modifying what would normally be the first route entry of the RIP message


Authentication

• RFC 1723 describes only simple password authentication • Cisco IOS provides the option of using MD5 authentication instead of

simple password authentication. • Cisco uses the first and last route entry spaces for MD5 authentication

purposes.• MD5 computes a 128-bit hash value from a plain text message of

arbitrary length and a password.


Authentication


MD5 Authentication (FYI) http://www.cisco.com/en/US/tech/tk713/tk507/technologies_tech_note09186a00800b4131.shtml

1 2

3 4

5 6


Same limitations of RIPv2 as with RIPv1

• Slow convergence and the need of holddown timers to reduce the possibility of routing loops.

Note: See CCNA 2 for review if needed.



• RIP v2 continues to rely on counting to infinity as a means of resolving certain error conditions within the network.

• Dependent upon holddown timers.• Triggered updates are also helpful.

Note: See CCNA 2 for review if needed.



• Perhaps the single greatest limitation that RIP v2 inherited from RIP is that its interpretation of infinity remained at 16.


Basic RIPv2 configuration

Other:For RIP and IGRP, the passive interface command stops the router from

sending updates to a particular neighbor, but the router continues to listen and use routing updates from that neighbor. (More later.)

Router(config-router)# passive-interface interface

Default behavior of version 1 restored: Router(config-router)# no version


Compatibility with RIP v1

NewYork

interface fastethernet0/0

ip address 192.168.50.129 255.255.255.192

ip rip send version 1

ip rip receive version 1


ip address 172.25.150.193 255.255.255.240

ip rip send version 1 2


ip address 172.25.150.225 225.255.255.240

router rip

version 2

network 172.25.0.0

network 192.168.50.0

• Interface FastEthernet0/0 is configured to send and receive RIP v1 updates.

• FastEthernet0/1 is configured to send both version 1 and 2 updates.

• FastEthernet0/2 has no special configuration and therefore sends and receives version 2 by default.

RIPv2


Discontiguous subnets and classless routing

• RIP v1 always uses automatic summarization.

• The default behavior of RIP v2 is to summarize at network boundaries the same as RIP v1.

router ripversion 2no auto-summary


Configuring authentication (EXTRA)

Router(config)#key chain RomeoRouter(config-keychain)#key 1Router(config-keychain-key)#key-string Juliet The password must be the same on both routers (Juliet), but the name of the key

(Romeo) can be different.

Router(config)#interface fastethernet 0/0Router(config-if)#ip rip authentication key-chain RomeoRouter(config-if)#ip rip authentication mode md5

• If the command ip rip authentication mode md5 is not added, the interface will use the default clear text authentication. Although clear text authentication may be necessary to communicate with some RIP v2 implementations, for security concerns use the more secure MD5 authentication whenever possible.


Show commands


show ip rip database

Router# show ip rip database172.19.0.0/16 auto-summary172.19.64.0/24 directly connected, Ethernet0172.19.65.0/24[1] via 172.19.70.36, 00:00:17, Serial1[2] via 172.19.67.38, 00:00:25, Serial0172.19.67.0/24 directly connected, Serial0172.19.67.38/32 directly connected, Serial0172.19.70.0/24 directly connected, Serial1172.19.86.0/24[1] via 172.19.67.38, 00:00:25, Serial0[1] via 172.19.70.36, 00:00:17, Serial1

• The show ip rip database command to check summary address entries in the RIP database.

• These entries will appear in the database if there are only relevant child or specific routes being summarized.

• When the last child route for a summary address becomes invalid, the summary address is also removed from the routing table.

Router#show ip rip database


Show commands


Debug commands


RIPv2 Example

Scenario:

• Discontiguous subnets

• VLSM

• CIDR

• Supernet to 207.0.0.0/8

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 the default auto-summary on ISP, it will load balance for all packets destined for 172.30.0.0/16


SantaCruz1

router rip

network 172.30.0.0

network 192.168.4.0

version 2

no auto-summary

SantaCruz2

router rip

network 172.30.0.0

network 192.168.4.0

version 2

no auto-summary

ISP

router 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

RIPv2 Example


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 rip

RIP protocol debugging is on

ISP#01:23:34: RIP: received v2 update from 192.168.4.22 on Serial1

01:23:34: 172.30.100.0/24 -> 0.0.0.0 in 1 hops

01:23:34: 172.30.110.0/24 -> 0.0.0.0 in 1 hops

ISP#

01:23:38: RIP: received v2 update from 192.168.4.26 on Serial0

01:23:38: 172.30.2.0/24 -> 0.0.0.0 in 1 hops

01:23:38: 172.30.1.0/24 -> 0.0.0.0 in 1 hops

ISP#

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 0

01:24:31: 172.30.1.0/24 -> 0.0.0.0, metric 2, tag 0

01:24:31: 172.30.100.0/24 -> 0.0.0.0, metric 2, tag 0

01:24:31: 172.30.110.0/24 -> 0.0.0.0, metric 2, tag 0

01:24:31: 192.168.4.24/30 -> 0.0.0.0, metric 1, tag 0

01:24:31: 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


Adding a default Routes to RIPv2

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

ISP

router rip

redistribute static

network 10.0.0.0

network 192.168.4.0

version 2

no auto-summary

default-information originate

ip route 207.0.0.0 255.0.0.0 null0

ip route 0.0.0.0 0.0.0.0 10.0.0.2 etherenet0


Other RIPv2 Commands (EXTRA)

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


RIPv2 Summary

Ch. 1 – Introduction to Classless Routing

CCNA 3 version 3.0

Rick Graziani

Cabrillo College

Documents

Ch. 1 – Introduction to Classless Routing