Autosummarization and Manual Summarization

Autosummarization and Manual Summarization Classless and Classful Routing Protocols and Convergence

classful routing, which is sometimes called a classful network. If you are using a classful routing protocol then all of your networks are in different classes and are separated by a router. Remember that the useable IPv4 address ranges are dived into classes. Class A 1-126, Class B 128-191, and Class C 192-223. So when we talk about a classful routing protocol it will only look at the IP address class and not the subnet mask. This is the problem; if you are using a classful routing protocol you cannot change the subnet mask in any of these networks. You have to use the default subnet mask, so an example is if you have a 172.16.X.X network the default subnet mask is a /16 or So in short you can’t have different size networks when using the same IP address class and a classful routing protocol. Remember
that classful routing does not support subnet information, and therefore lacking support for VLSM (variable length subnet masks).

With classless routing if you took a guess that it supports VLSM and different size networks within the same IP address class then you’re catching on ;) Because that is exactly what a classless routing protocol does, so for example we are using a class C address of with a standard subnet mask of or a /24. With a classless routing protocol you can split this network even more instead of having 254 usable hosts in one network; we could have 126 usable hosts with two networks. The ranges for the first network would start at and end at, and the second would start at and end at The subnet mask for both of these networks would be With a classful routing protocol it would only look at the class of the address in this case a class “C” and not look at the subnet mask and apply a default subnet mask of causing these addresses to be in the same network which in reality their not.

What Routing Protocol uses Classful and Classless Routing? Take a guess…

Classless No Yes Yes Yes Yes
Supports VLSM No Yes Yes Yes Yes
Send Subnet Mask No Yes Yes Yes Yes


IP Classless

Where the ip classless configuration command falls within the routing and forwarding processes is often confusing. In reality, IP classless only affects the operation of the forwarding processes in IOS; it doesn't affect the way the routing table is built. If IP classless isn't configured (using the no ip classless command), the router won't forward packets to supernets. As an example, let's again place three routes in the routing table and route packets through the router.

Note: If the supernet or default route is learned via IS-IS or OSPF, the no ip classless configuration command is ignored. In this case, packet switching behavior works as though ip classless were configured.

router# show ip route
.... is variably  subnetted, 2 subnets, 2 masks
D [90/4879540] via
D  [90/25789217] via
S* [1/0] via  

Remembering that the network includes the addresses through, and the network includes the addresses through, we can then try switching three packets through this routing table and see what the results are.

  • A packet destined to is forwarded to, since this is the longest prefix match.

  • A packet destined to is forwarded to, since this is the longest prefix match.

  • A packet destined to is forwarded to; since this network doesn't exist in the routing table, this packet is forwarded to the default route.

  • A packet destined to is dropped.

The surprising answer out of these four is the last packet, which is dropped. It's dropped because its destination,, is within a known major network,, but the router doesn't know about this particular subnet within that major network.

This is the essence of classful routing: If one part of a major network is known, but the subnet toward which the packet is destined within that major network is unknown, the packet is dropped.

The most confusing aspect of this rule is that the router only uses the default route if the destination major network doesn't exist in the routing table at all.

This can cause problems in a network where a remote site, with one connection back to the rest of the network, is running no routing protocols, as illustrated.


The remote site router is configured like this:

interface Serial 0
     ip address
   interface Ethernet 0
     ip address
   ip route
   no ip classless

With this configuration, the hosts at the remote site can reach destinations on the Internet (through the 10.x.x.x cloud), but not destinations within the 10.x.x.x cloud, which is the corporate network. Because the remote router knows about some part of the network, the two directly connected subnets, and no other subnet of 10.x.x.x, it assumes these other subnets don't exist and drops any packets destined for them. Traffic destined to the Internet, however, doesn't ever have a destination in the 10.x.x.x range of addresses, and is therefore correctly routed through the default route.

Configuring ip classless on the remote router resolves this problem by allowing the router to ignore the classful boundaries of the networks in its routing table and simply route to the longest prefix match it can find.



Classful routing protocols do not carry subnet mask information on their routing updates. This makes them unsuitable for hierarchical addressing that require Variable Length Subnet Mask (VLSM) and discontiguous network. Classless routing protocols do carry subnet mask information on their routing updates.

RIP v1 and IGRP are classful routing protocols. RIP v2, EIGRP, OSPF, and BGP are classless routing protocol.

When running a classful routing protocol on a network, make sure you use the same subnet mask everywhere. Otherwise, routing black holes can occur.

Classless routing protocols extend the standard Class A, B, or C IP addressing scheme by using a subnet mask or mask length to indicate how routers must interpret an IP network ID. Classless routing protocols include the subnet mask along with the IP address when advertising routing information. Subnet masks representing the network ID are not restricted to those defined by the address classes, but can contain a variable number of high-order bits. Such subnet mask flexibility enables you to group several networks as a single entry in a routing table, significantly reducing routing overhead. Classless routing protocols includes RIP v2 and OSPF, Border Gateway Protocol version 4 (BGP4) and Intermediate System to Intermediate System (IS-IS).



Convergence is the state of a set of routers that have the same topological information about the internetwork they operate in. For a set of routers to have converged, they must have collected all available topology information from each other via the implemented routing protocol, the information they gathered must not contradict any other router's topology information in the set, and it must reflect the real state of the network. In other words: In a converged network all routers "agree" on what the network topology looks like.

Convergence is an important notion for a set of routers that engage in dynamic routing; All Interior Gateway Protocols rely on convergence to function properly. To have converged it is the normal state of an operational autonomous system. The Exterior Gateway Routing Protocol BGP typically never converges because the Internet is too big for changes to be communicated fast enough.

Convergence process
When a routing protocol process is enabled, every participating router will attempt to exchange information about the topology of the network. The extent of this information exchange, the way it is sent and received, and the type of information required vary widely depending on the routing protocol in use, see e.g. RIP, OSPF, BGP4.

A state of convergence is achieved once all routing protocol-specific information has been distributed to all routers participating in the routing protocol process. Any change in the network that affects routing tables will break the convergence temporarily until this change has been successfully communicated to all other routers.

Convergence time
Convergence time is a measure of how fast a group of routers reach the state of convergence. It is one of the main design goals and an important performance indicator for routing protocols to implement a mechanism that allows all routers running this protocol to quickly and reliably converge. Of course, the size of the network also plays an important role, a larger network will converge slower than a small one.

RIP is a routing protocol that converges so slowly that even a network of a few routers can take a couple of minutes to converge. In case of a new route being advertised, triggered updates can speed up RIP's convergence but to flush a route that previously existed takes longer due to the holddown timers in use. OSPF is an example of a fast-converging routing protocol. A network of a few routers can converge in a matter of seconds.

Certain configuration and hardware conditions will prevent a network from ever converging. For instance, a "flapping" interface (an interface that frequently changes its state between "up" and "down") might cause conflicting information to propagate the network so that routers never agree on its current state. Under certain circumstances it might be desired to withhold routing information details from parts of the network via route aggregation, thereby speeding up convergence of the topological information shared by all routers.


As for manual summarization. This is used all the time in larger networks. Say you have a WAN with 30 sites. You give each site a /16, from the 10/8 space, which is then chopped up into several /24s (or smaller) for various VLANs at the site. There's no reason to advertise 11ty /24s into the WAN if the site is running a single, contiguous /16.

So 30 sites with a /16 per site, each site is using 30 /24s (conservative number considering how large a /16 really is). Without summarization, your WAN has 900 prefixes. If each site advertises only its /16, you have 30 prefixes.

-autosummarization ALWAYS summarizes to the class A, B, or C boundary and never summarizes a group a subnets?

-To summarize a group of subnets within a classful network you must use manual summariztion.

When you summarize routes in RIP, IGRP, EIGRP, or OSPF, you're replacing a series of routes with a summary route and mask. With RIP, IGRP, and EIGRP, this lessens the size of the routing update packet itself therefore multiple routes are replaced with the summary route. The routes,,, and can be summarized as So, only the summary address will be found in the update packet.

It can also make the routing table smaller, and permit complete IP connectivity when done correctly. As above the four more-specific routes will be replaced by the single summary route. Since the entire routing table is parsed before the routing process is complete, keeping the routing table as small as possible does help speed the routing process.



Within the scope of RIP, there's two things related to summarization you need to consider: 1) Sending RIP updates and 2) Receiving RIP updates.

The default for RIPv1 is to send automatically summarized network updates. So let's say on a three interface router you have the following networks:


So for a RIPv1 routing protocol update, only the following network will be advertized:


That's because that network address is the automatically summarized network for,, and But let's say you want to let the more specific subnetworks of and be advertised using RIP. That's where you need to configure the following:

Router#configuration terminal
Router(config)#router rip
Router(config-router)#no auto-summary

The "no auto-summary" part will DISable the automatic summarization that is sent using RIP because of the key word "no." So now that RIP sending updates has been covered, let's talk about RIP receiving updates.

By default, RIP routers listen for and receive both RIPv1 routing updates and RIPv2 routing updates. You can verify this by typing the following:

Router#show ip protocols

So in order to migrate a RIPv1 router fully to RIPv2, you should also specify that this RIP router will only listen for and accept RIPv2 routing updates by typing the following:

Router#configuration terminal
Router(config)#router rip
Router(config-router)#version 2

By configuring the above, you have now manually configured the RIP router to ONLY listen for and accept RIPv2 routing updates.




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