Jaycee's Networking

May 13, 2009

IP Routing Overview 1/2

Filed under: IOS — Tags: — Jaycee @ 12:31 am

IP routing works by comparing the destination addresses of IP packets to a list of possible destinations called the routing table. The destination address in a packet usually identifies a single host, which is called unicast routing.

1. A router needs information about how to reach various parts of the network. It can get this information through static routing or by exchanging routing information with other routers, which uses routing protocols.

2. An autonomous system is a collection of routers that is under the control of one organization.

a. OSPF uses a unique number called a process ID to identify the routing process on the router.

b. RIP doesn’t use AS numbers because it shares routes with any neighboring RIP router that will listen.

A. Subnet:

A subnet is a summary address representing a group of adjacent hosts. There are two different sets of rules for how groups of subnets can be summarized together: class and classless. (Classless is often referred to by CIDR (Classless Inter-Domain Routing).

You can turn on CIDR in Cisco routers with the global configuration command “ip classless“, which is on by default.

The biggest difference between classful and classless addressing: Classful addressing assumes that the first few bits of the address tell you how big the network is.

Class Range of network addresses Range in Binary Mask CIDR suffixMask bits
A 00000000-01111111 /8 8
B 10000000-10111111 /16 16
C 11000000-11011111 /24 24
D 11100000-11101111 not defined /4 32
E 11110000-11111111 not defined /4 32

Special Range

Router decides where to send a packet by comparing the destination address in the header of the IP packet with its routing table. This is the Longest Match Rule: router MUST always use the most specific match in the table, which is the entry has the most bits in its netmask.

Classless routing can use a mask of any length when looking for the best route to a destination, but classful routing cannot. For example, CIDR would allow the 4 networks,, and to be written as But a router using classful routing would not consider the destination address to be a part of because it knows that anything beginning with 192 must be a Class C network. If there was no specific route for or a subnet containing this destination, the router would skip straight to the default route.

B. Summary Routes:

CIDR allows more levels of route summarization, you can simplify your routing tables so they take up less memory in the routers. This can improve network performance.

Router will keep its summary route as long as any of its subnets exist. This means the summary route is as stable as the most stable route in the summarized range. Without summarization, if there is one route that repeatedly flaps up and down, the routing protocol must propagate every transition throughout the network.

A summary route can hide this instability from the rest of the network. The routing protocol doesn’t need to waste resources installing and removing the flapping route, which improves overall network stability.

C.”show ip route summary” command:

show ip route summary” command give useful information about the size of the routing table and how much memory the router has allocated to storing this information by breaking down each routing protocol.

sh ip route sum

1. A convenient way to estimate the routing table’s memory requirements. This can help you decide if you need route filtering or summarization mechanisms. Routers exchanging BGP routing information with the public Internet can have particularly serious memory utilization problems.

2. It shows how many routes are learned by each mechanism. You can easily check the stability of the routing table by seeing whether this number changes in time.

3. Easily see whether the routing table is getting the expect information. It’s a very quick and easy way to check if the router is installing floating static routes or external routes in its routing table.


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