Table of Contents
- About the Author
Chapter 1. Internet Protocol
- Basic Internet Protocol
- Variable-Length Subnet Masks (VLSM)
- Summarization and How to Configure Summarization
- IP Helper Address
- Scenario 1-1: Configuring a Cisco Router for IP
- Scenario 1-2: Efficiently Configuring a Network for IP
- Scenario 1-3: Configuring IP VLSM for a Large Network
- Scenario 1-4: Summarization with EIGRP and OSPF
- Scenario 1-5: Configuring IP Helper Address
- Practical Exercise: IP
- Review Questions
- Chapter 2. Routing Principles
- Chapter 3. Basic Open Shortest Path First
- Chapter 4. Advanced OSPF and Integrated Intermediate System-to-Intermediate System
- Chapter 5. Enhanced Interior Gateway Routing Protocol
- Chapter 6. Basic Border Gateway Protocol
- Chapter 7. Advanced BGP
- Chapter 8. Route Redistribution and Optimization
- Chapter 9. CCNP Routing Self-Study Lab
- A. Study Tips
- B. What to Do After CCNP?
- C. Answers to Review Questions
- D. CCIE Preparation—Sample Multiprotocol Lab
Scenario 1-4: Summarization with EIGRP and OSPF
In this scenario, given the address ranges in Table 1-9, you see how to configure summarization with EIGRP and OSPF.
Table 1-9 displays the IP address ranges to be summarized, as well as the binary representation of the third octet or the subnet port of the IP address space.
Table 1-9. IP Address Ranges
|IP Subnet||Subnet Mask||Binary Representation of Third Octet|
Before configuring EIGRP or OSPF summarization, you first need to decide whether summarization is possible at all. Table 1-9 displays 16 subnets, numbered from 1-16. The first 15 subnets all have one thing in common when viewed in binary: The first four bits or high-order bits are always 0. Therefore, you can summarize the first 15 networks using the subnet mask 255.255.255.240. (240 in binary is 1111000 where the first four bits are common.) The last four bits contains the networks 1 to 15 or in binary encompass all networks from 0000 to 1111.
The last remaining subnet 184.108.40.206 is the odd network out. Although it is contiguous, you cannot summarize it along with the first 15 network, because any summary address range encompasses networks beyond 220.127.116.11, which may reside in other parts of the network.
Configure EIGRP to summarize these routes out of a serial port (serial 0/0 in this example). Example 1-9 displays the configuration required to disable automatic summarization and the two required summary address commands on the serial 0/0 on a router named R1.
Example 1-9. EIGRP Summary
R1(config)#router eigrp 1 R1(config-router)# no auto-summary R1(config)#interface serial 0/0 R1(config-if)# ip summary-address eigrp 1 18.104.22.168 255.255.255.240 R1(config-if)# ip summary-address eigrp 1 22.214.171.124 255.255.255.0
In Example 1-9, the router R1 sends only two updates: one for the networks ranging from 126.96.36.199 to 188.8.131.52 and another for 184.108.40.206. These two are instead of 16 separate IP route entries. Even in a small scenario like this, you saved 14 IP route entries. Reducing IP routing tables means when a router performs a routing table search, the time it takes to determine the outbound interface is reduced allowing end-user data to be sent faster over a given medium.
With OSPF, you do not need to disable automatic summarization, because OSPF does not automatically summarize IP subnets. Hence, to summarize the same block of addresses of a router (OSPF ABR), you apply two commands under the OSPF process. Example 1-10 displays the summary commands required.
Example 1-10. OSPF Summary
R1(config)#router ospf 1 R1(config-router)#no area 1 range 220.127.116.11 255.255.255.240 R1(config-router)#area 1 range 18.104.22.168 255.255.255.0