Cisco Certified Support Technician (CCST) NetworkingIP ConnectivityHard
A network technician is configuring route summarization for an OSPF network. The goal is to advertise a single summary route for the networks 172.16.1.0/24, 172.16.2.0/24, 172.16.3.0/24, and 172.16.4.0/24 from Area 1 into Area 0. What is the most optimal summary route that can be configured?
- A172.16.0.0/21
- B172.16.1.0/21
- C172.16.1.0/20
- D172.16.0.0/22
Show answer & explanationAnswer & explanation
Correct answer: A. 172.16.0.0/21
To find the optimal summary route, convert the network addresses to binary and find the longest common prefix. 172.16.1.0/24 is 10101100.00010000.00000001.00000000. 172.16.2.0/24 is 10101100.00010000.00000010.00000000. 172.16.3.0/24 is 10101100.00010000.00000011.00000000. 172.16.4.0/24 is 10101100.00010000.00000100.00000000. The common prefix is 10101100.00010000.00000 (21 bits). This translates to 172.16.0.0/21.
Why the other options are wrong
- B. 172.16.1.0/21 is an incorrect network address for the summary; the summary must start at the lowest possible network address.
- C. 172.16.1.0/20 is also an incorrect network address and too broad; the summary would be 172.16.0.0/20 to include 172.16.1.0/24, but /20 is less optimal than /21.
- D. 172.16.0.0/22 (binary 10101100.00010000.000000xx.x) includes 172.16.0.0 through 172.16.3.255, missing 172.16.4.0/24.
Route Summarization (CIDR)
Route summarization, or CIDR (Classless Inter-Domain Routing), combines multiple contiguous subnetworks into a single, less specific route advertisement, reducing routing table size and improving routing efficiency.
- Reduces the number of entries in routing tables.
- Improves routing stability by isolating routing changes.
- Requires contiguous IP address blocks.
- Calculated by finding the longest common binary prefix.
Memory trick: Summarization is like putting many small boxes into one big box.