Summary Routes
Sections:
Use Case Scenarios:
- When there are contiguous IP subnets/networks
- To optimize routing in large networks
- To limit the information propagation of dynamic routing changes effecting convergence times (More on this in the Dynamic Routing Protocol section)
Overview:
- Summary routes are used to simplify routing tables and improve routing efficiency
- Summary routes achieve this by aggregating multiple more specific routes into a broader single route
- Primary purpose of summary routes is to reduce the size of the routing table and minimize the overhead of the router when making forwarding decisions
- In this lab scenario, we will define a summary route on R3 towards the private networks of 10.1.1.0/24 and 10.2.2.0/24 off of R1
Lab Topology
Configuration
Without Summary Routes: R1
Notes:
- From R3, we have 2 separate static route entries towards each private network off of R1
- In a real world scenario, routers will consist of hundreds of routes hence why summarization is essential to reduce the size of the routing table for performance, efficiency, and simplicity
With Summary Routes: R1
Notes:
- From R3, I have removed the 2 separate static route entries and configured only the one summary static route entry that matches the 10.1.1.0/24 and 10.2.2.0/24 networks reducing the size of the routing table
- Lets look closer at the ip route command: 'ip route 10.0.0.0 255.0.0.0 108.23.0.1'
- The subnet mask of the destination network '255.0.0.0' is the key factor in creating the summary route
- The subnet mask splits an IP address into two parts: the network portion and the host portion
- The network portion of an IP address represents the network to which the hosts belong to
- The host portion represents the number of hosts within the network
- Converting IP to Binary
- As an example let's break down the IP and subnet mask into binary
- IP: 10.0.0.0 - 00001010.00000000.00000000.00000000
- Subnet Mask: 255.0.0.0 - 11111111.00000000.00000000.00000000
- As an example let's break down the IP and subnet mask into binary
- Identifying Network and Host Portion
- The 1s in a subnet mask represent the network portion of an IP
- In this example, the first 8 bits of the IP address are used for the network portion = [10].0.0.0
- Note: Each IP address consists of a total of 32 bits with each of the 4 octets (x.x.x.x) consisting of 8 bits each
- The 0s in a subnet mask represent the host portion of an IP
- In this example, the remaining 24 bits of the IP address are used for the host portion = 10.[0.0.0]
- The 1s in a subnet mask represent the network portion of an IP
- Summarize
- After this calculation, the private networks of 10.1.1.0/24 and 10.2.2.0/24 off of R1 match the summary route of 10.0.0.0/8
Reachability
Scenario: In this scenario, we will test out full reachability between hosts PC1 and PC3 using ICMP ping using the new summary route we configured on R3.
Notes:
- From the ping results, PC3 is able to communicate with PC1
- Using traceroute, PC3 takes the direct path towards PC1 via R3 > R2 > R1 > PC1
- To bring the Summary Route concept home, a great way to verify the a route the router chooses is to specify it a destination IP address in the command 'show ip route <destination-IP>'
- As you can see, both IPs of 10.1.1.2 and 10.2.2.2 from the private subnets off of R1 will have R3 choose the summary route 10.0.0.0/8
