IP route summarization is used to make networks more flexible and efficient. Although some routing protocols such as RIPv1 and IGRP summarize only at the boundaries of major network numbers, others support route summarization (aggregation) at any bit boundary. Variable-length subnet masks enable routing protocols to summarize on bit boundaries. The following are the advantages to summarizing addresses into a hierarchy:
1. Reduces the amount of information stored in routing tables - Without summarization, a router needs to process every single route in the network. With summarization, routers can condense network addresses down to a single link advertisement, resulting in a reduction in both the resource load on the router and the overall network complexity. Route summarization is most effective in large networks.
2. Allocates an existing pool of addresses more economically - The available IP addresses are limited. Route summarization ensures that IP addresses are utilized efficiently.
3. Makes the routing process more efficient - With less overhead, routers are faster and more efficient.
4. Lowers the network convergence time - The network convergence time would reduce with route summarization.
5. Isolates topology changes - If any individual route changes, the change would be localized. The summary address may remain the same, thus saving unnecessary updates over the network.
6. Facilitates monitoring, reporting, and troubleshooting - A hierarchical address space is relatively easy to monitor and troubleshoot.
The reduction in route propagation and routing information overhead can be significant. Take a sample network of 172.16.1.0 /24. Without summarization, each router in a large enterprise network of 250 subnets (28 = 256 subnets with 28 - 2 = 254 hosts each) would need to know about 250 routes. With route summarization, you can quickly reduce the size of the routing tables by almost 75%. If the 172.16.0.0 Class B network used 7 bits of subnet address space (/23) instead of 8 bits (/24), the original 250 subnets could be broken up into two major subnetworks of about 125 each. Each router would still need to know all the routes for each subnet in its network number. However, that number would be reduced to 125 routes plus one additional route for the other major network. This process of collapsing many subnet routes into a single network route is a fundamental goal of route summarization.
Route Summarization: Route summarization means summarizing a group of routes into a single route advertisement. The net result of route summarization, and its most obvious benefit, is a reduction in the size of routing tables on the network. This in turn reduces the latency associated with each router hop since the average speed for routing table lookup will be increased due to the reduced number of entries. The routing protocol overhead can also be significantly reduced since fewer routing entries are being advertised. This can become critical as the overall network (and hence the number of subnets) grows.
IP Packets are transported from source network to the destination network by what is known as routing. Hop-by-hop routing model is used by the Internet for delivery of packets. At each hop, the destination IP address is examined, the best next hop is determined by the routing protocol (such as RIP, OSPF or BGP) and the packet is forwarded by one more hop through this route. The same process takes place at the next hop. During this process, the logical addresses remain same. In an IP network, the logical addresses are IP addresses. The hardware interface addresses, such as MAC address change with each hop.
Policy-based routing is applied to incoming packets. All packets received on an interface with policy-based routing enabled are considered for policy-based routing. The router passes the packets through enhanced packet filters called route maps. Based on the criteria defined in the route maps, packets are forwarded/routed to the appropriate next hop.
1. show policy -map control-plane: Verifies the applied policy map, the class maps in the order in which they will be applied, the match conditions of the class maps, and the policies that are applied to the traffic that is matched
2. show class-map: Displays all configured class maps on the router
3. show access-list: Displays all configured ACLs on the router
4. show policy-map: Displays all configured policy maps on the router
You can view policy-based decisions by enabling debugging functionality on policy-based routing with the command debug ip policy
ip vrf vrf-name: A global command that creates a VRF and enters VRF configuration mode.
ip vrf forwarding vrf-name: Interface or subinterface configuration command that assigns an interface or a subinterface to a VRF instance.
(Note: If the interface or subinterface already had an IP address assigned, this command will remove that address, and you will need to add it back.)
To configure a static router for a VRF, run the following:
Router(config)# ip route vrf <vrf-name> <network> <mask> {interface | next-hop}
