Cisco Configuring IP Unicast Routing User Manual PDF

Information About IP Unicast Routing

Configuring IP Unicast Routing Restrictions for IP Unicast Routing, on page 1 Information About IP Unicast Routing, on page 1 Information About IP Routing, on page 2 Configuration Guidelines for IP Routing, on page 9 How to Configure IP Addressing, on page 10 How to Configure IP Unicast Routing, on page 28 Configuration Example for Enabling IP Routing, on page 29 Monitoring and Maintaining IP Addressing, on page 30 Monitoring and Maintaining the IP Network, on page 30 Feature History for IP Unicast Routing, on page 31 Restrictions for IP Unicast Routing • On enabling IP routing, the VLAN configured as SVI will also learn broadcast ARP requests which are not self destined.

• The switch does not support tunnel interfaces for unicast routed traffic. • The number of routed ports and SVIs that you can configure is 2000. Exceeding the recommended number and volume of features being implemented might impact CPU utilization because of hardware limitations. • Subnetwork Access Protocol (SNAP) address resolution is not supported on this device. Information About IP Unicast Routing This module describes how to configure IP Version 4 (IPv4) unicast routing on the switch.

Note In addition to IPv4 traffic, you can also enable IP Version 6 (IPv6) unicast routing and configure interfaces to forward IPv6 traffic if the switch or switch stack is running the Network Essentials or Network Advantage license. Configuring IP Unicast Routing 1

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Types of Routing

Information About IP Routing In some network environments, VLANs are associated with individual networks or subnetworks. In an IP network, each subnetwork is mapped to an individual VLAN. Configuring VLANs helps control the size of the broadcast domain and keeps local traffic local. However, network devices in different VLANs cannot communicate with one another without a Layer 3 device (router) to route traffic between the VLAN, referred to as inter-VLAN routing. You configure one or more routers to route traffic to the appropriate destination VLAN. Figure 1: Routing Topology Example This figure shows a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20.

The router has an interface in each VLAN. When Host A in VLAN 10 needs to communicate with Host B in VLAN 10, it sends a packet that is addressed to that host. Switch A forwards the packet directly to Host B, without sending it to the router. When Host A sends a packet to Host C in VLAN 20, Switch A forwards the packet to the router, which receives the traffic on the VLAN 10 interface. The router checks the routing table, finds the correct outgoing interface, and forwards the packet on the VLAN 20 interface to Switch B. Switch B receives the packet and forwards it to Host C. Types of Routing Routers and Layer 3 switches can route packets in these ways:

• By using default routing • By using preprogrammed static routes for the traffic Default routing refers to sending traffic with a destination unknown to the router to a default outlet or destination. Static unicast routing forwards packets from predetermined ports through a single path into and out of a network. Static routing is secure and uses little bandwidth, but does not automatically respond to changes in the network, such as link failures, and therefore, might result in unreachable destinations. As networks grow, static routing becomes a labor-intensive liability. Dynamic routing protocols are used by routers to dynamically calculate the best route for forwarding traffic.

There are two types of dynamic routing protocols: • Routers using distance-vector protocols maintain routing tables with distance values of networked resources, and periodically pass these tables to their neighbors. Distance-vector protocols use one or a series of metrics for calculating the best routes. These protocols are easy to configure and use. • Routers using link-state protocols maintain a complex database of network topology, based on the exchange of link-state advertisements (LSAs) between routers. LSAs are triggered by an event in the network, which speeds up the convergence time or time that is required to respond to these changes.

Configuring IP Unicast Routing Information About IP Routing Configuring IP Unicast Routing 2

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IP Routing and Switch Stacks

Link-state protocols respond quickly to topology changes, but require greater bandwidth and more resources than distance-vector protocols. Distance-vector protocols that are supported by the switch are Routing Information Protocol (RIP), which uses a single distance metric (cost) to determine the best path and Border Gateway Protocol (BGP), which adds a path vector mechanism. The switch also supports the Open Shortest Path First (OSPF) link-state protocol and Enhanced IGRP (EIGRP), which adds some link-state routing features to traditional Interior Gateway Routing Protocol (IGRP) to improve efficiency.

Note On a switch or switch stack, the supported protocols are determined by the software running on the active switch. If the active switch is running the Network Essentials license, only default routing, static routing, and RIP are supported. All other routing protocols require the Network Advantage license. IP Routing and Switch Stacks A switch stack appears to the network as a single switch, regardless of which switch in the stack is connected to a routing peer. The active switch performs these functions: • It generates, maintains, and distributes the distributed Cisco Express Forwarding (dCEF) database to all stack members.

The routes are programmed on all switches in the stack bases on this database. • The MAC address of the active switch is used as the router MAC address for the whole stack, and all outside devices use this address to send IP packets to the stack. • All IP packets that require software forwarding or processing go through the CPU of the active switch. Stack members perform these functions: • They act as routing standby switches, ready to take over in case they are elected as the new active switch if the active switch fails. • They program the routes into hardware.

If an active switch fails, the stack detects that the active switch is down, and elects one of the stack members to be the new active switch. During this period, except for a momentary interruption, the hardware continues to forward packets with no active protocols. However, even though the switch stack maintains the hardware identification after a failure, the routing protocols on the router neighbors might flap during the brief interruption before the active switch restarts. Routing protocols such as OSPF and EIGRP need to recognize neighbor transitions.

The router uses two levels of nonstop forwarding (NSF) to detect a switchover, to continue forwarding network traffic, and to recover route information from peer devices: • NSF-aware routers tolerate neighboring router failures. After the neighbor router restarts, an NSF-aware router supplies information about its state and route adjacencies on request. • NSF-capable routers support NSF. When they detect an active switch change, they rebuild routing information from NSF-aware or NSF-capable neighbors and do not wait for a restart. The switch stack supports NSF-capable routing for OSPF and EIGRP. Configuring IP Unicast Routing IP Routing and Switch Stacks Configuring IP Unicast Routing 3

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Classless Routing

Upon election, the new active switch performs these functions: • It starts generating, receiving, and processing routing updates. • It builds routing tables, generates the CEF database, and distributes it to stack members. • It uses its MAC address as the router MAC address. To notify its network peers of the new MAC address, it periodically (every few seconds for 5 minutes) sends a gratuitous ARP reply with the new router MAC address. Note If you configure the persistent MAC address feature on the stack and the active switch changes, the stack MAC address does not change for the configured time period.

If the previous active switch rejoins the stack as a member switch during that time period, the stack MAC address remains the MAC address of the previous active switch. • It attempts to determine the reachability of every proxy ARP entry by sending an ARP request to the proxy ARP IP address and receiving an ARP reply. For each reachable proxy ARP IP address, it generates a gratuitous ARP reply with the new router MAC address. This process is repeated for 5 minutes after a new active switch election. Note When an active switch is running the Network Advantage license, the stack can run all supported protocols, including Enhanced IGRP (EIGRP), and Border Gateway Protocol (BGP).

If the active switch fails and the new elected active switch is running the Network Essentials license, these protocols will no longer run in the stack. Partitioning of the switch stack into two or more stacks might lead to undesirable behavior in the network. Caution If the switch is reloaded, then all the ports on that switch go down and there is a loss of traffic for the interfaces that are involved in routing, despite NSF/SSO capability. Classless Routing By default, classless routing behavior is enabled on the device when it is configured to route.

With classless routing, if a router receives packets for a subnet of a network with no default route, the router forwards the packet to the best supernet route. A supernet consists of contiguous blocks of Class C address spaces that are used to simulate a single, larger address space and is designed to relieve the pressure on the rapidly depleting Class B address space. In the following figure, classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of discarding the packet, the router forwards it to the best supernet route.

If you disable classless routing and a router receives packets that are destined for a subnet of a network with no network default route, the router discards the packet. Configuring IP Unicast Routing Classless Routing Configuring IP Unicast Routing 4

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