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