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