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
It initializes and configures
the routing protocols.
It sends routing protocol
messages and updates to other routers.
It processes routing protocol
messages and updates received from peer routers.
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
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
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
If a 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.
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.
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
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.
Partitioning of the switch
stack into two or more stacks might lead to undesirable behavior in the
If the switch is
reloaded, then all the ports on that switch go down and there is a loss of
traffic for the interfaces involved in routing.