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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 42 Configuring IP Unicast Routing
Understanding IP Routing
• The MAC address of the stack master 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 stack
master.
Stack members perform these functions:
• They act as routing standby switches, ready to take over in case they are elected as the new stack
master if the stack master fails.
• They program the routes into hardware. The routes programmed by the stack members are the same
that are downloaded by the stack master as part of the dCEF database.
If a stack master fails, the stack detects that the stack master is down and elects one of the stack members
to be the new stack master. 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 stack master 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 a stack master 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. For more information, see the
“OSPF NSF Capability” section on page 42-28 and the “EIGRP NSF Capability” section on page 42-39.
Upon election, the new stack master 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 stack master
changes, the stack MAC address does not change for the configured time period. If the
previous stack master rejoins the stack as a member switch during that time period, the stack
MAC address remains the MAC address of the previous stack master. See the “Enabling
Persistent MAC Address” section on page 5-20.
• 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 stack master election.
Note When a stack master is running the IP services feature set, the stack can to run all supported protocols,
including Open Shortest Path First (OSPF), Enhanced IGRP (EIGRP), and Border Gateway Protocol
(BGP). If the stack master fails and the new elected stack master is running the IP base feature set, these
protocols will no longer run in the stack.