Allied Telesis AT-S63 - Manual (Page 557)

AT-S63 Management Software Command Line Interface User’s Guide
Section VII: Internet Protocol Routing 557
The ECMP feature can be enabled and disabled on the switch. The
operating status of ECMP does not affect the switch’s ability to store
multiple routes to the same destination in its routing table. Rather, it
controls how many of the available routes the switch can use to route
packets to the same remote destination. When ECMP is enabled, the
default setting, the switch can use multiple routes to route packets to the
same remote destination, as explained above. When ECMP is disabled,
the switch selects only one route, based on preference value, metric value,
or age, to route packets to a remote subnet even when there are multiple
routes to the subnet.
A local subnet or directly connected network of a switch is usually
represented just once in the routing table by its routing interface. However,
in some situations a local subnet might have several routes to it if it is also
remotely reachable through other routing interfaces on the switch via other
routers. One of the routes would be the subnet’s routing interface and the
others could be RIP and static routes. Here, even if ECMP is enabled, the
switch uses only the routing interface to route packets to the local subnet,
because the routing interface is always the preferred route to a local
subnet. Any RIP or static routes to the local subnet are held in the standby
mode for fail-over protection. They are only used when the status of the
routing interface to the local subnet is down.
Routing Table
The switch maintains its routing information in a table of routes that tells
the switch how to find a local or remote destination. Each route is uniquely
identified in the table by its IP address, network mask, next hop, protocol,
and routing interface.
When the switch receives an IPv4 packet, it scans the routing table to find
the most specific route to the destination on an “up” interface where there
is at least one active port in the VLAN. If the switch does not find a direct
route to the remote destination and no default route exists, the switch
discards the packet and sends an ICMP message to that effect back to the
source.
The switch transmits its routing table every thirty seconds from those
interfaces that have RIP. The RIP timer is not adjustable. The switch also
transmits its routing table and resets the timer to zero whenever there is a
change to the table to ensure that the neighboring routers are immediately
informed of updates to the table.
Dynamic RIP routes are removed from the table when they are not kept up
to date (refreshed) by the neighboring routers. The metric of a route that is
not refreshed is increased to 16 to indicate an unreachable network. If the
route is not updated after 180 seconds, it is deleted from the table.