Changing The Router Id - Foundry Networks FESX Manual

The <num> parameter specifies the MTU. Ethernet II packets can hold IP packets from 576 – 1500 bytes long. If
jumbo mode is enabled, Ethernet II packets can hold IP packets up to 9216 bytes long. Ethernet SNAP packets
can hold IP packets from 576 – 1492 bytes long. If jumbo mode is enabled, SNAP packets can hold IP packets up
to 9216 bytes long. The default MTU for Ethernet II packets is 1500. The default MTU for SNAP packets is 1492.
Path MTU Discovery (RFC 1191) Support
Foundry devices support the path MTU discovery method described in RFC 1191. When the Foundry device
receives an IP packet that has its Don't Fragment (DF) bit set, and the packet size is greater than the MTU value of
the outbound interface, then the Foundry device returns an ICMP Destination Unreachable message to the source
of the packet, with the Code indicating "fragmentation needed and DF set". The ICMP Destination Unreachable
message includes the MTU of the outbound interface. The source host can use this information to help determine
the maximum MTU of a path to a destination.
RFC 1191 is supported on all interfaces.

Changing the Router ID

In most configurations, a Layer 3 Switch has multiple IP addresses, usually configured on different interfaces. As
a result, a Layer 3 Switch's identity to other devices varies depending on the interface to which the other device is
attached. Some routing protocols, including Open Shortest Path First (OSPF) and Border Gateway Protocol
version 4 (BGP4), identify a Layer 3 Switch by just one of the IP addresses configured on the Layer 3 Switch,
regardless of the interfaces that connect the Layer 3 Switches. This IP address is the router ID.
NOTE: Routing Information Protocol (RIP) does not use the router ID.
NOTE: If you change the router ID, all current BGP4 sessions are cleared.
By default, the router ID on a Foundry Layer 3 Switch is one of the following:
• If the router has loopback interfaces, the default router ID is the IP address configured on the lowest
numbered loopback interface configured on the Layer 3 Switch. For example, if you configure loopback
interfaces 1, 2, and 3 as follows, the default router ID is 9.9.9.9/24:
• Loopback interface 1, 9.9.9.9/24
• Loopback interface 2, 4.4.4.4/24
• Loopback interface 3, 1.1.1.1/24
• If the device does not have any loopback interfaces, the default router ID is the lowest numbered IP interface
configured on the device.
If you prefer, you can explicitly set the router ID to any valid IP address. The IP address cannot be in use on
another device in the network.
NOTE: Foundry Layer 3 Switches use the same router ID for both OSPF and BGP4. If the router is already
configured for OSPF, you may want to use the router ID that is already in use on the router rather than set a new
one. To display the router ID, enter the show ip CLI command at any CLI level or select the IP->General links
from the Configure tree in the Web management interface.
To change the router ID, enter a command such as the following:
FESX424 Router(config)# ip router-id 209.157.22.26
Syntax: ip router-id <ip-addr>
The <ip-addr> can be any valid, unique IP address.
NOTE: You can specify an IP address used for an interface on the Foundry Layer 3 Switch, but do not specify an
IP address in use by another device.
December 2005 © Foundry Networks, Inc.
Configuring IP
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