Configuration Guide User guide

976 FastIron Configuration Guide
53-1002494-02
Configuring IP parameters – Layer 3 Switches
The <num> parameter specifies the MTU. Ethernet II packets can hold IP packets from 576 through
1500 bytes long. If jumbo mode is enabled, Ethernet II packets can hold IP packets up to 10,240
bytes long. Ethernet SNAP packets can hold IP packets from 576 through 1492 bytes long. If jumbo
mode is enabled, SNAP packets can hold IP packets up to 10,240 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
FastIron X Series devices support the path MTU discovery method described in RFC 1191. When
the Brocade device receives an IP packet that has its Do not Fragment (DF) bit set, and the packet
size is greater than the MTU value of the outbound interface, then the Brocade 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 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 Brocade 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
Brocade 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 command at any CLI level or select
the IP->General links from the Configure tree in the Web Management Interface.