PowerConnect B-Series FCX Configuration Guide 80953-1002266-01Configuring IP parameters – Layer 3 Switches 26Path MTU discovery (RFC 1191) supportWhen the Dell PowerConnect 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 DellPowerConnect device returns an ICMP Destination Unreachable message to the source of thepacket, with the Code indicating "fragmentation needed and DF set". The ICMP DestinationUnreachable message includes the MTU of the outbound interface. The source host can use thisinformation to help determine the maximum MTU of a path to a destination.RFC 1191 is supported on all interfaces.Changing the router IDIn most configurations, a Layer 3 Switch has multiple IP addresses, usually configured on differentinterfaces. As a result, a Layer 3 Switch identity to other devices varies depending on the interfaceto 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 theIP addresses configured on the Layer 3 Switch, regardless of the interfaces that connect the Layer3 Switches. This IP address is the router ID.NOTERouting Information Protocol (RIP) does not use the router ID.NOTEIf you change the router ID, all current BGP4 sessions are cleared.By default, the router ID on a Layer 3 Switch is one of the following:• If the router has loopback interfaces, the default router ID is the IP address configured on thelowest numbered loopback interface configured on the Layer 3 Switch. For example, if youconfigure 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 lowestnumbered 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 inuse on another device in the network.NOTELayer 3 Switches use the same router ID for both OSPF and BGP4. If the router is already configuredfor OSPF, you may want to use the router ID that is already in use on the router rather than set a newone. To display the router ID, enter the show ip command at any CLI level or select the IP->Generallinks from the Configure tree in the Web Management Interface.To change the router ID, enter a command such as the following.PowerConnect(config)# ip router-id 209.157.22.26Syntax: ip router-id The can be any valid, unique IP address.
