Switch User Manual

190 IP Routing Configuration and Management
A mismatch in maximum transmission unit (MTU) sizes between the routers
usually causes this type of problem. For example, one router could be set
for a high MTU size and the other router a smaller value. Depending on the
size of the link state database, the router with the smaller value may not be
able to process the larger packets and thus be stuck in this state. To avoid
this problem, ensure that the MTU size value for both routers match. This
problem is usually encountered during interoperations in networks with
other vendor devices.
Note: The Nortel Ethernet Routing Switch 5500 Series automatically
checks for OSPF MTU mismatches.
In the Nortel Ethernet Routing Switch 5500 Series, the supported MTU size
for OSPF is 1500 bytes by default. Incoming OSPF database description
(DBD) packets are dropped if their MTU size is greater than this value.
Virtual Router Redundancy Protocol (VRRP) configuration
This section describes how to create a basic VRRP configuration on a
Nortel Ethernet Routing Switch 5500 Series.
VRRP uses an election process to select a master router that hosts use as
the default gateway. If the master router (the default gateway) fails, the
VRRP backup router automatically replaces the master router and becomes
the new default gateway. In either case, the default gateway IP address and
MAC address does not change, thereby providing transparent operation.
The Nortel Ethernet Routing Switch 5500 Series can be configured in
a master-master configuration for load-balancing applications that use
Split MultiLink Tr unking (SMLT). This configuration allows both switches to
respond to ARPs and forward traffic.
VRRP Priority settings can be configured to select the VRRP master router
for a specified VLAN. The VRRP Priority setting is an integer value, in the
range 1 and 255, where the highest value is used to elect the VRRP master
router. If two or more switches have the same priority value, the switch with
the highest numerical IP address value is selected and becomes the VRRP
master. The host is unaware of the entire process.
When a host sends traffic to a different subnet, it sends an ARP request for
the MAC address of the default gateway. In this case, the Nortel Ethernet
Routing Switch 5500 Series VRRP master router replies with its virtual
MAC address. The benefit of using a virtual MAC address is that, if the
master router fails, the VRRP backup router uses the same virtual MAC
address. The virtual MAC address on the Nortel Ethernet Routing Switch
5500 Ser ies does not need to be configured. The virtual MAC address is
automatically set as:
Nortel Ethernet Routing Switch 5500 Series
Configuration-IP Routing Protocols
NN47200-503 03.01 Standard
5.1 27 August 2007
Copyright © 2005-2007, Nortel Networks
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