- Enterasys Security Router User's Guide

IP Routing Protocols
XSR User’s Guide 5-17
OSPF Troubleshooting
XSR commands provide debugging of OSPF Version 2 control information including:
• Monitoring specific OSPF events from the CLI with
show ip ospf (with debugging enabled)
• Control Packets with
debug ip ospf packet
• LSA transmissions/receptions with debug ip ospf lsas
• Neighbor Events with debug ip ospf nbr
• Designated Router Events with debug ip ospf dr
Be aware that only one CLI debug session is permitted at a time.
Null Interface
The XSR provides a non-configurable null interface (null 0) so that discard routes can be
installed for OSPF LSA Type 3 and 5 route summarization. The null interface acts as a “bit bucket”
to dispose of unwanted packets while avoiding the problem of looping.
The null interface is unlike any other XSR port in that it does not need an IP interface to be enabled
- it is always up, it cannot be deleted, and it appears only when specifically referenced by the
show
ip interface null0
or show interface null0 commands. Other show commands do not
display the port. Additionally, the null interface is characterized by the following:
• It can be specified in the
ip route command as the next hop:
ip route 7.0.0.0 255.0.0.0 null0
• Routes which are unreachable through null0 cannot be redistributed
• It responds to the
interfaces.ifTable.ifEntry SNMP query with the value 0 or as
follows:
interfaces.ifTable.ifEntry.ifDescr.7 = Null0
Refer to the XSR CLI Reference Guide for more information on commands.
Route Preference
Route preference or administrative distance is a tool to decide which route to install in the global
routing table. Each routing protocol presents the global Routing Table Manager (RTM) with its
best selection of a route. If several routes to the same destination are offered (derived from
different protocols), installation occurs based on the protocol’s administrative distance. The route
for the protocol with the lowest distance is chosen from the table. The feature functions as follows:
• Administrative distance is configurable per protocol using the
distance command. Static
routes remain configurable per route.
• OSPF distance can be set for different values on intra-area, inter-area, and external routes.
• OSPF distance must follow this rule: Intra-area distance is less than inter-area distance which
is less than external distance.
• Static route distance can be set using the
ip route command. If distance is not specified, the
default distance is applied.
• Aggregation is not accepted in order to set the distance for several static routes at one time.
• Configuring the same distance across different protocols is permitted except for multiple
static routes. Tie breaks are resolved by the following default administrative distances:
– Connected routes: 0