3Com Router User Manual

							Routing Management Strategy407
Figure 143   Routing table illustration
3Com routers support not only static route configuration, but also dynamic 
routing protocols such as RIP, OSPF and BGP. Depending on the interface status 
and user configuration, a router can automatically obtain some direct routes 
during their operation.
Routing Management 
Strategy3Com routers support both manual configuration of a static route to a specific 
destination and dynamic routing protocol configuration which finds the route with 
the routing algorithm to interact with other routers in the network. Both static 
routes configured by the user and dynamic routes found by the routing protocol 
are uniformly administered in the router.
Routing Protocol and 
Routing PriorityDifferent routing protocols (including static routes) can find different routes to the 
same destination, but not all these routes are optimal. In fact, at a certain 
moment, the current route to a destination is determined only by a unique routing 
protocol. As a result, every routing protocol (including static route) is assigned a 
priority. When there are multiple route information sources, the route found by 
higher-priority routing protocols become the current route. The routing protocols 
and their default routing priorities (the less the value, the higher the priority) are 
shown in the 
Ta b l e 465.
Here, 0 stands for a directly connected route and 255 stands for any route from 
unknown sources or terminals.
Ta b l e 465   Routing Protocol and Routing Priority
The network of
destination
hostthe message is The router where
transferredThe port to
be passed
10.0.0.0
Direct2
11.0.0.0
Direct1
12.0.0.011.0.0.2  
13.0.0.0
Direct3
14.0.0.0    13.0.0.23
15.0.0.0
10.0.0.22
16.0.0.0  
2
15.0.0.0
16.0.0.0
10.0.0.0
13.0.0.0
12.0.0.0
14.0.0.0
11.0.0.0
R4
R1
3 R2
R6
R5
R7
R8
1
11.0.0.22
12.0.0.1 12.0.0.2
12.0.0.3 14.0.0.114.0.0.2
13.0.0.113.0.0.411.0.0.13 10.0.0.110.0.0.2 16.0.0.3
16.0.0.1
15.0.0.2
15.0.0.1
13.0.0.216.0.0.2
13.0.0.3
Routing table of Router R8
10.0.0.21
R
Routing Protocol or TypeCorresponding Routing Priority
Direct (Connected)0
OSPF10
STATIC60
RIP100
IBGP130 
						

							408CHAPTER 26: IP ROUTING PROTOCOL
Except for the direct route (Connected), the priority of each dynamic routing 
protocol can be manually configured according to specific requirements. In 
addition, each static route can have a different priority.
Support of the Route 
BackupA backup route allows a router to automatically select another route to transmit 
data packets when the line changes, and enhances the user network reliability. To 
implement route backup, you can set a different priority to the multiple routes to 
the same destination. In fact, a user can set the highest priority to the route 
passing the main path, and take turns to reduce the priority to the routes passing 
backup paths. Normally, the router will send data through the main path. When a 
fault occurs on the line, the route will be hidden, and router will select the backup 
route with second-highest priority for data transmission. In this way, the 
switchover from the active interface to the backup interface is implemented. 
When the main path is recovered, the router recovers the route and begins 
reselecting routes. Since the recovered route has the highest priority, it selects this 
main route to transmit data.
Sharing Routes LearnedAs different protocols find different routes due to the various algorithms adopted 
by each protocol, the problem of sharing the findings of different protocols is of 
concern. On 3Com routers, a route learned by a routing protocol can be imported 
to another routing protocol. Each protocol has its own route import mechanism. 
OSPF ASE150
EBGP170
Unknown255
Routing Protocol or Type Corresponding Routing Priority 
						

							27
CONFIGURING STATIC ROUTES 
This chapter covers the following topics:
■Static Route Overview 
■Configuring a Static Route
■Displaying and Debugging the Routing Table
■Static Route Configuration Example 
■Troubleshooting a Static Route Configuration
Static Route Overview A static route is a special route that allows a router to transmit packets over one 
path to a specified destination. Proper setting and application of the static route 
can guarantee network security effectively and at the same time, ensure 
bandwidth for important applications.
If the topology changes due to network failure or other problems, the static route 
cannot change automatically and requires the intervention of administrator.
The static route has the following attributes:
■Reachable route: Normally all routes are reachable and an IP packet is sent to 
the next hop according to the route identified by the destination -- a common 
application of static routes.
■Unreachable route: When a static route to a certain destination has the 
reject attribute, all IP packets to this destination are discarded and 
destination unreachable information is given.
■Black hole route: When a static route to a certain destination has black 
hole attribute, all IP packets to this destination will be discarded.
Here, the attributes reject and blackhole are normally used to control the scope 
of destinations reachable by this router, to facilitate network fault diagnosis.
Default Route
Default route is one type of static route that is used when no matching route is 
found or when there is no suitable route. In the routing table, the default route is 
the route to network 0.0.0.0 (mask is 0.0.0.0). You can check whether the default 
route is properly set through the result of 
display ip routing-table command. 
If the destination address of the message does not match any route item in the 
routing table, the default route is selected. If there is no default route, this 
message will be discarded and an ICMP message will be returned to the source 
terminal, indicating that the destination address or network is unreachable. 
						

							410CHAPTER 27: CONFIGURING STATIC ROUTES 
Default routes are very useful in network. In a typical network with hundreds of 
routers, dynamic routing protocols may consume lots of bandwidth resource. 
Using default route means that you can replace high bandwidth links with 
adequate bandwidth links to meet the requirements of communication for a large 
number of subscribers.
Configuring a Static 
RouteConfiguring static and default routes involves tasks described in the following 
sections: 
■Configuring a Static Route
■Configuring a Default Route
Configuring a Static 
RoutePerform the following configurations in system view.
Ta b l e 466   Configure a Static Route
The explanation of each parameter is as follows:
■IP address and network mask
IP address is shown in dotted decimal format. The 1s in the 32 bit mask must 
be continuous. The mask can also be presented in the dotted decimal format or 
by the mask length, that is, the number of “1”s in the mask. 
■Transmitting interface or next hop address 
In the configuration of static routes, the transmitting interface 
interface-type interface-number or the next hop address 
nexthop-address can be designated as required by the actual conditions.
You can specify the transmitting interfaces in the following cases:
■For interfaces that support resolution from the network address to the link 
layer address (like Ethernet interface supporting ARP), if a host address has 
been specified for Ip-address and mask (or mask-length), and if the destination 
address is in a network directly connected to this interface, then you can 
specify the transmitting interface.
■For a point-to-point type interface, specifying the transmitting interface implies 
specifying the address of next hop. In this case the address of the remote 
interface is considered the address of next hop. If the serial interface is 
encapsulated with the PPP protocol, the IP address of the node on other end 
can be determined through PPP consultation. Then you only need to specify 
the transmitting interface instead of the address of next hop.
When NBMA interfaces like the interface encapsulated with X.25 or frame relay or 
dial-up interface support point-to-multipoint mode, besides configuring the IP 
route, you must also set up the secondary route at the link layer and map from the 
OperationCommand
Configure a static routeip route-static ip-address { mask | mask-length } 
{ interface-type interface-number | 
nexthop-address } [ preference value ] [ reject | 
blackhole ]
Delete a static routeundo ip route-static  { all | ip-address { mask | 
mask-length } [ interface-type interface-number | 
nexthop-address ] [ preference value ] } 
						

							Displaying and Debugging the Routing Table411
IP address to the link layer address (such as dialer route ip, x.25 map ip or 
fr map ip commands, and so on). In this case, you cannot specify the 
transmitting interface for the static route and must configure the IP address of the 
next hop.
Actually, all the route items must mark the address of the next hop. According to 
the destination address of packets, an IP router searches for the matching route in 
the routing table. Only when the address of next hop is specified in the route, can 
the link layer find a corresponding address through this address and transfer 
packets.
However, in certain cases (such as PPP encapsulated in link layer), the address of 
the node on the other end may be unknown when the router is configured so that 
the sending interface has to be specified. In addition, if the sending interface has 
been specified, it is not necessary to change the routers configuration when the 
address of the node connected on the other end is changed
■Preference
Different preference configurations can achieve flexible route management. For 
example, when configuring multiple routes to the same destination, if the 
same preference is designated, load balancing can be realized. If different 
preferences are designated, route standby can be realized.
■Other parameters
The reject and blackhole attributes refer to unreachable routes and black 
hole routes respectively.
Configuring a Default 
RoutePerform the following configurations in system view.
Ta b l e 467   Configure a Default Route
The parameters of this command mean the same as those in static route 
configuration.
Displaying and 
Debugging the 
Routing TableTa b l e 468   Displaying and Debugging the Routing Table
OperationCommand
Configure a default 
routeip route-static 0.0.0.0 { 0.0.0.0 | 0 } { 
interface-type interface-number | nexthop-address 
} [ preference value ] [ reject | blackhole ]
Delete a default routeundo ip route-static 0.0.0.0 { 0.0.0.0 | 0 } [ 
interface-type interface-number | nexthop-address 
] [ preference value ]
OperationCommand
Display the abstract information of the 
routing tabledisplay ip routing-table
Display the information of specific routedisplay ip routing-table ip-address
Display the detailed information of the 
routing tabledisplay ip routing-table verbose
Display the radix information of the 
routing tabledisplay ip routing-table radix
Display the static routing tabledisplay ip routing-table static 
						

							412CHAPTER 27: CONFIGURING STATIC ROUTES 
Static Route 
Configuration 
Example By configuring a status route, any two hosts or routes can communicate with each 
other.
Figure 144   Example of static route configuration
To configure a static route:
1Configure the static route for RouterA:
[RouterA] ip route-static 1.1.4.0 255.255.255.0 1.1.6.2
[RouterA] ip route-static 1.1.5.0 255.255.255.0 1.1.2.2
2Configure the static routes for RouterB:
[RouterB] ip route-static 1.1.5.0 255.255.255.0 1.1.3.1
[RouterB] ip route-static 1.1.1.0 255.255.255.0 1.1.6.1
3Configure the static routes for RouterC:
[RouterC] ip route-static 1.1.1.0 255.255.255.0 1.1.2.1
[RouterC] ip route-static 1.1.4.0 255.255.255.0 1.1.3.2
Troubleshooting a 
Static Route 
ConfigurationThe status of the physical interface and link layer protocol is UP, but IP packets 
cannot be forwarded normally.
Troubleshooting:
■Use the display ip routing-table static command to check whether 
related static routes are configured correctly or not.
■Use the display ip routing-table command to see whether this static route 
is already effective or not.
■Check whether the next hop address is specified or specified correctly on the 
NBMA type interface.
■Check the secondary routing table of the link layer on the NBMA interface to 
see if the configuration is correct.
Host1  1.1.1.1Host2  1.1.4.2
Host3   1.1.5.1
E0
1.1.1.2s0
1.1.2.1s0
1.1.2.2E0
    1.1.5.2
s1
1.1.3.1
s0
1.1.3.2
E0
1.1.4.1 Router C
Router A Router B
s1
1.1.6.1s1
1.1.6.2 
						

							28
CONFIGURING RIP
This chapter covers the following topics:
■RIP Overview
■Configure RIP 
■Displaying and Debugging RIP 
■RIP - Unicast Configuration Example
■Troubleshooting RIP 
RIP OverviewThe Routing Information Protocol (RIP) is an interior gateway and dynamic routing 
protocol based on the Distance-Vector (D-V) routing algorithm. RIP uses User 
Datagram Protocol (UDP) packets to exchange routing information and adopts 
hop count to measure the distance from the destination, called the routing cost. In 
RIP, a hop count that is equal to or larger than 16 is defined as infinity (the 
destination network or host is unreachable) so RIP is generally applied to 
medium-sized networks, such as a campus network. RIP is not designed for 
complicated and large-sized networks.
RIP has two versions, RIP-1 and RIP-2. RIP-2 supports simple text authentication 
and MD5 authentication, as well as the variable-length sub-net masks.
To improve performance and prevent route loops, RIP supports split-horizon, 
poisoned reverse using triggered update. This allows the importation of routes 
that are obtained by other routing protocols.
Each router that runs RIP manages a database that includes route items of all 
reachable routers on the network. A route item includes the following 
information.
■Destination address: The address of the host or network.
■Next-hop address: The address of the next router through which this route 
passes to get to the destination.
■Interface: The interface where messages are forwarded.
■Metric value: The overhead for the router to get to the destination. It is an 
integer ranging from 0 to16.
■Timer: The last time the route item was modified.
■Route tag: The tag indicates whether it is an internal routing protocol route or 
an external routing protocol route. 
						

							414CHAPTER 28: CONFIGURING RIP
The procedure of running RIP can be described as follows:
1When a specific router is starting RIP for the first time, it broadcasts request 
messages to the neighbor routers. After receiving the request messages, the 
neighbor routers respond to the request and return response messages including 
local routing information.
2After receiving the response message, the router modifies the local routing table 
and sends triggered modified messages to the neighboring routers by 
broadcasting the route modification information. After receiving the triggered 
modified message, the neighboring routers forward them to their neighbors. After 
a series of triggered modification broadcasting, all routers can receive and 
maintain the latest routing information.
3At the same time, RIP broadcasts the local routing table to the neighbor routers 
every 30 seconds. The neighbor routers receive the message and maintain the 
local routes. Then they select the best route to broadcast the route modification 
information to their neighbor networks. In this way, the updated routing 
information can be globally effective. Also, RIP applies a timeout mechanism to 
dispose of an outdated route and to make sure that the route is real-time and 
effective.
Though RIP is widely used by most of the router manufacturers, it has limitations:
■It supports a very limited number of routers: RIP is only suitable to small 
autonomous systems, such as most campus networks and local networks with 
simple structure and high continuity.
■The route calculations depend on a fixed metric: RIP cannot update its metric in 
real time to adapt to network changes. The metric defined by an administrator 
remains constant until it is updated artificially.
■It may cost considerable network bandwidth to update its information: RIP 
broadcasts an update message every 30 seconds so it may cause low efficiency 
in a network with a lot of nodes.
Configure RIP Begin all configuration tasks by first enabling the RIP routing process and 
associating a network with an RIP routing process, then configure other functional 
features related to RIP protocol. The task of configuring the interface-related 
features is not subject to whether RIP has been enabled. 
The original interface parameters become invalid after the RIP is closed.
Configuring RIP includes tasks described in the following sections:
■Enabling RIP
■Enabling RIP at the Specified Network
■Defining a Neighboring Router
■Specifying RIP Version
■Configuring Check Zero Field of RIP Version 1
■Specifying the Status of an Interface
■Disabling Host Routes
■Enabling Route Summarization for RIP Version 2
■Configuring RIP-2 Packet Authentication on the Interface 
						

							Configure RIP 415
■Configuring RIP Horizontal Segmentation on the Interface
■Configuring Route Import for RIP
■Specifying Default Route Metric Value for RIP
■Specifying Additional Route Metric Values for RIP
■Setting Route Preference
■Configuring Route Distribution for RIP
■Resetting RIP
Enabling RIPTo enter RIP view, you must first enable RIP, then configure the parameters related 
to the RIP protocol. Interface-related parameters are not subject to enabling of RIP. 
Perform the following configurations in system view.
Ta b l e 469   Enabling RIP
By default, RIP is not enabled.
The parameters related to an interface are also invalid after RIP is turned off.
Enabling RIP at the 
Specified NetworkTo flexibly control RIP operation, you can configure a corresponding network 
segment to RIP network so that RIP messages can be received and transmitted 
through the specified interface.
Perform the following configurations in RIP view.
Ta b l e 470   Enable RIP at the Specified Network
The undo network command is associated with RIP by default after RIP is enabled.
After enabling RIP, you must specify a list of networks with the RIP, since RIP works 
only on the interface of specified network segment. RIP wont receive or forward a 
route on interfaces of non-specified network segments, and it functions as if these 
interfaces do not exist. The 
network-number attribute specifies the address of the 
enabled or disabled network or it can designate the network address of the 
interfaces.
When the network command is used for a specified address, the interface of the 
network segment of this address is enabled. For example:
 network 129.102.1.1, 
use either the 
display current-configuration or the display rip command, 
to see network 129.102.0.0.
OperationCommand
Enable RIP and enter the RIP viewrip
Disable RIPundo rip
OperationCommand
Specify a list of networks associated with 
RIPnetwork { network-number | all }
Delete a list of networks associated with 
RIPundo network { network-number | all } 
						

							416CHAPTER 28: CONFIGURING RIP
Defining a Neighboring 
Route
r
RIP is a broadcast protocol. It exchanges routing information with 
non-broadcasting networks in unicast mode.
Perform the following configurations in RIP view.
Ta b l e 471   Define a Neighboring Router
By default, no neighboring routers are defined.
Normally, this command is not recommended because the node on the other end 
does not need to receive two identical packets at the same time. Also when a peer 
sends messages, it is also subject to the restrictions of such commands as 
rip 
work
, rip output, rip input and network. 
Specifying RIP VersionRIP-2 has two sending modes, broadcasting and multicasting, with message 
multicasting as the default mode. The multicast address in RIP-2 is 224.0.0.9. The 
advantage of multicasting is that the host not running RIP in the network does not 
receive RIP broadcast messages. In addition, message multicasting can also prevent 
the host running RIP-1 from incorrectly receiving and processing the routes with 
subnet mask in RIP-2.
When RIP-1 is running on the interface, the interface receives and transmits the 
broadcast packets of RIP-1 and RIP-2 but does not receive RIP-2 multicast 
messages. When RIP-2 is running on the interface, the interface can receive and 
transmit RIP-1 and RIP-2 broadcast packets but cannot receive RIP-2 multicast 
packets. When the interface runs in RIP-2 multicast mode, it receives and transmits 
the RIP-2 multicast packets and does not receive the RIP-1 and RIP-2 broadcast 
packets.
Perform the following configurations in interface view.
Ta b l e 472   Specify RIP Version
By default, the interface runs RIP-1.
Configuring Check Zero 
Field of RIP Version 1The check zero command is used by the router to validate the version of the RIP 
Version 1 message. RFC 1058 stipulates that the ZERO FIELD in the RIP Version 1 
header must be set to zero. If the 
checkzero parameter is set and the router 
receives a message with the zero field not 0, the router will discard the RIP 
message because it is the wrong version. 
OperationCommand
Define a neighboring routerpeer ip-address
Cancel exchanging routing information 
with a neighboring router.undo peer ip-address
OperationCommand
Configure the interface to run RIP-1rip version 1
Configure the interface to run RIP-2rip version 2 [ broadcast | multicast 
]
Restore the default RIP version run on the 
interfaceundo rip version