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Configuration Guide - IP Routing 01

NE05E and NE08E V300R003C10SPC500

This is NE05E and NE08E V300R003C10SPC500 Configuration Guide - IP Routing
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Preventing Routing Loops

Preventing Routing Loops

RIP is based on the DV algorithm. RIP devices advertise their routing tables to their neighbors, and therefore, routing loops may occur.

Usage Scenario

RIP is a distance vector routing protocol.

RIP prevents routing loops through the following mechanisms:
  • Counting to infinity: RIP defines the cost 16 as infinity. If the cost of a route reaches 16 due to a routing loop, this route is considered unreachable.
  • Split horizon: Split horizon prevents a RIP-enabled interface from sending back the routes it learns, which reduces bandwidth consumption and prevents routing loops.
  • Poison reverse: Poison reverse allows a RIP-enabled interface to set the cost of the route that it learns from a neighbor to 16 (indicating that the route is unreachable) and then send the route back. After receiving this route, the neighbor deletes the useless route from its routing table, which prevents loops.
  • Suppression timers: Suppression timers can prevent routing loops and reduce the possibility of resulting in incorrect routing information due to the receiving of incorrect routes.
  • Disabling an interface from receiving and sending RIP packets: Similar to split horizon or poison reverse, this function filters out unreliable IP routes. However, routing information on the network may be incorrect because neighbors cannot receive packets from the local NE.
NOTE:

Counting to infinity is a basic feature of RIP, and therefore, it does not need to be configured. Split horizon and poison reverse, however, need to be configured. When both split horizon and poison reverse are configured, only poison reverse takes effect.

For details about these features, see the NE deviceMid-End RouterFeature Description- RIP.

Pre-configuration Tasks

Before configuring RIP to prevent routing loops on the network, complete the following tasks:

Configuration Procedure

Perform one or more of the following configurations as required.

Configuring Split Horizon

You can configure split horizon to prevent routing loops.

Procedure

  1. Run system-view

    The system view is displayed.

  2. Run interface interface-type interface-number

    The interface view is displayed.

  3. Run rip split-horizon

    Split horizon is enabled.

  4. Run commit

    The configuration is committed.

Follow-up Procedure

If both split horizon and poison reverse are configured, only poison reverse takes effect.

If an interface is connected to a Non Broadcast Multiple Access (NBMA) network, by default, split horizon is disabled on this interface.

Configuring Poison Reverse

You can configure poison reverse to prevent routing loops.

Procedure

  1. Run system-view

    The system view is displayed.

  2. Run interface interface-type interface-number

    The interface view is displayed.

  3. Run rip poison-reverse

    Poison reverse is enabled.

  4. Run commit

    The configuration is committed.

Follow-up Procedure

If both split horizon and poison reverse are configured, only poison reverse takes effect.

On a Non Broadcast Multiple Access (NBMA) network, poison reverse is enabled on interfaces by default.

Configuring Suppression Timers

Suppression timers can prevent routing loops and reduce the possibility of generating incorrect routing information due to the receiving of incorrect routes.

Context

When hop count of a route increases, a device starts suppression timers and accepts the Update packet of this route and updates the routing table until the suppression timers expire.

Suppression timers delays the addition of incorrect routes to the routing table and slows down route convergence on the entire network as well. Therefore, exercise caution when configuring the suppression timers.

Procedure

  1. Run system-view

    The system view is displayed.

  2. Run rip process-id

    A RIP process is created, and the RIP view is displayed.

  3. Run timers rip update age suppress garbage-collect

    The suppression timers are set.

    By default, the suppress timer is 0s.

  4. Run commit

    The configuration is committed.

Follow-up Procedure

RIP has four timers: update, age, suppress, and garbage-collect. The value of update is less than that of age, and the value of suppress is less than that of garbage-collect. Setting improper values for the timers affects RIP convergence speed and even causes route flapping on the network. For example, if the value of update is greater than that of age, a device cannot inform its neighbors of the change of RIP routes immediately.

For the configurations of update, age, suppress, and garbage-collect, see Configuring RIP Timers.

Verifying the Configuration of Routing Loop Prevention

After configuring routing loop prevention, verify the running status of RIP, information about interfaces, and RIP routing information.

Prerequisites

Routing loop prevention has been configured.

Procedure

  • Run the display rip [ process-id | vpn-instance vpn-instance-name ] command to check the running status and configuration of RIP.
  • Run the display rip process-id route command to check RIP routes.
  • Run the display rip process-id interface command to check information about RIP interfaces.

Example

Run the display rip command to view the running status and configuration of the RIP process.

<HUAWEI> display rip
  Public VPN-instance
    RIP process : 1
       RIP version : 1
       Preference : 100
       Checkzero : Enabled
       Default-cost : 0
       Summary : Enabled
       Host-route    : Enabled
       Maximum number of balanced paths : 3
       Update time   : 30 sec  Age time  : 180 sec
       Suppress time : 0 sec   Garbage-collect time : 120 sec
       Silent-interfaces : None
       Default Route : Disabled
       Verify-source : Enabled
       Networks :
       172.4.0.0
       Configured peers : None
       Number of routes in database : 4
       Number of interfaces enabled : 3
       Triggered updates sent : 3
       Number of route changes : 6
       Number of replies to queries : 1
       Description  : RIP 
  Total count for 1 process:
     Number of routes in database : 1
     Number of interfaces enabled :  1
     Number of routes sendable in a periodic update : 1
     Number of routes sent in last periodic update : 1

Run the display rip process-id route command to view all the routes learned by the RIP process.

<HUAWEI> display rip 1 route
 Route Flags: R - RIP
              A - Aging, S - Suppressed, G - Garbage-collect 
 ----------------------------------------------------------------------------
 Peer 192.4.5.1  on GigabitEthernet0/4/1
      Destination/Mask        Nexthop     Cost   Tag     Flags   Sec
        172.4.0.0/16         192.4.5.1      1    0        RA      15
        192.13.14.0/24       192.4.5.1      2    0        RA      15
        192.4.5.0/24         192.4.5.1      1    0        RA      15

Run the display rip process-id interface verbose command to view information about the RIP interface, for example, whether the RIP interface is disabled from receiving or sending RIP packets and whether split horizon and poison reverse are enabled on the interface.

<HUAWEI> display rip 1 interface verbose
GigabitEthernet0/1/0 (1.1.1.2)
  State    : UP              MTU: 0
  Metricin : 0
  Metricout: 1
  Input    : Disabled        Output: Disabled     
  Protocol : RIPv2 Multicast                                   
  Send     : RIPv2 Multicast Packets                           
  Receive  : RIPv2 Multicast Packets                           
  Poison-reverse                : Enabled                
  Split-Horizon                 : Enabled                
  Authentication type           : None                          
  Max Packet Length             : 512                           
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Updated: 2019-01-14

Document ID: EDOC1100058916

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