RIP & DEFAULT Routes


HELLO ALL,

        Again I am just writing this to check my logic and to ensure that I have everything straight in my mind. I have just wrote up a little review on RIP and the default-information Originate command. Please give me feed back so I can fill in any knowldge gaps I might have.

Respectfully,

AntRal

RIP and DEFAULT-INFORMATION Originate

The Basic Issue-

 Using the Default-information originate command is a good way to advertise a default route within a network using RIP as the IGP, but there is an issue when you implement it in a network that has any kind of mesh topology. Unchecked the default route will just loop around the network incrementing its hop count until the count of 16 then it will flush the route and start over; a “count to infinity”.  To fix this we need to block the default route from coming back into the originating router. This can be accomplished by creating a prefix list on one of the routers going out the interface into the originating router or on the originating router you can create a static route of the 0.0.0.0 directed to null0.  

Topology-

Router1—f0/2àRouter2—f0/0 à Router3--f0/1à Router1

Example 1 -

Router1

Router rip

Default-information originate

!

Router3

IP prefix-list DEFAULT-FILTER seq 10 deny 0.0.0.0/0

IP prefix-list DEFAULT-FILTER seq 15 permit 0.0.0.0/0 le 32

!

Router rip

Distribute-list prefix DEFAULT-FILTER out f0/1

!

 Example 2 –

Router1

Router rip

Default-information originate

!

IP route 0.0.0.0 0.0.0.0 null

!

Default Route with Route-Map

   You can use a route-map with the default-information originate command to generate a default route only if the route-map conditions are satisfied. You can also use it to set the interface you want to advertise the default route. The advantage of making the default route contingent on a route-map is that you can ensure that the route will stop being advertised if the connection that is being tracked by the route-map goes down. Note: You cannot use extended access-lists with the configuration of default-information originate.

Example 1-

Topology-

Router1—f0/2àRouter2—f0/0 à Router3--f0/1à Router1—f0/0à Network_Edge_RT—Loopback0 2.1.1.1/32

Router1

IP prefix-list EDGE_LOOP seq 10 permit 2.1.1.1/32

!

Route-map CHECK_EDGE_LOOP permit 10

Match ip address prefix-list EDGE_LOOP

!

Router rip

Default-information originate route-map CHECK_EDGE_LOOP

!

!

Router3

IP prefix-list DEFAULT-FILTER seq 10 deny 0.0.0.0/0

IP prefix-list DEFAULT-FILTER seq 15 permit 0.0.0.0/0 le 32

!

Router rip

Distribute-list prefix DEFAULT-FILTER out f0/1

!

Example 2-

Topology-

Network to outside 201.1.1.0/24-Router1—f0/2àRouter2—f0/0 à Router3

Router1

Access-list 5 permit 201.1.1.0 0.0.0.255

Route-map SET_DEFAULT_INTER permit 10

Match ip address 5

Set interface f0/2

!

Router rip

Default-information originate route-map SET_DEFAULT_INTER

!

!

Default on passive-interfaces

You can also set your default route to only be generated on passive interfaces. This would be a good implementation for a stub like router.  

Topology-

Router1—f0/2àRouter2—f0/0 à Router3--f0/1à Router1—f0/0à Router4

Router1

Router rip

Passive-interface f0/0

Default-information originate on-passive

!

!

Comments

  • Good Afternoon Sir,

        Thank you. The on-passive has been added on the IOS 15.1(2) and later so it is a newer feature, but it does add more simplistic configurations than using route maps.

    Respectfully,

    AntRal

  • Ture it is the worst for most designs, but I could still see a few designs that it would work in still. It is no EIGRP or OSPF that is for sure.

  • All that the blue print says is:

    2.4 RIP v2

    • 2.4.a Implement and troubleshoot RIPv2

    So minimal at best I would think.

     

  • Hi AntRal

    On my CSR1000V "on-passive" command dose not work properly.

    Example:

    interface GigabitEthernet1.101
     encapsulation dot1Q 101
     ip address 154.1.0.1 255.255.255.0
    !
    interface GigabitEthernet1.102
    encapsulation dot1Q 102
     ip address 154.1.1.1 255.255.255.0
    !
    router rip
     version 2
     passive-interface GigabitEthernet1.101
     network 154.1.0.0
     default-information originate on-passive
     no auto-summary

    R6#debug ip rip
    RIP protocol debugging is on
    R6#
    *Aug  4 21:51:23.153: RIP: sending v2 update to 224.0.0.9 via GigabitEthernet1.101 (154.1.0.1)
    *Aug  4 21:51:23.153: RIP: build update entries
    *Aug  4 21:51:23.153:   0.0.0.0/0 via 0.0.0.0, metric 1, tag 0
    R6#
    *Aug  4 21:51:25.523: RIP: sending v2 update to 224.0.0.9 via GigabitEthernet1.102 (154.1.1.1)
    *Aug  4 21:51:25.523: RIP: build update entries
    *Aug  4 21:51:25.524:   0.0.0.0/0 via 0.0.0.0, metric 1, tag 0
    *Aug  4 21:51:25.524:   154.1.0.0/24 via 0.0.0.0, metric 1, tag 0

    R6 advertises default route from not only passive interface but also non-passive interface.
    Is there any other version of IOS-XE that can advertise default route only from passive interfaces?
    Or did I make configurational mistake?

    Additional Information:

    R6#sh ver
    Cisco IOS XE Software, Version 03.13.02.S - Extended Support Release
    Cisco IOS Software, CSR1000V Software (X86_64_LINUX_IOSD-UNIVERSALK9-M), Version 15.4(3)S2, RELEASE SOFTWARE (fc3)
    (snip)

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