5.20 ip prefix-list NOT_FROM_R4 deny 155.1.146.4/32 or 155.1.146.4/24???

I noticed they used prefix-list e.g 155.1.146.4/32 throughout this workbook when applying prefix-list to 155.1.0.0/16 subnets.  I believe the length is 24 and it should be 155.1.146.4/24.  Does anyone have an idea on this?

Comments

  • beejay,

    In 5.20 the 155.x.146.4/32 is used to match the gateway - this should be a host /32 prefix. If a /24 were matched, prefixes from R6 on VLAN 146 would also be denied. This may be acceptable in this scenario, but read the tasks carefully. The wording "from R4 on the VLAN 146 segment" makes the /32 match sound more correct than a /24.

    Also, if you did switch to a /24 it should be 155.x.146.0/24 - most IOS versions will give an error message with .4/24.

    I noticed they used prefix-list e.g 155.1.146.4/32 throughout this workbook when applying prefix-list to 155.1.0.0/16 subnets.  I believe the length is 24 and it should be 155.1.146.4/24.  Does anyone have an idea on this?

     

  • Thanks Darell!  One more thing, the number used in 5.22 i.e

    offset-list 1 in 2147483647 fa0/3

    Is it just an arbitrary number as long as the composite metric via 155.1.37.3 is greater than the one via 155.1.67.6 or there is a particulay way to calculate this.

  • beejay,

    The offset value is the max permitted by IOS:

    c3640a(config)#router eigrp 100      
    c3640a(config-router)#offset
    c3640a(config-router)#offset-list in ?
      in   Perform offset on incoming updates
      out  Perform offset on outgoing updates

    c3640a(config-router)#offset-list 1 in ?
      <0-2147483647>  Offset

    This makes it most likely that any other route is preferred.

    Since the max EIGRP metric is 2147611647, the route can still be used if the preferred link goes down.

    Thanks Darell!  One more thing, the number used in 5.22 i.e

    offset-list 1 in 2147483647 fa0/3

    Is it just an arbitrary number as long as the composite metric via 155.1.37.3 is greater than the one via 155.1.67.6 or there is a particulay way to calculate this.

     

  • From reading the instructions for 5.20 I did the following for R1:

    ip prefix-list PL_NOT_R4 seq 10 deny 0.0.0.0/0 le 32
    router eigrp 100
    distribute-list prefix PL_NOT_R4 in FastEthernet0/0

    I see the perscribed solution results in

    Rack1R1#sh ip route | inc 146
    C       155.1.146.0 is directly connected, FastEthernet0/0
    D       155.1.67.0 [90/5120] via 155.1.146.6, 00:18:47, FastEthernet0/0
    D EX 200.0.0.0/24 [170/642560] via 155.1.146.6, 00:18:47, FastEthernet0/0
    D EX    54.1.1.0 [170/514560] via 155.1.146.6, 00:18:47, FastEthernet0/0
    D EX 200.0.1.0/24 [170/642560] via 155.1.146.6, 00:18:47, FastEthernet0/0
    D EX 200.0.2.0/24 [170/642560] via 155.1.146.6, 00:18:47, FastEthernet0/0
    D EX 200.0.3.0/24 [170/642560] via 155.1.146.6, 00:18:47, FastEthernet0/0
    D       150.1.6.0 [90/130560] via 155.1.146.6, 00:18:47, FastEthernet0/0
    Rack1R1#

    I my simplistic original implementation wrong?   I am missing something so any help appreciated.

     

  • Hi,

       Your configuration filters anything received on VLAN 146, regardless of the source of the update. The task states to filter all prefixes received from R4, so you need to filter also based on the gateway.

    Good luck with your studies!

  • Gentlemen, 

    I have used following :

    distribute-list gateway r4 in FastEthernet0/0

    ip prefix-list r4 seq 5 deny 155.1.146.4/32
    ip prefix-list r4 seq 6 permit 0.0.0.0/0 le 32

     

    This is  suggested solution :

    distribute-list prefix-list ALL gateway r4 in FastEthernet0/0

    ip prefix-list r4 seq 5 deny 155.1.146.4/32

    ip prefix-list r4 seq 6 permit 0.0.0.0/0 le 32

    ip prefix-list ALL seq 5 permit 0.0.0.0/0 le 32

     

    Difference in how distribute-list operates here :

    distribute-list gateway r4 in FastEthernet0/0     - block R4 host route + allow all

    distribute-list prefix-list ALL gateway r4 in FastEthernet0/0    - Allow all from anything BUT the host route R4 + allow all  ?

     

    Is this correct understanding ? Thanks

     

     

     

    P.S

    I'm also curious on rwalker post :

    ip prefix-list PL_NOT_R4 seq 10 deny 0.0.0.0/0 le 32
    router eigrp 100
    distribute-list prefix PL_NOT_R4 in FastEthernet0/0

    I see the perscribed solution results in


    Rack1R1#sh ip route | inc 146
    C       155.1.146.0 is directly connected, FastEthernet0/0
    D       155.1.67.0 [90/5120] via 155.1.146.6, 00:18:47, FastEthernet0/0
    D EX 200.0.0.0/24 [170/642560] via 155.1.146.6, 00:18:47, FastEthernet0/0    <--- how are you getting this External EIGRP route ?

     

     

     

     

  • unsobill -
    "distribute-list gateway r4 in FastEthernet0/0
    "distribute-list prefix-list ALL gateway r4 in FastEthernet0/0"
    - Both does the same thing.

    "I'm also curious on rwalker post :
    D EX 200.0.0.0/24 [170/642560] via 155.1.146.6, 00:18:47,
    FastEthernet0/0 < how are you getting this External EIGRP route
    ?"
    - I don't know what he's done, but with the config he's provided that
    would not work.

    Andrius
  • Hi All,

     

    In my case R1 prefers R3:

     

    Rack1R1#sh ip route | i 3(0|1).[0-3].0.0
         31.0.0.0/16 is subnetted, 4 subnets
    D EX    31.3.0.0 [170/2180096] via 155.1.13.3, 00:00:02, Serial0/1
    D EX    31.2.0.0 [170/2180096] via 155.1.13.3, 00:00:02, Serial0/1
    D EX    31.1.0.0 [170/2180096] via 155.1.13.3, 00:00:02, Serial0/1
    D EX    31.0.0.0 [170/2180096] via 155.1.13.3, 00:00:02, Serial0/1
         30.0.0.0/16 is subnetted, 4 subnets
    D EX    30.2.0.0 [170/2180096] via 155.1.13.3, 00:00:02, Serial0/1
    D EX    30.3.0.0 [170/2180096] via 155.1.13.3, 00:00:02, Serial0/1
    D EX    30.0.0.0 [170/2180096] via 155.1.13.3, 00:00:02, Serial0/1
    D EX    30.1.0.0 [170/2180096] via 155.1.13.3, 00:00:02, Serial0/1
    Rack1R1#

     

     

     

    Rack1R1#sh ip eigrp topology 30.2.0.0 255.255.0.0 
    IP-EIGRP (AS 100): Topology entry for 30.2.0.0/16
      State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2180096
      Routing Descriptor Blocks:
      155.1.13.3 (Serial0/1), from 155.1.13.3, Send flag is 0x0
          Composite metric is (2180096/35840), Route is External
          Vector metric:
            Minimum bandwidth is 1544 Kbit
            Total delay is 20400 microseconds
            Reliability is 255/255
            Load is 1/255
            Minimum MTU is 1500
            Hop count is 4
          External data:
            Originating router is 150.1.4.4 
            AS number of route is 0
            External protocol is RIP, external metric is 1
            Administrator tag is 0 (0x00000000)
      155.1.0.5 (Serial0/0.1), from 155.1.0.5, Send flag is 0x0
          Composite metric is (2684416/2172416), Route is External
          Vector metric:
            Minimum bandwidth is 1544 Kbit
            Total delay is 40100 microseconds
            Reliability is 255/255
            Load is 1/255
            Minimum MTU is 1500
            Hop count is 2
          External data:
            Originating router is 150.1.4.4 
            AS number of route is 0
            External protocol is RIP, external metric is 1
            Administrator tag is 0 (0x00000000)
    Rack1R1#sh int se0/0.1 | i DLY
      MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
    Rack1R1#sh int se0/1 | i DLY 
      MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
    Rack1R1#sh ip pre
    Rack1R1#sh ip prefix-list
    ip prefix-list NOT_FROM_R4: 2 entries
       seq 5 deny 155.1.146.4/32
       seq 10 permit 0.0.0.0/0 le 32
    ip prefix-list PERMIT_ALL: 1 entries
       seq 5 permit 0.0.0.0/0 le 32
    Rack1R1#
    Rack1R1#sh run | sectio router eigrp
    router eigrp 100
     network 150.1.1.1 0.0.0.0
     network 155.1.0.1 0.0.0.0
     network 155.1.13.1 0.0.0.0
     network 155.1.146.1 0.0.0.0
     distribute-list prefix PERMIT_ALL gateway NOT_FROM_R4 in
     no auto-summary
    Rack1R1#

     

    R3 has better metric than R5

     

    Rack1R3#sh ip eigrp topology 30.2.0.0 255.255.0.0
    IP-EIGRP (AS 100): Topology entry for 30.2.0.0/16
      State is Passive, Query origin flag is 1, 1 Successor(s), FD is 35840
      Routing Descriptor Blocks:
      155.1.37.7 (FastEthernet0/0), from 155.1.37.7, Send flag is 0x0
          Composite metric is (35840/33280), Route is External
          Vector metric:
            Minimum bandwidth is 100000 Kbit
            Total delay is 400 microseconds
            Reliability is 255/255
            Load is 1/255
            Minimum MTU is 1500
            Hop count is 3
          External data:
            Originating router is 150.1.4.4 
            AS number of route is 0
            External protocol is RIP, external metric is 1
            Administrator tag is 0 (0x00000000)
    Rack1R3#

     

     

     

     

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