From: Ahsan Mohiuddin (ahsan.mohiuddin@gmail.com)
Date: Tue Oct 21 2008 - 05:30:45 ARST
the dreaded routing loop.. clichi, isn't it :)
Point is, you mustn't change the pre-config i.e. the loop should remain in
place. Steps 1-8 are part of the pre-config; we have to find solution to the
"TASK".
Interesting thing is that if you disable CEF and enable process switching on
R3, you will have 100% reachability to 6.6.1.0/24; one packet will go
directly out R3's s1/1, the next will go out s1/0.35 but eventually will
follow the path out R3 itself, that is after looping thru R6 then R4
(R3--->R5--->R6--->R4--->R3--->R1). Although not ideal but it works
perfectly well. However, the task asks not to disable CEF on R3.
On Tue, Oct 21, 2008 at 11:25 AM, Fahad Khan <fahad.khan@gmail.com> wrote:
> The routing loop is being occured in between R3 and R5
>
> R3 : D EX 6.6.1.0 [170/2425856] via *6.6.35.5*, 00:01:37,
> Serial1/0.35 -----> next hop is R5
> [170/2425856] via 6.6.13.1, 00:01:37, Serial1/1
>
>
> R5 : D EX 6.6.1.0 [170/2937856] via *6.6.35.3*, 00:01:23,
> Serial1/0.53 ------> next hop is R3
>
> The problems like routing loops and route feedback are handled by
>
> 1- Tagging
> 2- Modifying AD
> 3- Modifying Metric
>
> Ist solution:
>
> On R4, set a tag when redistribution is done from EIGRP to OSPF and On R5 ,
> configure NOT to send the tagged routes back to R3.
>
> 2nd solution: (modifying AD seems not possible in this case; since In eigrp
> we cant modify specific external routes)
>
> well,
>
> On R3 , see if you can modify the AD of the route coming from R5 (distance
> 180 x.x.x.x 0.0.0.0 <acl>)
>
> (lab it n tell us)
>
> HTH
>
> best regards,
> On 10/21/08, Ahsan Mohiuddin <ahsan.mohiuddin@gmail.com> wrote:
>
>> Here's the topology:
>>
>> R1---R3----R4
>> | |
>> R5----R6
>>
>> All links are frame-relay P2P, except R1-R3 which is HDLC serial.
>>
>> (1) R1 is advertising its Lo0 int (6.6.1.1/24) in RIP. R1 is also running
>> EIGRP 100 and redistributing RIP into EIGRP 100.
>> (2) R1 & R3 are EIGRP neighbors over serial link (6.6.13.0/24).
>>
>> (3) R3 is learning the route to 6.6.1.0/24 via 6.6.13.1:
>>
>> D EX 6.6.1.0 [170/2425856] via 6.6.13.1, 00:00:02, Serial1/1
>>
>> R3 is able to ping destination 6.6.1.1:
>>
>> Sending 5, 100-byte ICMP Echos to 6.6.1.1, timeout is 2 seconds:
>> !!!!!
>> Success rate is 100 percent (5/5), round-trip min/avg/max = 4/12/32 ms
>>
>> (4) R3 is advertising this route to its EIGRP 100 neighbors R4 & R5.
>>
>> At R4:
>> D EX 6.6.1.0 [170/2937856] via 6.6.34.3, 00:07:26, Serial1/0.43
>> At R5:
>> D EX 6.6.1.0 [170/2937856] via 6.6.35.3, 00:01:23, Serial1/0.53
>>
>> (5) R4, R5, R6 are running in OSPF Area 0. Note that R4 & R5 do not have a
>> direct link, and both these routers are OSPF neighbors to R6.
>> (6) Now on R4, we configure redistribution of EIGRP 100 into OSPF. As a
>> result, R6 learns the route 6.6.1.0/24:
>>
>> O E2 6.6.1.0 [110/20] via 6.6.46.4, 00:03:06, Serial1/0.64
>>
>> (7) R6 advertises this route to R5. As a result, R5 removes it original
>> route (distance 170) and installs O E2 (distance 90) route:
>>
>> O E2 6.6.1.0 [110/20] via 6.6.56.6, 00:00:01, Serial1/0.56
>>
>> (8) We configure R5 to redistribute OSPF into EIGRP 100. This causes R3 to
>> learn an additional path to 6.6.1.0/24:
>>
>> At R3:
>> D EX 6.6.1.0 [170/2425856] via 6.6.35.5, 00:01:37, Serial1/0.35
>> [170/2425856] via 6.6.13.1, 00:01:37, Serial1/1
>>
>> (9) Now at R3, if we ping 6.6.1.1..
>>
>> Sending 5, 100-byte ICMP Echos to 6.6.1.1, timeout is 2 seconds:
>> .....
>> Success rate is 0 percent (0/5)
>>
>> *TASK*
>> Ensure that R3 can reach 6.6.1.0/24. Do not disable CEF on R3. Do not
>> create
>> any tunnels. You are allowed to change IP Addressing on a single link
>> (subnet) in the topology.
>>
>>
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>>
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>>
>>
>>
>>
>>
>>
>>
>
>
> --
> Fahad Khan
Blogs and organic groups at http://www.ccie.net
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