From: Scott Vermillion (scott_ccie_list@it-ag.com)
Date: Fri Mar 07 2008 - 15:25:04 ARST
Oh, those naughty, naughty little routers! Paul, this is way cool man.
Thanks for taking the time to put this together in your lab and to post all
of these results. Very nice.
One final question:
Is there a chance of this happening if you simply reboot R3? Or must you
follow this exact sequence to wind up with the looped topology?
Cheers,
Scott
-----Original Message-----
From: Paul Cosgrove [mailto:paul.cosgrove@heanet.ie]
Sent: Thursday, March 06, 2008 6:13 PM
To: Scott Vermillion
Cc: 'Timothy Chin'; 'John'; 'Sadiq Yakasai'; 'Hash Aminu'; 'Carlos Alberto
Trujillo Jimenez'; ccielab@groupstudy.com
Subject: Re: redistribution
Hi Scott,
Thanks for your help with the additional information about the topology
in this scenario. I setup a similar (smaller) scenario in my lab, using
a loopback on my switch to originate 204.12.1.0/24 into RIP to R1. R1
redistributes into a full mesh point-to-multipoint OSPF area containing
R1, R2, R3 & R4. R2 and R3 share an ethernet interface running EIGRP.
I've used the same OSPF config which was mentioned in the earlier emails.
router eigrp 10
redistribute ospf 1 metric 100000 10 255 1 1500
network 132.1.23.0 0.0.0.255
auto-summary
!
router ospf 1
log-adjacency-changes
redistribute eigrp 10 subnets
network 132.1.0.0 0.0.0.255 area 0
distance ospf external 171
distance 110 0.0.0.0 255.255.255.255 EXT_OSPF
!
ip access-list standard EXT_OSPF
permit 132.1.17.0
permit 204.12.1.0
!
The distance ospf external 171 doesn't actually do anything in this case
as the E2 routes from R1 are matched by the ACL. Left it there anyway
for completeness.
I've ran a few tests and found that the issue does occur, but it seems
harder to reproduce on point-to-multipoint than with a point-to-point
link on the router where I shutdown the link. The topology in NMC lab 1
has a point-to-point interface which I was shutting down, and that
reproduces the issue every time, point to multipoint seems to be much
less reliable at breaking things (about 50/50). I guess this is because
it comes down to a timing issue of who advertises to who first, but
don't see a clear explaination in the RFC.
I'm using the following method to try to trigger the routing loop:
- shutdown the s0 interface on R3
- clear ip ospf process on R3
- no shut s0 again.
To reset between attempts I was shutting down the ethernet interface I
have EIGRP running on before repeating the same.
In the topologies I've been using the loop is permanent, as once R1
overrides its own OSPF E2 route with one learned from R3, it will not
reinstall its own until the R3 is removed (e.g. by shutting down an
interface).
The following example may help to clarify what I've done:
Paul.
Router3(config-if)#do sh ip ro
O E2 204.12.1.0/24 [110/20] via 132.1.0.1, 00:00:08, Serial0
132.1.0.0/16 is variably subnetted, 6 subnets, 2 masks
O 132.1.0.4/32 [110/128] via 132.1.0.1, 00:00:08, Serial0
[110/128] via 132.1.0.2, 00:00:08, Serial0
O 132.1.0.1/32 [110/64] via 132.1.0.1, 00:00:08, Serial0
C 132.1.0.0/24 is directly connected, Serial0
O 132.1.0.2/32 [110/64] via 132.1.0.2, 00:00:08, Serial0
C 132.1.23.0/24 is directly connected, Ethernet1
O E2 132.1.17.0/24 [110/20] via 132.1.0.1, 00:00:09, Serial0
Router3(config-if)#int s0
Router3(config-if)#shut
Router3(config-if)#do c
Mar 7 00:04:23.743: %OSPF-5-ADJCHG: Process 1, Nbr 132.1.23.4 on
Serial0 from FULL to DOWN, Neighbor Down: Interface do
wn or detached
Mar 7 00:04:23.751: %OSPF-5-ADJCHG: Process 1, Nbr 172.16.102.1 on
Serial0 from FULL to DOWN, Neighbor Down: Interface
down or detached
Mar 7 00:04:23.755: %OSPF-5-ADJCHG: Process 1, Nbr 172.16.101.1 on
Serial0 from FULL to DOWN, Neighbor Down: Interface
down or detachedlear ip o
Mar 7 00:04:25.735: %LINK-5-CHANGED: Interface Serial0, changed state
to administratively down
Mar 7 00:04:26.735: %LINEPROTO-5-UPDOWN: Line protocol on Interface
Serial0, changed state to downspf process
Reset ALL OSPF processes? [no]: y
Router3(config-if)#do sh ip ospf da | b Type-5
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
132.1.0.0 172.16.103.1 64 0x80000001 0x00B441 0
132.1.0.1 172.16.103.1 64 0x80000001 0x00AA4A 0
132.1.0.4 172.16.103.1 64 0x80000001 0x008C65 0
132.1.17.0 172.16.103.1 64 0x80000001 0x00F8EB 0
132.1.23.0 172.16.103.1 64 0x80000001 0x00B628 0
204.12.1.0 172.16.103.1 64 0x80000001 0x007928 0
Router3(config-if)#no shut
Router3(config-if)#
Mar 7 00:05:44.815: %LINK-3-UPDOWN: Interface Serial0, changed state to up
Mar 7 00:05:45.815: %LINEPROTO-5-UPDOWN: Line protocol on Interface
Serial0, changed state to up
Mar 7 00:06:10.231: %OSPF-5-ADJCHG: Process 1, Nbr 172.16.102.1 on
Serial0 from LOADING to FULL, Loading Done
Mar 7 00:06:13.091: %OSPF-5-ADJCHG: Process 1, Nbr 172.16.101.1 on
Serial0 from LOADING to FULL, Loading Done
Mar 7 00:06:18.387: %OSPF-5-ADJCHG: Process 1, Nbr 132.1.23.4 on
Serial0 from LOADING to FULL, Loading Done
Router3(config-if)#
Router3(config-if)#
Router3(config-if)#do sh ip ro
D EX 204.12.1.0/24 [170/284160] via 132.1.23.2, 00:02:08, Ethernet1
132.1.0.0/16 is variably subnetted, 6 subnets, 2 masks
O 132.1.0.4/32 [110/64] via 132.1.0.4, 00:00:07, Serial0
O 132.1.0.1/32 [110/64] via 132.1.0.1, 00:00:07, Serial0
C 132.1.0.0/24 is directly connected, Serial0
O 132.1.0.2/32 [110/64] via 132.1.0.2, 00:00:07, Serial0
C 132.1.23.0/24 is directly connected, Ethernet1
O E2 132.1.17.0/24 [110/20] via 132.1.0.1, 00:00:07, Serial0
Router3(config-if)#do sh clock
00:06:43.027 UTC Fri Mar 7 2008
Router3#traceroute 204.12.1.1
Type escape sequence to abort.
Tracing the route to 204.12.1.1
1 132.1.23.2 4 msec 4 msec 4 msec
2 132.1.0.3 32 msec 16 msec 12 msec
3 132.1.23.2 20 msec 12 msec 12 msec
4 132.1.0.3 28 msec 24 msec 24 msec
5 132.1.23.2 24 msec 24 msec 24 msec
6 132.1.0.3 36 msec 36 msec 36 msec
7 132.1.23.2 36 msec 36 msec 32 msec
Router3#
Router3#sh ip ro
D EX 204.12.1.0/24 [170/284160] via 132.1.23.2, 01:07:33, Ethernet1
132.1.0.0/16 is variably subnetted, 6 subnets, 2 masks
O 132.1.0.4/32 [110/64] via 132.1.0.4, 00:06:36, Serial0
O 132.1.0.1/32 [110/64] via 132.1.0.1, 00:06:36, Serial0
C 132.1.0.0/24 is directly connected, Serial0
O 132.1.0.2/32 [110/64] via 132.1.0.2, 00:06:36, Serial0
C 132.1.23.0/24 is directly connected, Ethernet1
O E2 132.1.17.0/24 [110/20] via 132.1.0.1, 00:06:36, Serial0
Router3#sh clock
01:12:06.479 UTC Fri Mar 7 2008
Router3#
Router1#sh ip ro
O E2 204.12.1.0/24 [110/20] via 132.1.0.3, 01:04:51, Serial0/0/0
132.1.0.0/16 is variably subnetted, 6 subnets, 2 masks
O 132.1.0.4/32 [110/64] via 132.1.0.4, 01:04:51, Serial0/0/0
C 132.1.0.0/24 is directly connected, Serial0/0/0
O 132.1.0.3/32 [110/64] via 132.1.0.3, 01:04:51, Serial0/0/0
O 132.1.0.2/32 [110/64] via 132.1.0.2, 01:04:51, Serial0/0/0
O E2 132.1.23.0/24 [110/20] via 132.1.0.3, 01:04:51, Serial0/0/0
[110/20] via 132.1.0.2, 01:04:51, Serial0/0/0
C 132.1.17.0/24 is directly connected, FastEthernet0/0
Router1#
-- Paul Cosgrove HEAnet Limited, Ireland's Education and Research Network 1st Floor, 5 George's Dock, IFSC, Dublin 1 Registered in Ireland, no 275301 tel: +353-1-660 9040 fax: +353-1-660 3666 web: http://www.heanet.ie/
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