From: NET HE (he_net@hotmail.com)
Date: Mon Sep 03 2007 - 18:07:12 ART
Hi, there
I have another question.
As you all know, while using network type point-to-multipoint for
frame-relay links, ospf generates host routes. But when host routes are
redistributed into eigrp, on top of redistributed host routes, a classful
route is generated by eigrp as well. Do I have to prevent that classful
route from entering routing table? If not, there will be host routes
pointing to ospf domain, and a classful route pointing to eigrp domain.
And I don't see any negative effect if I don't prevent the classful route,
because routers always look for the longest match.
Best Regards,
Net (Xin) He
>
>Xin He;
>
>I use tagging to prevent route feedback. This will happen when you are
>re-distributing one protocol into another in at least two places and the
>RECEIVING protocol (like OSPF/RIP) doesn't have a build in mechanism to
>detect inferior redistributed routes (EIGRP does this pretty well by
>default, with EX = 170).
>
>I use "distance xxx source ACL" when a single path must be preferred which
>supersedes what the redistribution decided was the best path on its own,
>and
>the task wont let us touch cost, bandwidth, etc.
>
>Also you will find yourself touch distance often when preferring two routes
>(and two sources) from the same protocol (i.e. two ospf paths to
>123.0.1.0/24, etc.)
>
>If you find you are stuck on redistribution, DRAW it OUT. THINK like each
>router at EACH hop. Consider all information available at each hop, prefix
>length, AD, Metric, and as you pointed out, order of learning the route
>(what will be in the routing table when the redistribution command looks
>for
>the routes to redistribute)
>
>-Joe
>
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of NET
>HE
>Sent: Thursday, August 30, 2007 10:24 PM
>To: bit.gossip@chello.nl; ccielab@groupstudy.com
>Subject: RE: nasty redistribution
>
>Hi,
>
>I was under the same inpression as well. I was always using tagging to
>prevent routing loop it there were multiple redistrition points between 2
>routing domains. Recently I found a solution, but inputs from group are
>very
>
>appreciated.
>
>Since CCIE lab is small, it's very easy to know th biggest metric in a
>routing domain, for example, maximum hops in a rip domain, minimum
>bandwidth
>
>and biggest acummulated delay in eigrp domain, and biggest acummulated cost
>in ospf domain. I would put that biggest metric as default-metric under the
>routing protocol. When routes are redistributed from another routing
>domain,
>
>all those routes will have the biggest metric at that redistribution point.
>This would simply prevent any routing loop without any tagging.
>
>
>Best Regards,
>Net (Xin) He
>
> >
> >Group,
> >so far I was under the impression that I could tackle every scenario of
> >simple
> >and mutual redistribution by tagging and filtering at redistribution
> >points.
> >So I focused and practiced a lot to become proficient and familiar with
> >this
> >technique.
> >Until the simple scenario that I describe below shattered all my
>confidence
> >and pushed me back to square 0 :-(
> >Tagging and filtering is not enough: in some cases it doesn't work; in
>some
> >cases admin disctance must be manipulated.
> >
> >The problem with this simple network is the redistribution of R7 lo0
> >170.1.7.7
> >from RIP to OSPF via 2 distribution points: R5 and R6. In normal
>condition
> >the
> >network is stable in the following status:
> >- either R5 or R6, whoever is faster, redistributes 170.1.7.7 from RIP to
> >OSPF; let's assume R6 is faster
> >- R6 will redistribute 150.1.7.7 into OSPF and keep the original RIP
>route
> >in
> >its routing table
> >R6#show ip route | i 170.1.7.7
> >R 170.1.7.7/32 [120/1] via 170.1.200.7, 00:00:23, FastEthernet0/0
> >
> >- R5 instead learns 170.1.7.7 via OSPF so that the OSPF route overwrites
> >the
> >RIP route due to better admin distance
> >R5#show ip route | i 170.1.7.7
> >O E2 170.1.7.7/32 [110/20] via 170.1.100.9, 00:17:30, Serial1/0.1
> >
> >- the tagging that I put in place prevents the route from ricirculating:
> >the
> >network is perfectly stable, even if maybe routing is not optimal but who
> >cares now.....
> >
> >~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> >The fun starts if I shutdown the OSPF session between R5 and R2, for
> >instance
> >putting passive one interface long enogh for R5 to re-learn the route via
> >RIP
> >from R7. At this point, if we reestablish OSPF, both R5 and R6 try to
> >redistribute the route from RIP to OSPF and 170.1.7.7 ping-pongs for ever
> >between R5 and R6.
> >
> >~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> >The first nasty thing here is that I would have not spotted this
>condition
> >at
> >the real lab because in normal condition the network is stable and even
> >after
> >a reload the network comes up stable. Only what I described above can
> >trigger
> >the instability.
> >The second nasty thing is that now I am back to the original question:
>when
> >should I use tagging? When admin-distance? When both?
> >In this case I guess both are needed!
> >
> >The reason why I tried to avoid manipulating admin distance so far is the
> >following:
> >
> >Let's say that to solve the instability here I increase the admin
>distance
> >of
> >OSPF external to 121: the problem of 170.1.7.7 is solved because both R5
> >and
> >R6 will prefer the RIP route and only R2 will see the O E2
> >But while I solved the problem in the direction RIP -> OPSF, now I have
> >create
> >the same potential problem in the opposite direction; so, if R2 were to
> >redistribute its loopback as connected, I am exactly in the same
>situation
> >as
> >I was at the beginning but in a mirrored form. I have not solved the
> >problem
> >but just thrown in over the fence !
> >
> >I think that the only way out here is to increase the admin distance only
> >of
> >specific selected routes when redistributed; in this case only 170.1.7.7
> >shold
> >be changed from O E2 [110/20] to O E2 [121/20]
> >
> >Am I completely out of track?
> >PS: sorry for the long post!
> >
> >
> >~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > _____ R5 _|
> > / | rip
> > / |
> > / |
> >R2(ospf) |--- R7 - lo0 170.1.7.7
> > \ |
> > \ |
> > \_____ R6 _ |
> >
> >
> >~~~~~~~~
> >
> >hostname R2
> >!
> >interface Serial1/0
> > no ip address
> > encapsulation frame-relay
> > serial restart-delay 0
> > no frame-relay inverse-arp
> >!
> >interface Serial1/0.5 point-to-point
> > ip address 170.1.100.9 255.255.255.252
> > frame-relay interface-dlci 205
> >!
> >interface Serial1/0.6 point-to-point
> > ip address 170.1.100.13 255.255.255.252
> > frame-relay interface-dlci 206
> >!
> >router ospf 1
> > log-adjacency-changes
> > network 170.1.100.0 0.0.0.255 area 0
> >
> >~~~~~~~~
> >
> >hostname R5
> >!
> >interface FastEthernet0/0
> > ip address 170.1.200.5 255.255.255.224
> > duplex full
> >!
> >interface Serial1/0
> > no ip address
> > encapsulation frame-relay
> > serial restart-delay 0
> > no frame-relay inverse-arp
> >!
> >interface Serial1/0.1 point-to-point
> > ip address 170.1.100.10 255.255.255.252
> > frame-relay interface-dlci 502
> >!
> >router ospf 1
> > log-adjacency-changes
> > redistribute rip subnets route-map RO
> > network 170.1.100.0 0.0.0.255 area 0
> >!
> >router rip
> > version 2
> > redistribute ospf 1 metric 2 route-map OR
> > passive-interface default
> > no passive-interface FastEthernet0/0
> > network 170.1.0.0
> > no auto-summary
> >!
> >route-map RO deny 10
> > match tag 96 115 116 125 126
> >!
> >route-map RO permit 20
> > set tag 125
> >!
> >route-map OR deny 10
> > match tag 96 115 116 125 126
> >!
> >route-map OR permit 20
> > set tag 115
> >
> >~~~~~~~~~~
> >
> >hostname R6
> >!
> >interface FastEthernet0/0
> > ip address 170.1.200.6 255.255.255.224
> > duplex full
> >!
> >interface Serial1/0
> > no ip address
> > encapsulation frame-relay
> > serial restart-delay 0
> > no frame-relay inverse-arp
> >!
> >interface Serial1/0.1 point-to-point
> > ip address 170.1.100.14 255.255.255.252
> > frame-relay interface-dlci 602
> >!
> >router ospf 1
> > log-adjacency-changes
> > redistribute rip subnets route-map FROM-RIP
> > network 170.1.100.0 0.0.0.255 area 0
> >!
> >router rip
> > version 2
> > redistribute ospf 1 metric 2 route-map FROM-OSPF
> > passive-interface default
> > no passive-interface FastEthernet0/0
> > network 170.1.0.0
> > no auto-summary
> >!
> >no ip http server
> >no ip http secure-server
> >!
> >!
> >!
> >logging alarm informational
> >!
> >!
> >!
> >route-map FROM-OSPF deny 10
> > match tag 96 115 116 125 126
> >!
> >route-map FROM-OSPF permit 20
> > set tag 116
> >!
> >route-map FROM-RIP deny 10
> > match tag 96 115 116 125 126
> >!
> >route-map FROM-RIP permit 20
> > set tag 126
> >!
> >~~~~~~~~~
> >
> >hostname R7
> >!
> >interface Loopback0
> > ip address 170.1.7.7 255.255.255.255
> >!
> >interface FastEthernet0/0
> > ip address 170.1.200.7 255.255.255.224
> > duplex full
> >!
> >router rip
> > version 2
> > network 170.1.0.0
> > no auto-summary
> >!
> >~~~~~~~~~~~
> >R5#debug ip routing
> >IP routing debugging is on
> >R5(config-router)#passive-interface s1/0.1
> >R5(config-router)#
> >*Aug 30 21:07:49.147: %OSPF-5-ADJCHG: Process 1, Nbr 170.1.100.13 on
> >Serial1/0.1 from FULL to DOWN, Neighbor Down: Interface down or detached
> >*Aug 30 21:07:54.651: RT: del 170.1.100.12/30 via 170.1.100.9, ospf
>metric
> >[110/128]
> >*Aug 30 21:07:54.655: RT: delete subnet route to 170.1.100.12/30
> >*Aug 30 21:07:54.659: RT: NET-RED 170.1.100.12/30
> >*Aug 30 21:07:54.667: RT: del 170.1.7.7/32 via 170.1.100.9, ospf metric
> >[110/20]
> >*Aug 30 21:07:54.671: RT: delete subnet route to 170.1.7.7/32
> >*Aug 30 21:07:54.675: RT: NET-RED 170.1.7.7/32
> >R5(config-router)#
> >R5(config-router)#
> >*Aug 30 21:08:07.395: RT: add 170.1.100.12/30 via 170.1.200.6, rip metric
> >[120/1]
> >*Aug 30 21:08:07.399: RT: NET-RED 170.1.100.12/30
> >R5(config-router)#no passive-interface s1/0.1 <<<<<<<<<<< let's the
>show
> >begin !
> >R5(config-router)#
> >*Aug 30 21:08:18.071: %OSPF-5-ADJCHG: Process 1, Nbr 170.1.100.13 on
> >Serial1/0.1 from LOADING to FULL, Loading Done
> >*Aug 30 21:08:18.363: RT: add 170.1.7.7/32 via 170.1.200.7, rip metric
> >[120/1]
> >*Aug 30 21:08:18.367: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:08:33.111: RT: closer admin distance for 170.1.100.12,
>flushing
> >1
> >routes
> >*Aug 30 21:08:33.115: RT: NET-RED 170.1.100.12/30
> >*Aug 30 21:08:33.119: RT: add 170.1.100.12/30 via 170.1.100.9, ospf
>metric
> >[110/128]
> >*Aug 30 21:08:33.123: RT: NET-RED 170.1.100.12/30
> >*Aug 30 21:08:33.127: RT: closer admin distance for 170.1.7.7, flushing 1
> >routes
> >*Aug 30 21:08:33.127: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:08:33.127: RT: add 170.1.7.7/32 via 170.1.100.9, ospf metric
> >[110/20]
> >*Aug 30 21:08:33.127: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:08:33.279: RT: del 170.1.7.7/32 via 170.1.100.9, ospf metric
> >[110/20]
> >*Aug 30 21:08:33.279: RT: delete subnet route to 170.1.7.7/32
> >*Aug 30 21:08:33.279: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:08:45.515: RT: add 170.1.7.7/32 via 170.1.200.7, rip metric
> >[120/1]
> >*Aug 30 21:08:45.519: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:08:45.615: RT: closer admin distance for 170.1.7.7, flushing 1
> >routes
> >*Aug 30 21:08:45.615: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:08:45.615: RT: add 170.1.7.7/32 via 170.1.100.9, ospf metric
> >[110/20]
> >*Aug 30 21:08:45.619: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:08:50.751: RT: del 170.1.7.7/32 via 170.1.100.9, ospf metric
> >[110/20]
> >*Aug 30 21:08:50.751: RT: delete subnet route to 170.1.7.7/32
> >*Aug 30 21:08:50.751: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:09:14.455: RT: add 170.1.7.7/32 via 170.1.200.7, rip metric
> >[120/1]
> >*Aug 30 21:09:14.459: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:09:14.599: RT: closer admin distance for 170.1.7.7, flushing 1
> >routes
> >*Aug 30 21:09:14.603: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:09:14.607: RT: add 170.1.7.7/32 via 170.1.100.9, ospf metric
> >[110/20]
> >*Aug 30 21:09:14.611: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:09:19.699: RT: del 170.1.7.7/32 via 170.1.100.9, ospf metric
> >[110/20]
> >*Aug 30 21:09:19.699: RT: delete subnet route to 170.1.7.7/32
> >*Aug 30 21:09:19.699: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:09:43.395: RT: add 170.1.7.7/32 via 170.1.200.7, rip metric
> >[120/1]
> >*Aug 30 21:09:43.399: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:09:43.527: RT: closer admin distance for 170.1.7.7, flushing 1
> >routes
> >*Aug 30 21:09:43.531: RT: NET-RED 170.1.7.7/32
> >*Aug 30 21:09:43.535: RT: add 170.1.7.7/32 via 170.1.100.9, ospf metric
> >[110/20]
> >*Aug 30 21:09:43.539: RT: NET-RED 170.1.7.7/32
> >
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