From: Ajit (ajitmohanraj@vsnl.com)
Date: Fri Nov 29 2002 - 16:05:24 GMT-3
To put it simply .... its a case of sheep (RIP) dressed up in wolves
clothing (OSPF) and the wiley gatekeeper knowing them both.
When RIP is redistributed into OSPF, it changes its 'clothes' so to speak in
the OSPF domain, cause earlier it had an AD value of 120 (sheep's clothing)
and now, after being redistributed into the OSPF world has a better AD value
of 110 (Wolve's clothing)
So when you do the turn-around redistribution or "mutual distrobuition" as
its called , Router A is just ensuring that the so-called wolves (RIP
dressed up as AD 110 (wolves clothing) are filtered out in his routing table
updates to his other RIP friends ....cause Router A already knows about his
own routes right ....so the real 'wolves' (OSPF) are the only ones allowed
to go past Router A into his RIP domain.
Now to explain it from both routing protocol sides on Router A
1. RIP
Since Router A's RIP process advertises its routing table to neighbhors, yu
will find the distribute list OUT .....keyword in operation, basically
ensuring that the sheep-dressed-in-wolves clothing DO NOT make into the
routing table and only the real WOLVES (the pure OSPF routes ) are allowed
in. Important point to remember here is that the OUT keyword filtering the
routes that make it to the ROUTING TABLE.
2. OSPF
Router's A OSPF process and the command distribute list OUT rip ..... RTR A
has just ensured that only the RIP routes get installed in the OSPF DATABASE
(that the key here ). Thats the key dfference b/t the distribute -list OUT
option working on a DV protocol and the OUT option working on a LINK state
protocol.
One affects routes going into the ROUTE TABLE and the other affects routes
going inot the OSPF DATABASE ....(so be careful when yu are reading
distribute-list x out routing-protocol ubder OSPF)
I hope this clears up the logic behind it all ... the commands may make more
sense now when yu reading this back.
Ajit
router rip
default-metric 10
network 130.10.0.0
passive-interface serial 0
passive-interface serial 1
redistribute ospf 109
distribute-list 10 out ospf 109
!
router ospf 109
network 130.10.62.0 0.0.0.255 area 0
network 130.10.63.0 0.0.0.255 area 0
redistribute rip subnets
distribute-list 11 out rip
!
access-list 10 deny 130.10.8.0 0.0.7.255
access-list 10 permit 0.0.0.0 255.255.255.255
access-list 11 permit 130.10.8.0 0.0.7.255
access-list 11 deny 0.0.0.0 255.255.255.255
----- Original Message -----
From: "Emmanuel Guilain" <emmanuel.guilain@cgey.com>
To: "CCIE Lab (E-mail)" <ccielab@groupstudy.com>
Sent: Friday, November 29, 2002 9:46 PM
Subject: Redistribution question (Cisco site)
> Hello
>
> I'm having problems reading this document:
> http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/cs001.htm#xtocid57174
>
> Especially with the access-lists 10 and 11 in the routerA configuration.
>
> For me,when filtering redistribution from OSPF to RIP you have to prevent
> routes learned from RIP into OSPF to return into the RIP domain. And the
> following lines seems to permit only these routes:
> router ospf 109
> distribute-list 11 out rip # this means "filters the routes distributed
out
> to RIP" right? Or Am I this wrong?
> access-list 11 permit 130.10.8.0 0.0.7.255
> access-list 11 deny 0.0.0.0 255.255.255.255
>
> If you want only the RIP routes to be advertised in OSPF you have to
filter
> everything but these routes.
> And the access-list 10 does the opposite.
>
> Please, can you tell me where I'm wrong?
>
> Thanks
>
> Emmanuel Guilain
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