BGP question

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Apr 15th, 2010
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guys suppose we are having 10 or 20 routes on WAN router learned from EIGRP


the wan routers has 10.15.15.0/24 to 10.15.X.0/24 etc (learned from EIGRP) and if we want to advertise a router in BGP can we use


10.15.0.0/16 route rather then putting 20 routes secondly do we have to put static null route for 10.15.0.0/16 or not


thanks for looking

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lamav Thu, 04/15/2010 - 07:19
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You can advertise that /16, yes. You can also suppress the advertisement of the specific prefixes.


ip route 10.15.0.0 255.255.0.0 null0


router bgp 1

aggregate-address 10.15.0.0 255.255.0.0 summary-only


HTH


Victor

comsats Thu, 04/15/2010 - 07:27
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Thanks for the answer now i have a question do we really have to advertise the null0 route......if eigrp connection breaks

up even then the routes would be advertised in BGP ??? am i right or wrong secondly my main question remains stands

if we dont do null0 routes would this work or not??? thanks again

Collin Clark Thu, 04/15/2010 - 07:40
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In order to advertise a route into BGP, the local router must have that route in its local routing table. The null0 route is not advertising, it's putting the /16 route into the local routing table so BGP can advertise it.

Collin Clark Thu, 04/15/2010 - 07:50
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Is it possible for you to summarize the routes before your MPLS router? Then your MPLS router will already have the /16 route in its routing table and you don't have to worry about creating any to null0.

Giuseppe Larosa Thu, 04/15/2010 - 08:05
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Hello Comsat,


the route to null0 is not strictly required you can use


network command for each component route


+ aggregate-address for the aggregate if you want to create the aggregate


usually the static to null0 is combined with a network command that matches directly the aggregate



Hope to help

Giuseppe

comsats Thu, 04/15/2010 - 09:02
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Thanks guys for your help now one more and final question.......actually there are two wan routers (the quetion is not abt the second one)


i am advertising  three networks in BGP 10.4.1.0/24


10.5.1.0/24

10.6.1.0/24


the aggregation will be look like this (correct me if i am wrong)


aggregate-address 10.4.0.0 255.255.0.0 summary-only
aggregate-address 10.5.0.0 255.255.0.0 summary-only
aggregate-address 10.6.0.0 255.255.0.0 summary-only


is it correct ?


and if i want to prepend 10.6.1.0/24 (this the requirment)


so all i have to create a prefix-list and it should look like this


ip prefix-list prepend_network_to_isp seq 10 permit 10.6.0.0/16 (it shd be /16 not 24 right????)

route-map my_prepend permit 10
match ip address prefix-list prepend_network_to_isp 
set as-path prepend X.X.X


now the imp question is shd i add permit staement in the route map (like same route _map for two other as i have read that then those two will be denied)

if yes how it shd look like

guys thanks again for looking

Jon Marshall Thu, 04/15/2010 - 09:17
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i am advertising  three networks in BGP 10.4.1.0/24

10.5.1.0/24

10.6.1.0/24

the aggregation will be look like this (correct me if i am wrong)

aggregate-address 10.4.0.0 255.255.0.0 summary-only
aggregate-address 10.5.0.0 255.255.0.0 summary-only
aggregate-address 10.6.0.0 255.255.0.0 summary-only


As Giuseppe pointed out with the aggregate-address command you don't need a matching route in the routing table you just need at least one of the subnets of the aggregated address.


As for the above if you have 3 /24 networks why do you want to advertise them as 3 /16 summary addresses. I mean you can but is this really what you want to do eg.


10.4.0.0/16 as a summary address would be 10.4.1.0 -> 10.4.255.0, do you really want to do that ?


Jon

comsats Thu, 04/15/2010 - 09:48
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Jon you are right the main thing is which i cant get my head around is that i have around

70 networks 10.4.1.0 to 10.4.200.0/24 its mix actually now i am advertising in bgp


router bgp 1

neigbour X.X.X.C remote_as X route-map traffic_out out

network 10.4.1.0 mask 255.255.255.0 (its 24 not 16)


and then


aggregate-address 10.4.0.0 255.255.0.0 summary-only


!

ip prefix-list WAN_list seq 10 permit 10.4.0.0/16

route-map traffic_out permit 10

match ip address prefix-list WAN_list


This is the best i could think of


now my question is will it advertise all my 70 networks


Yes or no???


now shd i use null0 or not??? as i am getting all teh routes from IGP (all /24)


kindly guide me guys

Jon Marshall Thu, 04/15/2010 - 09:55
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comsats wrote:


Jon you are right the main thing is which i cant get my head around is that i have around

70 networks 10.4.1.0 to 10.4.200.0/24 its mix actually now i am advertising in bgp


router bgp 1

neigbour X.X.X.C remote_as X route-map traffic_out out

network 10.4.1.0 mask 255.255.255.0 (its 24 not 16)


and then


aggregate-address 10.4.0.0 255.255.0.0 summary-only


!

ip prefix-list WAN_list seq 10 permit 10.4.0.0/16

route-map traffic_out permit 10

match ip address prefix-list WAN_list


This is the best i could think of


now my question is will it advertise all my 70 networks


Yes or no???


now shd i use null0 or not??? as i am getting all teh routes from IGP (all /24)


kindly guide me guys


It won't advertise all 70 routes, it will advertise a summary route of 10.4.0.0/16 to EBGP peers.


Also because you are using the summary-only keyword i'm not sure what the route-map is meant to achieve because all the more specific 10.4.x.0/24 networks will not be advertised anyway.


You do not need to use Null0.


Jon

comsats Thu, 04/15/2010 - 10:17
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JON

       so if /16 would be summarized still ppl can reach 70 networks that what i meant to ask......as i cant add 70 network commands in BGP


what is the solution am i doing wrong or what???


task is 70 routes with 10.4.X.X /24


so i have add 10.4.1.0/24 (which is the first subnet) and then aggregating them with /16


the route-map is cuz i am using 10.4.1.0/24 in network statement under BGP so i thought to use 10.4.0.0/16 in route-map tied up with prefix-list


look i am here as i m having problem and i m unsure


what do u think


and thanks again

Jon Marshall Thu, 04/15/2010 - 10:22
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comsats wrote:


JON

       so if /16 would be summarized still ppl can reach 70 networks that what i meant to ask......as i cant add 70 network commands in BGP


what is the solution am i doing wrong or what???


task is 70 routes with 10.4.X.X /24


so i have add 10.4.1.0/24 (which is the first subnet) and then aggregating them with /16


the route-map is cuz i am using 10.4.1.0/24 in network statement under BGP so i thought to use 10.4.0.0/16 in route-map tied up with prefix-list


look i am here as i m having problem and i m unsure


what do u think


and thanks again


The "summary-only" keyword you are using with your aggregate address stops any individual subnets that makes up the summary address being advertised so only 10.4.0.0/16 will be advertised.


Yes, people should be able to reach all 70 networks because the summary address covers all those 70 networks but be aware it also covers a lot more than just 70 /24 networks.


Jon

comsats Thu, 04/15/2010 - 10:27
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Thanks JON 100 out of 100 (you deserve that) :-)


so am i doing the right thing putting 10.4.1.0/24 under BGP and doing aggregation with /16


r u happy with route-map???? does it make sence now???

Jon Marshall Thu, 04/15/2010 - 10:28
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comsats wrote:


Thanks JON 100 out of 100 (you deserve that) :-)


so am i doing the right thing putting 10.4.1.0/24 under BGP and doing aggregation with /16


r u happy with route-map???? does it make sence now???


I still don't think you need the route-map because your router will only advertise out the /16 summary address. However it's not doing any harm.


Jon

comsats Thu, 04/15/2010 - 23:51
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Jon

     Now one last question buddy and i guess it takes me to square one........i hav ethose 10.4.1.0/24 to 200 (its mix) so all togather 70 routes on my WAN router.......we have dual home BGP connection with one AS though........no IBGP is running and dont ask me why :-) as it was before i joined.....


now i have another 40 routes in WAN router (from EIGRP) which i dont want to advertise and excpet one which is 10.5.1.0/24 (there are 10 of them)


the requirment is that 10.4.1.0/24 till 10.4.200.0/24 takes ISP one and 10.5.1.0/24 to 10.5.200.0/24 takes ISP 2 as primary.


now as discussed earlier what shd be my config


under bgp 1

neigbour X.X.X.X remote-as 2 route map WAN_out out

network 10.4.0.0 mask 255.255.255.0

network 10.5.0.0 mask 255.255.255.0

aggregate-address 10.4.0.0 255.255.0.0 summary-only

aggregate-address 10.5.0.0 255.255.0.0 summary-only


Now shd i use network 10.4.1.0 massk 255.255.255.0 or the one which is above is right

or shd i just redistribute EIGRP into BGP with no network statement (Man i am confused)


now two things i have to do prepend for 10.5.0.0 network and advertise only these two supernets as i have other networks which i dont want to advertised in BGP



so ip prefix-list shd be

ip prefix-list Filter seq 20 permit 10.4.0.0/16

ip prefix-list Filter seq 21 permit 10.5.0.0/16


route-map WAN_out out permit 10

match ip address prefix-list Filter


now how i will do prepend for 10.5.0.0/16 network on this router


Jon Thanks again Dude

ozzyosbu1 Fri, 04/16/2010 - 00:42
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hello experts

just one doubt, for the aggregate to be announced at least one specific network should be there in the bgp table.

now as per the configuration, only one specific /24 will be there in the bgp table and if that specific goes down there wont be any more specifics in the BGP table( it will still be there on the routing table though( through IGP)). so this would mean that the aggregate will not be advertised. And no reachabilty from out side to any of the networks..!

Giuseppe Larosa Thu, 04/15/2010 - 09:16
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Hello Comsats,


at list one component router per aggregate-address has to be in the BGP table


the prefix-list should match the prefix you want to be prepended


ip prefix-list tobeprepended permit 10.6.0.0/16


you need a second empty route-map block or you will send out only one prefix


You should prepend only your own AS number one or more time.


Hope to help

Giuseppe

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