Re: Advertising subnets of existing networks ?

Thanks Brian, This is exactly what I was looking for .

On Wed, Jun 29, 2011 at 1:07 PM, Brian McGahan <bmcgahan_at_ine.com> wrote:

> The same way, just point the routes towards an interface instead of towards
> Null0. Here's an example, where R2 has a LAN interface 10.0.0.0/24, but
> advertises it to R1 as two /25s
>
> R2#sh ip route
> Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
> D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
> N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
> E1 - OSPF external type 1, E2 - OSPF external type 2
> i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
> ia - IS-IS inter area, * - candidate default, U - per-user static
> route
> o - ODR, P - periodic downloaded static route
>
> Gateway of last resort is not set
>
> O 200.0.16.0/24 [110/65] via 200.0.12.1, 21:37:06, Serial0/0.201
> C 200.0.12.0/24 is directly connected, Serial0/0.201
> O 200.0.13.0/24 [110/128] via 200.0.12.1, 21:37:16, Serial0/0.201
> 10.0.0.0/24 is subnetted, 1 subnets
> C 10.0.0.0 is directly connected, FastEthernet0/0
>
> R2#conf t
> Enter configuration commands, one per line. End with CNTL/Z.
> R2(config)#ip route 10.0.0.0 255.255.255.128 f0/0
> R2(config)#ip route 10.0.0.128 255.255.255.128 f0/0
> R2(config)#end
> R2#sh ip route
> Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
> D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
> N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
> E1 - OSPF external type 1, E2 - OSPF external type 2
> i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
> ia - IS-IS inter area, * - candidate default, U - per-user static
> route
> o - ODR, P - periodic downloaded static route
>
> Gateway of last resort is not set
>
> O 200.0.16.0/24 [110/65] via 200.0.12.1, 21:37:20, Serial0/0.201
> C 200.0.12.0/24 is directly connected, Serial0/0.201
> O 200.0.13.0/24 [110/128] via 200.0.12.1, 21:37:30, Serial0/0.201
> 10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
> S 10.0.0.0/25 is directly connected, FastEthernet0/0
> C 10.0.0.0/24 is directly connected, FastEthernet0/0
> S 10.0.0.128/25 is directly connected, FastEthernet0/0
>
> R2#conf t
> Enter configuration commands, one per line. End with CNTL/Z.
> R2(config)#router ospf 1
> R2(config-router)#redistribute static subnets
> R2(config-router)#end
> R2#
>
> R1#show ip route
> Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
> D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
> N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
> E1 - OSPF external type 1, E2 - OSPF external type 2
> i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
> ia - IS-IS inter area, * - candidate default, U - per-user static
> route
> o - ODR, P - periodic downloaded static route
>
> Gateway of last resort is not set
>
> C 200.0.16.0/24 is directly connected, FastEthernet0/0
> C 200.0.12.0/24 is directly connected, Serial0/0.102
> C 200.0.13.0/24 is directly connected, Serial0/0.103
> 10.0.0.0/25 is subnetted, 2 subnets
> O E2 10.0.0.0 [110/20] via 200.0.12.2, 00:00:07, Serial0/0.102
> O E2 10.0.0.128 [110/20] via 200.0.12.2, 00:00:07, Serial0/0.102
>
> HTH,
>
> Brian McGahan, CCIE #8593 (R&S/SP/Security)
> bmcgahan_at_INE.com
>
> Internetwork Expert, Inc.
> http://www.INE.com
>
>
> From: Jacek [mailto:q.192.168.1.0_at_gmail.com]
> Sent: Wednesday, June 29, 2011 10:06 AM
> To: Brian McGahan
> Cc: Cisco certification
> Subject: Re: Advertising subnets of existing networks ?
>
> Thanks for your answer. I think that you are referring to what Pakinstani
> ISP did wrong. I am interested how You Tube reacted to the problem.
> As axplained in RIPEs case study Pakistani ISP started advertising
> 208.65.153.0/24.
> To fix the problem YouTube started announcing 208.65.153.0/25 and
> 208.65.153.128/25, Because of the longest match rule the /25 are preferred
> over /24 and routers that received 2 x /25 routes sent traffic to YouTube.
>
> My question is how can you quickly advertise 2 smaller subnets, in this
> case 2 x /25 instead of one /24 ? I can only suppose that 208.65.153.0/24is redistributed from an IGP where servers reside. How can you then take a
> /24 route from for example OSPF and adverise it via BGP as two /25 routes ?
>
> RIPE case study is here:
>
> http://www.ripe.net/internet-coordination/news/industry-developments/youtube-hijacking-a-ripe-ncc-ris-case-study
>
> On Tue, Jun 28, 2011 at 5:36 PM, Brian McGahan <bmcgahan_at_ine.com> wrote:
> They were trying to Null route it. All you have to do is say:
>
> 208.65.153.128 255.255.255.128 null0
> 208.65.153.0 255.255.255.128 null0
> !
> router bgp 1
> network 208.65.153.128 mask 255.255.255.128
> network 208.65.153.0 mask 255.255.255.128
>
> Blackholes like this are support to be community no-export, but they
> screwed up their config and they leaked the advertisements to their EBGP
> peers.
>
> Brian McGahan, CCIE #8593 (R&S/SP/Security)
> bmcgahan_at_INE.com
>
> Internetwork Expert, Inc.
> http://www.INE.com

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Received on Wed Jun 29 2011 - 15:33:43 ART