NULL 0

Step 5: Configure EBGP
Configure ISP to run EBGP with SanJose1 and SanJose2. Enter the following
commands on ISP:
ISP(config)#router bgp 200
ISP(config-router)#neighbor 192.168.1.6 remote-as 64512
ISP(config-router)#neighbor 192.168.1.2 remote-as 64512
ISP(config-router)#network 192.168.100.0

Step 6: Verify BGP Neighbors
Configure SanJose1 as an EBGP peer to ISP:
SanJose1(config)#ip route 172.16.0.0 255.255.0.0 null0
SanJose1(config)#router bgp 64512
SanJose1(config-router)#neighbor 192.168.1.5 remote-as 200
SanJose1(config-router)#network 172.16.0.0

Step 7: View BGP Summary Output
Configure SanJose2 as an EBGP peer to ISP:
SanJose2(config)#ip route 172.16.0.0 255.255.0.0 null0
SanJose2(config)#router bgp 64512
SanJose2(config-router)#neighbor 192.168.1.1 remote-as 200
SanJose2(config-router)#network 172.16.0.0

i could not understand the job of static route null 0 here could someone take a look and explain plz

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Comments

Plazma

It's a loop prevention mechanism... see the link below:

http://www.cisco.com/en/US/tech/tk364/technologies_tech_note09186a00801c9a6e.shtml

wirerat

I was just going over this lab in the BSCI Lab Portfolio book and I believe the static route is used to define a supernet for the 172.16.1.0/24, 172.16.32.0/24, and 172.16.64.0/24 networks that are in AS64512 that will be advertised by BGP to AS200 as 172.16.0.0/16.

The static route is used to place the supernet route in the routing table so BGP will advertise it to the other AS as the supernet.

Sorry if this doesn't make sense but still figuring it out myself. If anyone could explain this a little better I would appreciate a deeper understanding.

Thanks.

dtlokee

It can be used as a placeholder or a loop prevention mechanism. If you look at how the network statement functions in BGP it is different than how the network statement functions with an IGP. In the case ob BGP to advertise a route with the network statement you must have a corresponding route in the IGP routing table (this can include static and connected routes). In this case if there is no route to 172.16.0.0/16 in the routing table adding one to null 0 will allow BGP to advertise the path to neighboring routers and since null 0 is a black hole for packets (bit bucket) when packets reach this router there needs to be more specific routes to reach inside networks.

An alternitive to this type of configuration is to use the aggragate address command to advertise an aggregate to the BGP peers.

cisco_trooper

Agree with dtlokee here. If you want to advertise a network via BGP, you need to have a route to that network in your routing table. That being said, the static route to null 0 in this scenaria appears to fulfill that requirement so the network can be advertised via BGP...