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New Member

BGP auto-summary

I am testing in a lab and my question is: When I inject a specific route (subnet) into the BGP via the network command with auto-summary disabled, the Next hop to that subnet appears as 0.0.0.0, so the BGP router itself. However, when I disable suto-summary, the Next hop changes to the router within my AS which is the actual gateway into the subnet

RTA#sh ip bgp 30.30.30.0

BGP routing table entry for 30.30.30.0/24, version 132

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Flag: 0x210

Advertised to non peer-group peers:

10.1.1.2 172.16.12.2 172.26.16.2

Local

20.20.0.2 from 0.0.0.0 (172.16.12.1)

Origin IGP, metric 11, localpref 100, weight 32768, valid, sourced, local, best

20.20.0.2 being the actual gateway to the subnet 30.30.30.0

can anyone explain this behavior?

2 REPLIES
New Member

Re: BGP auto-summary

Hi,

BGP speakers do not advertise prefix if there not found in/knowed by IGP. If the specific route is known by IGP and the network statement in BGP is configure (with auto-sum enabled) it will: Summarizes subprefixes to the classful network boundary when crossing classful network boundaries. (with auto-sum disabled): By default, BGP does not accept subnets redistributed from Interior Gateway Protocol (IGP). To advertise and carry subnet routes in BGP, use no auto-summary command.

All can ofcourse depend on how your other BGP speakers are configured with syncronization and auto-summary and your IGP.

New Member

Re: BGP auto-summary

Thanks for zour reply! Anyway, I am aware of how auto-summary works in BGP, I am just a bit confised why the Next hop of a subnet configured with network command (auto-summary enanled) is 0.0.0.0 (the local router), but WHY is changes to the actual IGP Next hop with auto-summary DISabled...Can you help with this one?

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