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How to BGP works

Unanswered Question
Mar 24th, 2006
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I've got two data centre sites. Each site has is connected by the same ISP, and BGP is is setup between the ISP and eash site:

The diagram looks like this:




The problem I have is when I do a traceroute from the external router in site1 (router1) to the external router in site2(2), the traceroute shows one hop, as though they are directly connected.

How is that possible, shouldn't the traceorute show the route through the ISP's router aswell.




ISP router=

In my router I have the following config:

router bgp dddd

no synchronization

bgp log-neighbor-changes

network vvv.vvv.vvv.0 mask

network xxx.xxx.xxx.0 mask

network zzz.zzz.zzz.96 mask

neighbor vvv.vvv.vvv.1 remote-as nnnn

neighbor vvv.vvv.vvv.1 soft-reconfiguration inbound

neighbor vvv.vvv.vvv.1 route-map isp-to-se in

neighbor vvv.vvv.vvv.1 route-map se-to-isp out

neighbor xxx.xxx.xxx.2 remote-as dddd

neighbor xxx.xxx.xxx.2 soft-reconfiguration inbound

ip prefix-list se-src seq 10 permit xxx.xxx.xxx.0/27

ip prefix-list se-src seq 11 permit zzz.zzz.zzz.96/27

route-map se-to-isp permit 10

match ip address prefix-list se-src

set metric 300


route-map isp-to-se permit 10

set metric 20


Does the above play some part in hiding the actual traceroute? How does BGP play a part in this?



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Harold Ritter Fri, 03/24/2006 - 04:25
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This is in no mean related to BGP. Your SP is probably providing you with a L2VPN service of some sort, which explains your two routers seeing themselves as directly connected.

Hope this helps,


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