How does BGP work?

Hi

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:

ISP--Site1 | Site2

The problem I have is when I do a traceroute from the external router in site1 (router1) to the external router in site2 (router 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.

note: router1=yyy.yyy.yyy.1 router2=xxx.xxx.xxx.2 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 255.255.255.252 network xxx.xxx.xxx.0 mask 255.255.255.224 network zzz.zzz.zzz.96 mask 255.255.255.224 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?

Thanks Dan

Reply to
Dan_track
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Hi

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:

ISP--Site1 | Site2

The problem I have is when I do a traceroute from the external router in site1 (router1) to the external router in site2 (router 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.

note: router1=yyy.yyy.yyy.1 router2=xxx.xxx.xxx.2 ISP router=vvv.vvv.vvv.1

In my router I have the following config:

router bgp dddd no synchronization bgp log-neighbor-changes network vvv.vvv.vvv.0 mask 255.255.255.252 network xxx.xxx.xxx.0 mask 255.255.255.224 network zzz.zzz.zzz.96 mask 255.255.255.224 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?

Thanks Dan

Reply to
Dan_track

It's possible if for some reason your first packet leaves with TTL=2, or if ISP router doesn't change TTL. The later can be, for example, if your ISP has provisioned your lines as MPLS L2VPN. BGP doesn't really have anything to do in hiding hops, TTL is the trick why you do or don't see intermediate hops. Have a look at page 19 of

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Kind regards, iLya

Reply to
Charlie Root

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