how does route-map affect network traffic?

[dan_track]

Hi All,

I've got two data centre sites. Each site is connected by the same ISP, and BGP is setup between the ISP and each site:

The diagram looks like this:

ISP--Site1

|

Site2

The router in site1 (router1) has the following configuration:

note:

router1=yyy.yyy.yyy.1

router2=xxx.xxx.xxx.2

ISP router= vvv.vvv.vvv.1

In router1 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

!

The router in site2 (router 2) has pretty much the same as router 1 except the route-map is different, here it says:

route-map se-to-isp permit 10

match ip address 1

set metric 8000

set local-preference 50

!

route-map isp-to-se permit 10

set metric 50

!

My question is how is the difference in the route-map changing the flow of traffic? Why is traffic for xxx.xxx.xxx.0 going down to site 1 while traffic destined to zzz.zzz.zzz.0 goes to site 2?

Thanks in advance.

Dan

[Richard Burts]

Dan

Going by what you have posted (unless there is something you ommitted that changes the functioning - and I note that you do give the prefix-list from site 1 but not the access list 1 from site 2) it looks like both sites are advertising both network xxx.xxx.xxx.0 and zzz.zzz.zzz.0 with equal metrics from that site. So the difference in what is sent to site 1 and what is sent to site 2 is determined by how the ISP is configured, and that is not under your control.

In fact it is generally true of running BGP that you have control of how you sent traffic to your providers and to the Internet. But you have no real control over how the Internet and your provider send traffic to you (they control that).

HTH

Rick