BGP Looking Glass

[jkaftan]

I have a dual homed internet connection with Level3 and ATT. I have the ATT side prepended to make it less desirable and thus my secondary. When I go out to the looking glass sites I expect to see both my internet connections as options on the ATT path showing my AS number twice.

However I do not see that. I just see my the route to my Primary (Level3) connection.

Any idea why I am not seeing both routes? I believe I did see both back when I first set this up last summer.

If I login to a global crossings router and do a sh ip bgp 72.237.4.150 I get the following

Not advertised to any peer
3356 26808, (received & used)
67.17.64.89 from 67.17.80.155 (67.17.80.155)
Origin IGP, metric 100, localpref 200, valid, internal, best
Community: 3549:2524 3549:31826
Originator: 67.17.80.178, Cluster list: 0.0.0.111
3356 26808, (received & used)

If I log into an ATT router and do the same I get the following.


BGP routing table entry for 72.237.4.0/24, version 17668211
Paths: (18 available, best #18, table Default-IP-Routing-Table)
Not advertised to any peer
7018 26808 26808 26808, (received & used)
12.123.45.252 from 12.123.45.252 (12.123.45.252)
Origin IGP, localpref 100, valid, external
Community: 7018:2000 7018:37232
7018 26808 26808 26808, (received & used)
12.123.137.124 from 12.123.137.124 (12.123.137.124)
Origin IGP, localpref 100, valid, external
Community: 7018:2000 7018:37232
7018 26808 26808 26808, (received & used)
12.123.25.245 from 12.123.25.245 (12.123.25.245)
Origin IGP, localpref 100, valid, external
Community: 7018:2000 7018:37232
7018 26808 26808 26808, (received & used)


So I know ATT is receiving my advertisement for the subnet in question. I would just expect that other routers would see both routes.


Thanks

 

[jneiberger]

To make matters more interesting, I just checked Qwest's looking glass and it sees only your Global Crossing route, so it is being advertised.

Internet routing can be a weird thing. You never really know how other organizations are going to handle your traffic or your routing advertisements. As you can see, it's pretty difficult to control how people reach your network. You can try to influence it, but you have no real control over it. Outbound load sharing is easy. Inbound is far trickier.

Are you seeing incoming traffic on both links?

 

[UCAtech]

ATT's route server sees and prefers it's own session with you as the best hop, I'm guessing other ISP's trust L3 over ATT. I tried from Sprint's looking glass and they go through your L3 connection.

ATT wouldn't accept L3's advertisement and L3 wouldn't accept ATT's advertisement because they both have better ways to get to you than the other. Why would ATT want to say "to get to our customer, who we peer with, please go through L3". Likewise with L3.

Ben Boyd
Network Engineer/Programmer/Student
Windstream Communications
Linuxbox Server Stats: 700mhz, 1024 MB RAM, 120 GB HD, Fedora 3
Laptop Stats: 2.0 Ghz, 1024MB RAM, 40GB HD, Windows XP Professional
Desktop Stats: 2.6 Ghz, 4096MB RAM, 3 x 120 GB HD, 8500GT, Windows XP 64bit Edition

 

[CholoE]

ATT has received your route but is not advertising it to any BGP peer. That is why you are not seeing it at any route servers. You'll need to contact ATT to let them know you want your IP block advertised out in order to correct this.

As far as UCAtechs statement "Why would ATT want to say "to get to our customer, who we peer with, please go through L3". BGP decides routes by AS Path, since the route via ATT is prepended, the hosts router will find out that it has a longer AS Path to go thru ATT than it is via Level3 therefore will prefer to take Level3 path. So, filtering advertisements between them, in this specific example, is completely unnecessary.

 

[BuckWeet]

The ATT looking glass is that..Part of ATT.. As its BGP neighbors will only advertise their best route to the looking glass you are at the discretion of the neighbors.. What you are seeing is expected..

You might try to find some independent universities that have looking glass systems connected to multiple ISP's and see its results.


BuckWeet