Multi-homed implementation and BGP convergence time

Mailman List Mirror (Read Only)
1

Hello - my company currently has two connections with a single tier 1 ISP.
We are using the AS from our ISP at this time. In the next month we will
be implementing a third connection with a second tier 1 ISP, so we will now
be using our own AS number on all three routers. My question is when we
implement the new connection and update our existing connections to use are
own AS number, how much downtime will there be? So far the second ISP has
only said that it could be hours for BGP to fully converge. We are looking
for more detail about how long the outage will be and how widespread.

Will it be relatively short to our customers that are on one of the ISPs we
are directly connected to? Is downtime less for customers on other tier 1
ISPs versus tier 2, etc. ISPs?

We will only be receiving a default route on each of the three connections.
Our routers will be advertising a small number of routes - 6 to 8.

Thank you.

Andy Claybaugh

2

Hello - my company currently has two connections with a single tier 1 ISP.
We are using the AS from our ISP at this time. In the next month we will
be implementing a third connection with a second tier 1 ISP, so we will now
be using our own AS number on all three routers. My question is when we
implement the new connection and update our existing connections to use are
own AS number, how much downtime will there be? So far the second ISP has
only said that it could be hours for BGP to fully converge. We are looking
for more detail about how long the outage will be and how widespread.

It should not take several hours. Typically less than 15 minutes.

I would suggest that you first ensure that your networks and ASN are in the routing registries. Then schedule a downtime with your present ISP and begin advertising using your ASN.

If you're not presently speaking BGP with your existing ISP, set that up first advertising your network(s) with your own ASN.

Will it be relatively short to our customers that are on one of the ISPs we
are directly connected to? Is downtime less for customers on other tier 1
ISPs versus tier 2, etc. ISPs?

There may be a short downtime when you switch to originating from your own ASN. With sufficient clue on your part and that of your current ISP, and assuming that either of the two connections can handle all of your traffic, you may be able to eliminate most or all of it. Adding the second ISP won't result in significant downtime especially if you're just taking default routes and your routers don't need to build large BGP tables.

"Tier 1", "tier 2" etc. are terms used primarily by salespeople, and don't have a lot to do with technical matters.

3

The time should be measured in seconds for your BGP advertised prefixes
to propagate to most of the Internet. It may take longer for some
isolated ISP's to receive the routes. If you use the longest prefix
method to advertise to your preferred ISP, a convergence to the backup
ISP (where shorter prefixes are advertised) may take 30 seconds or so
max. Converging back to the preferred ISP should take a few seconds max.

4

Jay Hennigan wrote:

"Tier 1", "tier 2" etc. are terms used primarily by salespeople, and
don't have a lot to do with technical matters.

Sure it does. If you're multihoming it will increase your AS path length.

~Seth

5

Hello - my company currently has two connections with a single tier 1 ISP.
We are using the AS from our ISP at this time. In the next month we will
be implementing a third connection with a second tier 1 ISP, so we will now
be using our own AS number on all three routers. My question is when we
implement the new connection and update our existing connections to use are
own AS number, how much downtime will there be? So far the second ISP has
only said that it could be hours for BGP to fully converge. We are looking
for more detail about how long the outage will be and how widespread.

Hours? No way. It's more like minutes.

Will it be relatively short to our customers that are on one of the ISPs we
are directly connected to? Is downtime less for customers on other tier 1
ISPs versus tier 2, etc. ISPs?

Doesn't matter.

We will only be receiving a default route on each of the three connections.
Our routers will be advertising a small number of routes - 6 to 8.

I strongly encourage you to reconsider and take more than a default if
you're multihoming and your routers have enough memory. Remember to
create a full mesh on your BGP routers.

And as already said, if you're totally new to BGP and multihoming, hire
someone with experience in such matters to set it up.

~Seth

6

Seth Mattinen wrote:

Jay Hennigan wrote:

"Tier 1", "tier 2" etc. are terms used primarily by salespeople, and
don't have a lot to do with technical matters.

Sure it does. If you're multihoming it will increase your AS path length.

There is no general correlation between AS path length and whether
or not a network pays to exchange traffic.

There is a noticeable correlation between cost and local-preference,
as-path prepending, metric setting and other ways networks control
how they send you traffic. This is affected by peering selectivity
as well as transit prices.

- Kevin

7

Kevin Loch wrote:

Seth Mattinen wrote:

Jay Hennigan wrote:

"Tier 1", "tier 2" etc. are terms used primarily by salespeople, and
don't have a lot to do with technical matters.

Sure it does. If you're multihoming it will increase your AS path length.

There is no general correlation between AS path length and whether
or not a network pays to exchange traffic.

That has nothing to do with what I was trying to say. When one mixes
shorter/longer paths, you will generally see most of your traffic come
in via the shorter path.

It's like if I were to multihome with the local cable co (who has Level3
as an upstream) with a connection to Level3 myself. It's not likely that
traffic inbound to me is going to choose the longer route unless the
shorter one is down, and it's pointless as a backup because a regional
outage affecting Level3 may kill the cable co's link to them as well.

~Seth

8

Hello - my company currently has two connections with a single tier 1 ISP. We are using the AS from our ISP at this time. In the next month we will be implementing a third connection with a second tier 1 ISP, so we will now be using our own AS number on all three routers.

Does this mean that right now, you BGP peer with your ISP on a private ASN which they have given you ?

I also assume that you have your own PI, and that you are not deaggregating some of your providers' addressing ....

My question is when we implement the new connection and update our existing connections to use are own AS number, how much downtime will there be? So far the second ISP has only said that it could be hours for BGP to fully converge. We are looking for more detail about how long the outage will be and how widespread.

It will be hours if you don't plan the work in advance, but if you partner with someone who rolls this stuff out all of the time to plan and execute the work, then there will be a short amount of downtime.

If your kit supports local-as, then I would roll this out in a few phases.

  - Migrate to your new ASN for ibgp, use local-as to announce via the old asn on your ebgp session with ISP1. This is the bit where the service disruption will be. By keeping the scope of this window small, you increase the chances of this disruptive maintenance working fine.
  - Turn up isp2. Test, thoroughly.
  - Migrate isp1 from the private asn to your new public asn. All traffic should pass through isp2, so disruption should be limited. Test, thoroughly.

Will it be relatively short to our customers that are on one of the ISPs we are directly connected to? Is downtime less for customers on other tier 1 ISPs versus tier 2, etc. ISPs?

Downtime is less the more competent your ISP. :slight_smile: "Tierness" is not a measure of this.

Sorry for the late reply, if this still needs to be rolled out, then we can help.

Best wishes
Andy