Load balancing BGP Johan Nicklasson KTHNOC/NADA · DD2491, p1 2008 Load balancing BGP Johan...

DD2491, p1 2008

Load balancing BGP

Johan Nicklasson KTHNOC/NADA

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DD2491 p1 2008

DD2491, p1 2008

Dual home

• When do you need to be dual homed?

• How should you be dual homed?

– Same provider.

– Different providers.

• What do you need to have in place to do dual homing?

– AS number

– PI vs. PA

– BGP?

DD2491, p1 2008

Single provider

• You can do dual homing to

the same provider.

– What kind of redundancy do

we have in this setup?

– BGP does not load balance

across multiple links.

DD2491, p1 2008

Single provider

• What level of redundancy do we need?

– Do we need redundant routers?

– Do we need to connect to different POPs?

– What about the local loop?

• Can we load balance over redundant links?

– Maybe. In the previous example we could make use of an IGP to load

balance packets over the two links.

– It is not likely that the provider wants to do that. A provider wants his

edge to be as static as possible.

– What about different routers?

DD2491, p1 2008

Single provider

• Redundant routers.

– We can loose 1 router and still

be connected to the internet.

– If the ISP router dies our

connections goes with it.

– The local loop may or may not

be redundant.

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Single provider

• Redundant routers and

redundant POPs

– We can loose one of our

routers and still have

connectivity.

– We can loose one provider

router/POP and still have

connectivity.

– The local loop may or may not

be redundant.

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Single provider

• Dual homed to the same provider.

– We can have different levels of redundancy. Depending on our

needs.

• Do we need BGP to dual home to the same provider?

– Even in the setup with 2 routers and 2 POPs we can use a static

default route to get to the internet.

– We inject the default route into our IGP and the node will send it's

traffic to the nearest exit point.

– Can the ISP load balance traffic to us?

DD2491, p1 2008

Single provider

• If we use BGP on our connections to our ISP

– We must have an iBGP connection between our edge routers.

– We can still use a static default route to the internet.

– The provider can send us a default route via BGP.

– We can use MED, AS prepend or communities to try to get the ISP to

send us traffic to different prefixes over different links.

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Single provider

• If we get a full table from our ISP

– We need our hardware to handle ~230 000 prefixes.

– We can use policies to have the traffic leave on different links

depending on the destination.

– If we don't make our IGP aware of those routes we could end up with

suboptimal routing, depending on the network topology.

DD2491, p1 2008

Single provider

• Addressing and AS numbers

– The provider will assign IP address space to us.

– We don't have to have our own AS number. We can use a private AS,

that have to be assigned to us by our provider.

– Private AS numbers are 64512 to 65535.

– The provider have to remove private AS's from prefixes on their

eBGP peerings.

DD2491, p1 2008

Dual providers

• Our address space becomes an issue.

– We need PI (Provider independent) space

• We need a public AS number.

– How to get an AS number will be covered later.

• BGP is a must.

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Dual providers

DD2491, p1 2008

Dual providers

• If we should use IP space provided by ISP A, 10.1.1.0/24

• That /24 is just a portion of the space provided to the ISP by

the RIR. 10.1.0.0 /19

• We get ISP B to announce “our” /24 (most ISPs will never

announce part of another ISP aggregate).

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Dual providers

DD2491, p1 2008

Dual providers

• Which ISP will attract all our traffic?

– Longest prefix match

• One solution to this problem would be to have ISP A

announce 10.1.1.0/24 and 10.1.0.0/19.

DD2491, p1 2008

Dual providers

DD2491, p1 2008

Dual providers

• Another solution to this problem is to get Provider

Independent (PI) IP space from a RIR (Regional Internet

Registry).

– To use PI space will also make it much easier to switch ISP.

– With PA space the ISP “owns” the IP addresses you use. If you move

to another ISP the first one will make you return the borrowed space.

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Dual providers

• Load balancing the egress

– Using BGP attributes and IGP cost you can prefer one prefix set over

one ISP and another set over the other ISP.

– This will not balance the load equally over the two upstreams.

– If you monitor your traffic patterns you could try to balance the load

more.

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Dual providers

• Load balancing the ingress

– Is it possible to use MED when dual homing with two IPSs?

– Is it possible to use AS prepend?

– How about announcing more specific routes to attract traffic?

• You have to have a good dialog with your ISPs when you are

doing any kind of traffic engineering.

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Symmetry/asymmetry and the internet

• When you have more the one way to reach a destination

symmetry can not be guaranteed.

• Some hardware dealing with state and flow needs symmetry

to work properly.

DD2491, p1 2008

Symmetry/asymmetry

DD2491, p1 2008

Symmetry/asymmetry

• We have to have traffic leaving a firewall return over the

same one.

– We could use AS prepend.

– We could advertise more specific routes

– What if the firewalls exchanged their current flow and state tables?

DD2491, p1 2008

Questions?