BGP - Using same AS different sites

[carl_townshend]

Hi All

We have 2 main hub sites, we have just ordered a second MPLS circuit for the other site.

Can we use the same AS number on each site? or is it advised to have different AS numbers?

The 2 sites do have a connection between them

I am guessing it will work with same or different AS on each site?

if using the same, would it be best to use IBGP between them or would it not matter?

cheers

[Julio E. Moisa]

Hi

It can works but all depends of your engineering how the traffic will flow, if you are going to have one site as primary and other as backup or if they are going to be independent, the link between them must support the bandwidth capacity from each other site. 

Do you have a diagram or could you please provide more details about the network?

:-)

[carl_townshend]

I don't have a diagram yet

The link between the sites is 100Mbit, the MPLS is 32Mbit on each site.

[Julio E. Moisa]

Thank you, you could have eBGP between your router and MPLS and iBGP between the sites or redistribute the prefixes learned by BGP into a IGP like EIGRP or OSPF on your network. 

[carl_townshend]

it is not requirement to have ibgp between the sites? can we run without it?

would you ask the isp for same AS on each site or 2 different AS numbers ?

[Julio E. Moisa]

The communication between the site is through a dedicated link? Please correct me but I see your topology like:

      ISP A (mpls)                                             ISP B (mpls)

        |                                                               |

      ebgp                                                        ebgp

        |                                                               |

     Router A  ---------- ibgp ----------- Router B

        |                                                               |

   internal network                                         internal network

[mahditalebi]

Dear Carl,

According to your requirements, your connection to Internet could be differ. But the following options are usually used:

Option 1:

Different sites / Single AS / no iBGP connections between sites : In this situation you need subnet your IP address range to multiple lower size subnets and advertise each subnet from one site.

Option 2: 

Different Site/ Single AS / iBGP connections between sites: In this situation you can advertise your whole IP Address range from each site and according to clients distances from your sites, they will use your specific site. (This option is most popular)

HTH

Mahdi

[carl_townshend]

Hi

What if we did not have IBGP but wanted to advertise both sites through both links, would this still be OK?

I guess we would need to use AS prepend with a route map to match certain subnets that we want to route inbound ?

[Mark Malone]

This is just off my head without knwoning full setup but could you not use Local AS feature , i use it with dual internet EBGP circuits for same ISP as it was causing issues coming up but this managed us to have dual circuits same address space advertisecd out both but as you said we also use AS path preference to chose one path for some subnets first and vice versa

#The local-AS feature allows a router to appear to be a member of a second autonomous system (AS), in addition to its real AS. This feature can only be used for true eBGP peers. You cannot use this feature for two peers that are members of different confederation sub-ASs.

[carl_townshend]

What about having different AS number on each hub site?

[Mark Malone]

having a diff AS is fine , if you have same AS though on different sites with same ISP the ISP will drop it as it will think its a loop unless you use neighbor allow-as in command , so both CE routers can speak to each other ,or else speak with teh ISP and get them to use the override command on the PE side , either should allow you to have same AS in different locations and allow them to speak to each other without being dropped

[carl_townshend]

Hi Mark

What I need to achieve is basically to have site A and site B, but I need to advertise site A's subnets at site B if the site is lost.

How would I best to do this, use different AS on each site?

and use some sort of advertise map on site B that only announces site A's subnets if site A is down?

is that possible ?

Another question would be, if I advertised same subnet from different AS, what would happen?

cheers

[cofee]

What I need to achieve is basically to have site A and site B, but I need to advertise site A's subnets at site B if the site is lost .How would I best to do this, use different AS on each site?

- If possible you can advertise a longer match from your primary site and a shorter match from standby site.

Or

Use AS prepending.

 use some sort of advertise map on site B that only announces site A's subnets if site A is down?is that possible ?

- You can do it as long as there is a condition to trigger this like losing a prefix from site A

Another question would be, if I advertised same subnet from different AS, what would happen?

This should depend on BGP route attributes and will most likely cause unpredictable behavior. 

[Mark Malone]

- If possible you can advertise a longer match from your primary site and a shorter match from standby site.

Yes you can , we advertise say a /16 from US but then override locally in EU with /24 as an example , AS prepend will be important if you have dual links on same site advertising same /24 , you would need to make 1 less preferred

I would not advertise exact same subnet from 2 sites that will cause an issue , could blackhole traffic for that subnet , using longer shorter prefix match would be cleaner , than trying to match routes in table and remove them if invalid at 1 location