BGP and multiple ISPs

[WILLIAM STEGMAN]

I'm trying to design a backup solution that uses two different ISPs and two different routers.  I was thinking of using HSRP on the routers, Rtr-A and Rtr-B, and pointing my firewall to that gateway.  I want to be able to advertise my public networks to both ISPs simultanously both have ISP-A preferred.  Other than appending additional AS paths, is there a way local preference could be used?  I don't see it since my routers are not connected to both ISPs, they are only connected to one, so the remote-as is a different # in each case. 

ISP-A          ISP-B

  |                    |

Rtr-A - hsrp -  Rtr-B

  |                    |

ASA            ASA-failover

  |                    |

LAN              LAN

thank you,

Bill

[Vaibhava Varma · Accepted solution]

Hi Bill

Regarding the above setup for the redundancy perspective we need to look at  two directions ie Forward and Reverse Traffic. We will talk about both them one by one

Forward Traffic

1. Using HSRP in the LAN will provide GW Level redundancy on the LAN in Active:Standby Mode and sends the Traffic to RTRA (assuming RTRA has higher HSRP priority).

2. Now RTRA has an eBGP session with ISPA and no eBGP session with ISPB.Now to have Forward redundancy on RTRA we need to have an iBGP session with RTRB which will inturn send the eBGP routes learned from ISPB to RTRA which will act as a Backup. So we get redundancy here for Forward Traffic without any manipulation. This solution serves the simple purpose of sending out the traffic from that RTR which is the HSRP Master in question.Had it been the requirement that always a particular ISP has to be sending outbound traffic then we need to set the LP of the eBGP routes received on that router to something higher than 100.

Reverse Traffic

As you have mentioned above AS-Path prebpending is the easiest one to use and will work perfectly fine. Had we been dual homed to same ISP only MED would have been another consideration though but personally its more challenging to me :-)

Hope this helps to answer your query.

Regards

Varma

[WILLIAM STEGMAN]

Hi Varma, and thank you.  If using iBGP between my routers, would it be necessary to have either or both routers learn more than the default route from the ISPs?  If so, do you know how much memory or what model router is best suited to take on learning all the Internet routes?

[Vaibhava Varma]

Hi Bill

Default route from both ISPs is way more than enough for Outbound Traffic and we need not learn the full routing table...

Now thinking of the case when we consider to get the full routing table ( not needed though just calculating)

To my best knowledge around 2-3 years back to handle the Full Internet Routing Table Size we were recommending to our Customerrs min 128Mb RAM..With the exponenetil growth in Internet and the current estimate of ~375000 Prefixes and that too receiving from two sides eBGP and iBGP my guess would be 512 Mb RAM minimum but I may be wrong
From Hardware perspective we might need to look at the Cisco Portfolio but guess ISG2 might be helpful.

Hope this helps in your traffic requirement.

Regards

Varma

[WILLIAM STEGMAN]

great, thank you.