Dual Internet with BGP Failover

[mikegrous]

1 Router Has:

     2 /24 Public IP blocks.

     2 ISPs.

     Both public blocks will get sent up the the ISP. (We will call them...Public Subnet Block A and Block B)

So for Block A or B (depending where you are on the internet) your traffic will take 1 ISP path or the other. When 1 ISP Peer fails how long will it take for the internet to reconverge so that all traffic for public subnet block A and B only come through that 1 remaining ISP?

Are we talking seconds/ minutes/ hours

[Vaibhava Varma]

Hi mikegrous

My viewpoint on above scenario

Lets assume the below setup which will resemble the Interet Scenario

     --eBGP------ISP1----------iBGP-------ISP1----------eBGP------

(X)CE1                                                                              CE2

     ---eBGP------ISP2----------iBGP------ISP2---------eBGP------

Lets say Prefix X is sent via ISP1 as preferred and via ISP2 as Backup and received by CE2 in same way. Now when the CE1-Link_to-ISP1 goes down the below events would be required at CE2 to get updated of the Prefix X withdrwal via ISP2

a) At ISP2 (facing CE1) Next-Hop_Tracking to delcare eBGP session with CE1 as  down and withdraw the Prefix X (Default 5 seconds)

b) ISP2 (facing CE1 advertising the withdrwal of Prefix X in the iBGP Update ( Default advertsiement interval 5 seconds)

c) ISP2 (facing CE2) updating its RIB with Prefix X withdrwal ( Default BGP Scan Time 60 seconds )

d) ISP2 (facing CE2 advertising the withdrwal of Prefix X in the eBGP Update ( Default advertsiement interval 30 seconds)

e) CE2  updating its RIB with Prefix X withdrwal from ISP2 ( Default BGP Scan Time 60 seconds )

So in my personal view everything happens at T0 = 0 seconds the max time should be ( 5 + 5 + 60 + 30 +60 =160 seconds = 2 mins 40 secs)

So, as it seems from above the worst case convergence would be ~ 3 mins.

Hope this helps something in your understanding. Would be nice to hear someone's else view on this interesting questions and get a better understanding on anything missed.

Regards

Varma

[Edison Ortiz]

It depends on the ISP and how aggressive their BGP scan intervals are. With that said, reconverging should take a minute or two max.