BGP Questions

[carl_townshend]

Hi All

I have a few questions around BGP

- I have always read that the first path that gets installed in BGP will be the one that is always used even if a newer better path comes in, however I read in another document that the best path will always overwrite the current path, which one is true here?

-If there are multiple paths to a destination and 1 is in use, if this path disappears, how long does it take BGP to use the backup path? does it have to run a computation like ospf etc or at the point its in the bgp table this has already been done and it uses it straight away?

Cheers

[Giuseppe Larosa]

Hello Carl,

the rule of the oldestbest path  is valid between eBGP routes in this case the best route is kept the oldest. For iBGP routes this criteria is not used and if an new best path is determined it can take over.

>> If there are multiple paths to a destination and 1 is in use, if this path disappears, how long does it take BGP to use the backup path? does it have to run a computation like ospf etc or at the point its in the bgp table this has already been done and it uses it straight away?

First of all, a BGP update with withdraw section containing the current best path must arrive at the BGP node then there are other timers involved like BGP scanner time .

There is not a computation like in OSPF. However for BGP to scale to so many routes, it works using scheduled tasks like BGP scannner that focus on checking validity of BGP next-hops and there are also timers for propagating a received update that is not flooded but it is sent again after a timer expires (this gives the opportunity to collect multiple updates before sending).

It is not immediate and total time depends on how many BGP routers are in between,

Once the local node knows the previous best path is not valid anymore it will choice a new best path using the BGP timers.

Hope to help

Giuseppe

[carl_townshend]

Thanks Giuseppe,

To clarify your second point, for example what would happen if the next hop went down without the bgp withdraw message? would you still have to wait for the scanner? what is the default time for the scanner?

On average, if we had 2 routes to a destination and if one was lost, how long does it normally take for the other backup route to take over?

cheers

[Giuseppe Larosa]

Hello Carl,

BGP scanner timer should be 60 seconds for IPv4 unicast standard prefixes.

>> what would happen if the next hop went down without the bgp withdraw message? would you still have to wait for the scanner? what is the default time for the scanner?

Really depends on how the interconnection is performed if there is a device in the middle like a LAN Switch the local node will not be able to detect that eBGP peer has failed and will wait the hol

d timer 180 seconds before declaring it down.

If they are directly connected the local router will see the interface down and it should turn down the session with eBGP session and remove the received prefixes.

Hope to help

Giuseppe