I recently got a task on a practice lab that was obviously regarding BGP auto summary. I'm well-practiced in BGP on production systems, but who the heck uses auto-summary any longer? It then occurred to me that I'd never even turned it on.
My first attempt was to:
int lo5
ip address 5.5.5.5 255.255.255.0
router bgp 100
auto-summary
network 5.5.5.0 mask 255.255.255.0
I peered it up with another router, and expected to see "5.0.0.0/8" in the BGP table of the other router.
No such luck, I ended up with 5.5.5.0/24.
After some googling, I found two methods to make this work:
int lo5
ip address 5.5.5.5 255.255.255.0
router bgp 100
auto-summary
network 5.0.0.0
That will produce 5.0.0.0 in both the local BGP table and anyone it peers to.
You can also:
int lo5
ip address 5.5.5.5 255.255.255.0
router bgp 100
auto-summary
redistribute connected
That will also get you 5.0.0.0 in both the local BGP table and anyone it peers to.
Of interesting note, if you:
int lo5
ip address 5.5.5.5 255.255.255.0
int lo6
ip address 5.5.6.6 255.255.255.0
router bgp 100
auto-summary
network 5.5.0.0 mask 255.255.0.0
That will also produce 5.0.0.0/8.
Not a complex topic, but it works differently than the way IGPs do, and I thought it was worth mentioning.
Happy studying!
Jeff
