BGP conditional advertisement with non-exist-map - can we modify logic

[owaisberg]

Hi,

I need to implement BGP conditional advertisement with non-exist-map, but in a different logic.

As by the book, BGP will advertise prefixes listed in advertise-map only if the routes matched in

non-exist-map do not exist, but if at least one of them exists - advertisement will be withdrawn.

Now, I am trying to find a way to initiate advertisement through BGP if  ANY of the routes in the 

non-exist-map doesn't exist. Other words, if at least one of the prefixes within the non-exist-map

doesn't exist, BGP will start advertisement.

Tried to play with a non-exist-map route-map using multiple seq. entries for different routes - no

success. Tried to use multiple "neighbor x.x.x.x addvertise-map" lines with same advertise-map

and different non-exist-map route-maps to reflect multiple routes looks like no success either,

as soon as one of the conditions matches to be withdrawn, next one even if matches to be as

advertised, has no effect and BGP sends no advertisements. Hope there is a trick to go around

this limitation.

Thanks in advance

[owaisberg]

Yes, first statement is correct, if MPLS goes down, then no prefixes are to be redistributed into the branch LAN on CE and all the prefix exchange shifts to iBGP backup (I've just updated the diagram, marking where is what for clarity).

Your second statement: it could be a potential work around, however the CEs are owned by the SP so it requires their involvement plus they may simply refuse. As well that will have to be done on all the CEs and we are talking multiple sites, pretty painfull thing.

Thanks again.

[Jon Marshall]

No problem I understand.

So again, just thinking out loud, is there any chance within the branch you can advertise a summary route for the branch subnets via IBGP across the backup link ?

That way when the CE interface came back up it would have a summary for it's own branch subnets from EBGP but more specific EIGRP routes from within the branch itself.

Jon

[owaisberg]

Unfortunately this is not an option, as each branch router has a bunch of networks and subnets that cannot be summarized as they are all over the map  :(

[Jon Marshall]

Just out of interest you mentioned GRE tunnels and running EIGRP everywhere, can you not run the tunnel from the branch router itself to the hub router ie. why does it need to be between the CE devices ?

That aside one final idea for the day before logging off.

Use EEM on the branch router and if the CE EIGRP neighbor goes down then configure a "redistribute eigrp <AS no>" under your BGP configuration on the branch router.

If it comes back up then remove that command.

There would be a delay for the routes to propagate but there would be anyway with your original idea.

If I think of anything else tomorrow I'll post back :)

Jon

[Tyler Conrad]

Little late to the party on this one, but found it while searching for something similar and decided to take a shot.

Try this - doing it from memory, so cli syntax may be a little off.

neighbor x.x.x.x advertise-map ADVERTISEDROUTES non-exist-map TRACKEDROUTE

track 5 ip route 192.0.2.50/32 reachability

track 6 ip route 192.0.2.51/32 reachability 

track 10 list boolean AND

 object 5

 object 6

ip route 192.0.2.1/32 track 10

route-map ADVERTISEDROUTES permit 10

match ip address prefix-list YOURADVERTISEDROUTES

route-map TRACKEDROUTE permit 10

match ip address prefix-list TRACKED

ip prefix-list TRACKED permit 192.0.2.1/32

Redistribute that static /32 route using a route-map to BGP, and then if either of the tracked objects go down, the /32 will be withdrawn, and your conditional advertisement will be advertised.