BGP Route Announcement

[StarTAC]

hi all.. i have a quick question..

i am going to start multihoming with my existing provider and a second new provider.. i have a /22 prefix that i am currently advertising thru ISP A, and ISP A is originating default route to me..

ISP B, my backup ISP, will also originate default route to me.. and i will use weights to prefer ISP A's connection as long as it's always up... if they go down, and their default route gets deleted from my routing table, ISP B's default route will be installed from BGP's Loc-RIB into my active routing table..

my question is - when both ISP A and ISP B are up and running, and i am announcing my /22 to both of them, is there any chance that traffic i send out thru ISP A will return thru ISP B's link, even when ISP A is still up..?..

also, as i advertise thru ISP B, do they also announce to the rest of the world that they know how to reach me..?.. if so, how does the world treat that..?.. if i send a request to a network closer to ISP B than ISP A, will the response come in via ISP A or ISP B's link, even when i need it to come thru the longer path, ISP A..?.. how do i ensure i have all this in check..?..

all help appreciated..

 

[intelwizrd]

I had a similar problem. there are two ways you can solve this, prepend your as number a bunch of times to your route announcements or alter the metric on your route announcements.

here is a sample of what i did

replace your ip's with my sample ones as needed

192.168.0.0/22 (your network)
192.168.25.31 (secondary isp)
192.168.29.47 (primary isp)

what "show run" should show
router bgp (your as number here)
network 192.168.0.0 mask 255.255.252.0
neighbor 192.168.25.31 remote-as (as for neighbor 192.168.25.31)
neighbor 192.168.25.31 route-map setmetric out
neighbor 192.168.29.47 remote-as (as for neighbor 192.168.25.47)
!
ip route 0.0.0.0 0.0.0.0 192.168.29.47
!
access-list 10 permit 192.168.0.0 0.0.3.255
!
route-map setmetric permit 10
match ip address 10
set metric 50


with this configuration, all traffic should come back in your primary ISP. this will also provide fail over (though as with any line failure it will take some time for remote routers to remove the routes to your network (through your primary isp) from their routing tables. if you try this and it works backwards move the routemap statement to the other neighbor. I had to adapt this as i have quite a few networks and multiple connections to different isp's

 

[baddos]

I would do the as path prepending. Your more likely that all the routers on the net will behave the way you want.

I would also make an inbound route-map for your two peers and set the local preference. This way you know for sure which route you will be using for outbound when they are both up.

Also... I would use a prefix-list since they are a little bit faster.

Here's a modified version of intelwizard's config:

192.168.0.0/22 (your network)
192.168.25.31 (secondary isp)
192.168.29.47 (primary isp)

what "show run" should show
router bgp (your as number here)
network 192.168.0.0 mask 255.255.252.0
neighbor 192.168.25.31 remote-as (as for neighbor 192.168.25.31)
neighbor 192.168.25.31 route-map isp1out out
neighbor 192.168.25.31 route-map isp1in in
neighbor 192.168.29.47 remote-as (as for neighbor 192.168.25.47)
neighbor 192.168.29.47 route-map isp2out out
neighbor 192.168.29.47 route-map isp2in in
!
ip prefix-list mynet seq 5 permit 192.168.0.0/22
!
route-map isp1in permit 10
set local-preference 100
!
route-map isp1out permit 10
match ip address prefix-list mynet
set metric 50
set as-path prepend (your as number here) (your as number here) (your as number here) (your as number here)
!I.e. it would look something like set as-path prepend 100 100 100 100
!
route-map isp2in permit 10
set local-preference 50
!
route-map isp2out permit 10
match ip address prefix-list mynet
set metric 100


One thing here to note is that I wouldn't put in a static default route in this instance. This is because if the peer goes down, that static is still a good route and removes redundancy.

 

[StarTAC]

the AS prepend path looks like a more workable solution...

the route-map, isp1in that sets their default information to local pref 100 is ok, but according to the example, it corresponds to the neighbor who is ISP 2.. however, the isp1out route-map looks correct, to send a longer as path to the backup ISP, although the route-map says isp1out, yet it shd be saying isp2out, i guess...

plz clarify on the example as well as placements of the route-maps based on the same.. seems a little out of place to me..

all help appreciated..