BGP config to advertise 16 bit subnet, for 24 bit subnets

[wilson_1234_2]

I have a remote branch that is connected to my Main site via BGP.

The BGP is up and active.

I was unable to avertise a 16 bit subnet to the main branch for my 24 bit VLAN addresses that have been created.

I had the below config at first and the router did not advertise my subnet to the main branch until I changed the mask on the router interface to a 16 bit subnet (10.1.0.0).

The goal is to advrtise a 16 bit subnet so I don't have every route from the remote branch in the main branch route table and I can communicate with all subnets.

Can this be done?

Router

interface FastEthernet0/0

description Link to switch

ip address 10.1.1.1 255.255.255.0

!

router bgp 65004

no synchronization

bgp log-neighbor-changes

network 10.1.0.0 mask 255.255.0.0

network 172.16.254.1 mask 255.255.255.255

network 1.2.1.52 mask 255.255.255.252

neighbor 1.2.1.53 remote-as 65000

neighbor 1.2.1.53 version 4

neighbor 1.2.1.53 soft-reconfiguration inbound

no auto-summary

Switch

VLan 1

10.1.1.2 255.255.255.0

Vlan 2

10.1.10.1 255.255.255.0

Vlan 3

10.1.100.1 255.255.255.0

Vlan 4

10.1.200.1 255.255.255.0

[Johnson_Mo]

I had the exact same issue with EIGRO where I have 192.168.2.1/16 and 192.168.1.1/24 directly connected. on the neighbor router I had 192.168.0.0/16 installed not 192.168.1.0/24, so I added redistributed connected under EIGRP and /24 installed successfully. thanks for the wonderful explanation! 

[Jon Marshall]

"I have an image on the 3560 that I could configure ospf between the switch and router, if I do that would you suggest distributing all routes into BGP?"

You can but as i say it doesn't make a lot of difference in your scenario as long as you use the example Edison gave ie. network 10.1.1.0 mask 255.255.255.0.

If you used any of your other subnets under the BGP config instead then yes it could make a difference.

But as i say if 10.1.1.0/24 is lost then you can't get to the switch subnets anyway from outside so it doesn't make much difference. If in the future though you end up with redundancy in the branch with more than one path to the L3 switch then yes running an IGP like OSPF between the L3 switch and router(s) would make sense.

Jon

[wilson_1234_2]

I was just thinking about this,

couldn't I have solved the problem if I had just put a static route in the router.

For example, if my link to the switch is

Router Switch

10.1.1.1/30 --> 10.1.1.2/30

Put a static route in the router:

10.1.0.0 255.255.0.0 10.1.1.2

[Edison Ortiz]

Yes, with a static route will solve the issue without having an aggregate.

It's actually a perfect solution

1) You instruct the router about the other subnets in the switch

2) It allows you to enter the network 10.1.0.0 mask 255.255.0.0 under BGP.

Good job Richard :)

Remember to have a default route configured on the switch pointing to the router LAN interface.

[wilson_1234_2]

thanks jon and edison,

you guys mentioned the static route first.

thanks for all of the help.