Partial Mesh multiple path routing design

adam · ‎10-10-2008

I'm looking for input pertaining to best practice for establishing a redundant backhaul system.

We have a primary wireless link from the office to the first tower. That tower then links to two other towers and those link to others creating a partial mesh of linked Wireless towers which then backhaul to Tower 1 and over to the main office before being sent out the OC3.

We are attempting to create a routing solution so that when a wireless backhaul link fails, it reroutes the traffic over the next available wireless link. Some towers would also have tertiary T1 links available when it or it's segment gets isolated from the rest of the mesh. The T1 links terminate to the core router at the office.

We use Cisco routers at each location and we are looking for assistance in determining the best routing protocol to accomplish this and how to implement it.

Initial thoughts were leaning to weighted static routing or possibly OSPF. I've also included an image for further clarification.

I do not have any experience with OSPF routing, but it is configured from the previous engineer on some of the T1 links we plan to use for tertiary backhaul.

I'm hoping for input as to which routing protocol would be best for this, as well as details such as what type of ospf networks to use, should they all be area 0, which routers would require "redistribute static subnets or connected subnets" config lines. Should a second ospf area be established and if so which point would be best to assign as ASBR? Is OSPF really the best protocol for this type of design?

Management would like for us to deploy VOIP over this at some point, but right now it is to be used for standard wireless Internet subscribers from 256k to 3mb.

R0 and R1 are 7200VXRs

The towers all have 1700s currently.

Initial tests, indicated that when enabling OSPF on all interfaces with 'network 0.0.0.0 255.255.255.255 area 0' at each location, the problem seems to be that OSPF doesn't know to direct off-network traffic out the OC3. Local services work fine, but requests off-net never get forwarded out the OC3.

Giuseppe Larosa · ‎10-10-2008

Hello Adam,

OSPF can be used in your scenario.

For Internet routing you need the router attached to internet to generate a default route and propagate it in the OSPF domain

R0:

router ospf 10

default-information originate metric-type type-1

an O E1 0.0.0.0/0 will be generated and sent out interfaces to other routers.

An O E1 is to be preferred over the O E2 in this case.

(needs a default route to be in R0 routing table)

to build primary, secondary, tertiary paths you can do in two way:

a) change /modif bandwidth

int type x/y

bandwidth xxx [kbps]

the ip ospf cost will be 10^8 /BW [bps]

so an ethernet link has cost 10

an FE cost 1

a T1 cost 64

b) modify the ip ospf cost directly

int type x/y

ip ospf cost value

I think that keeping all backbone links in area 0 is fine.

You can consider to use additional areas one for each tower/building

Hope to help

Giuseppe