! Allow use of zero'th subnet
ip subnet-zero
! Disable obsolete IP address classfulness assumptions
ip classless

interface serial 0/0
  desc T1 to ViaNet PacBell Circuit 77HCGF924831-001.pt
  ip address 209.81.51.2 255.255.255.252
  no shutdown

interface fastether  0/0
  desc Local ethernet LAN
  ip address 209.81.100.1 255.255.255.0
  no ip directed-broadcast
  no shutdown

! default route
ip route 0.0.0.0 0.0.0.0 serial 0/0

router bgp 65535
 no synchronization
 bgp always-compare-med
 bgp log-neighbor-changes
 ! announcing just 1 /24
 network 209.81.100.0 255.255.255.0
 neighbor 10.100.1.1  remote-as 4200
 neighbor 10.100.1.1  desc AGIS
 neighbor 209.81.51.1 remote-as 7091
 neighbor 209.81.51.1 desc ViaNet

There are several ways to do this, but probably the best way to do this is to use communities. A community is an arbitrary number (or series of numbers) that you can associate with an individual route prefix.

Using route-maps, we can tag a route that we learn from a BGP neighbor with a community (actually, we can tag routes that we learn from any source.) and we can selectively filter out routes from routing updates that we send to BGP neighbors. These are the two basic properties of route-maps and communities that we're going to use to control route propogation.

Improved config

#
# This configuration treats ViaNet as the Primary network feed, and AGIS
# is relegated to being a backup connection.
#
#
ip subnet-zero 
ip classless
! Display communities as AS:Number instead of 0XXXXXXXXX
ip bgp-community new-format

interface serial 0/0
 desc T1 to ViaNet
 ip address 209.81.51.2 255.255.255.252
 no shutdown

interface serial 0/1
 desc T1 to AGIS
 ip address 10.100.1.2 255.255.255.252
 no shutdown

interface fastether 0/0
 ip address 209.81.100.1 255.255.255.0
 no ip directed-broadcast
 no shutdown

! Here is where we declare our AS number
autonomous-system 65535

router bgp 7091
 no synchronization
 bgp always-compare-med
 bgp log-neighbor-changes

 ! Attach a community to local net blocks that we advertise to the outside world
 network 209.81.100.0 mask 255.255.255.0 route-map ntwk-to-bgp

 ! AGIS BGP peering session
 neighbor 10.100.1.1 remote-as 4200
 neighbor 10.100.1.1 desc AGIS 
 neighbor 10.100.1.1 version 4
 neighbor 10.100.1.1 route-map agis-in in
 neighbor 10.100.1.1 route-map agis-out out

 ! ViaNet Peering Session
 neighbor 209.81.51.1 remote-as 7091
 neighbor 209.81.51.1 desc ViaNet 
 neighbor 209.81.51.1 version 4
 neighbor 209.81.51.1 route-map vianet-in in
 neighbor 209.81.51.1 route-map vianet-out out
 
! Default route points to ViaNet
ip route 0.0.0.0 0.0.0.0 ser 0/0

! Test to see if we should export a route to an upstream ISP
! We only want to advertise our own statically allocated blocks
ip community-list 1 permit 65535:300


! Attach a community value to statically allocated IP blocks
! (e.g. assigned by upstream ISP or ARIN)
route-map ntwk-to-bgp permit 10
 set nlri unicast multicast
 set origin igp
 set community 65535:300

! Each incoming route prefix from AGIS is run through this route-map.
route-map agis-in 10
 set local-preference 100
 set community 65535:100

! Each route prefix that we advertise to AGIS is run through 
! this route-map. If the match against the 'ip community-list'
! passes, the subsequest route-map statements are executed
! and the route prefix is accepted. Otherwise, the route prefix
! is discarded.
route-map agis-out 10
 match community 1
 set metric-type internal

! Each incoming route prefix from ViaNet is run through this route-map.
route-map vianet-in 10
 set local-preference 200
 set community 65535:100

route-map vianet-out 10
 match community 1
 set metric-type internal