A few examples of advertising a default-route within RIPv2 using different techniques, some was a bit tricky to figure out. The requirements were as follows (three separate labs):
- From R6 – Advertise a default-route via RIP only outbound on Vl146, you are not allowed to use any access/prefix-lists
- From R4 – Advertise a default-route via RIP as long as R4 has a route to R9s loopback
- From R1 – Advertise a default-route via RIP as long as R1 has reachability to R7s LAN-interface 188.8.131.52, otherwise withdraw route
First lab – Advertise by outbound interface
Advertising a default-route on a specific interface without filtering by accesss/prefix-list we could instead use a route-map.
! R6 route-map FILTER permit 10 set interface Gi1.146 router rip default-information originate route-map FILTER
Big difference from ex. OSPF is that RIP doesn’t require the route to be in it’s actual routing-table to advertise it, which in turn leads to a routing loop in our topology. R6 advertise the route to R1 & R4, R1 will in turn advertise it to R7 who will forward it to R6. R6 will accept the route as it dosen’t have a default-route in it’s table and advertise that.
R6#sh ip rip database 0.0.0.0 0.0.0.0 0.0.0.0/0  via 184.108.40.206, 00:00:07, GigabitEthernet1.67 R6#sh ip rip database 0.0.0.0 0.0.0.0 0.0.0.0/0  via 220.127.116.11, 00:00:00, GigabitEthernet1.67 R1#sh ip route | beg Gate Gateway of last resort is 18.104.22.168 to network 0.0.0.0 R* 0.0.0.0/0 [120/13] via 22.214.171.124, 00:00:02, GigabitEthernet1.146
This can be solved in many ways, I chose to insert a dummy default-route to null0, but you could also use filtering etc.
! R6 ip route 0.0.0.0 0.0.0.0 null0
R6 will now ignore the default-route advertisement from R7 and not propagate it any further.
R6#sh ip rip database 0.0.0.0 0.0.0.0 0.0.0.0/0 redistributed  via 0.0.0.0, R8#sh ip rip database 0.0.0.0/0 auto-summary 0.0.0.0/0  via 126.96.36.199, 00:00:09, GigabitEthernet1.58
Second lab – Conditional default-route
This lab requires us to originate a default-route from R4 as long as it has a route to R9s loopback0, the final solution looked like this for me:
! R4 ip prefix-list R9 permit 188.8.131.52/32 route-map R9_TRACKING permit 10 match ip address prefix-list R9 ip route 0.0.0.0 0.0.0.0 null0 router rip default-information originate route-map R9_TRACKING
The logic is that as long as our route-map matches the prefix-list of R9s loopback it will advertise the default-route, and we add a static route to avoid routing loops via the DMVPN-hub R5 (no split-horizon). Let’s verify to be sure.
R5#sh ip route | inc 0.0.0.0 Gateway of last resort is 184.108.40.206 to network 0.0.0.0 R* 0.0.0.0/0 [120/1] via 220.127.116.11, 00:00:05, GigabitEthernet1.45
If we shut R9s loopback the default-route should time out eventually.
! R9 int Lo0 shut R4#sh ip route 18.104.22.168 % Subnet not in table R5#sh ip route 0.0.0.0 % Network not in table
Third lab – IP SLA & default-route
This lab requires us to advertise a default-route as long as R1 has reachability to R7s LAN-interface 22.214.171.124, otherwise withdraw route. So obviously we’re looking at setting up IP SLA to start with.
! R1 ip sla 1 icmp-echo 126.96.36.199 frequency 5 ip sla schedule 1 start-time now life forever track 1 ip sla 1 R1#sh track 1 Track 1 IP SLA 1 state State is Up
I couldn’t figure out how to use our tracker in RIP however, eventually I found a pretty neat solution that might not be the prettiest, but it does the trick. First we create a “dummy-route” together with our tracker.
ip route 169.254.254.1 255.255.255.255 null0 track 1
Next step we borrow from our second lab, we create a prefix-list matching our dummy-route together with a route-map that we then use as a condition for our default-route advertising.
! R1 ip prefix-list DUMMY_FILTER permit 169.254.254.1/32
route-map DUMMY permit 10 match ip address prefix-list DUMMY_FILTER router rip default-information originate route-map DUMMY
The logic is, when our tracker (testing icmp-reachability to 188.8.131.52) goes down, our dummy static route will be removed from the routing table. This will in turn make rip stop advertising (or rather poison-reverse) the default as our route-map no longer has any match. Let’s try it!
! R7 interface Gi1.7 shut ! R1 R1#debug ip rip RIP protocol debugging is on R1# %TRACK-6-STATE: 1 ip sla 1 state Up -> Down R1# RIP: sending v2 flash update to 184.108.40.206 via Loopback0 (220.127.116.11) RIP: build flash update entries 0.0.0.0/0 via 0.0.0.0, metric 16, tag 0 18.104.22.168/24 via 0.0.0.0, metric 16, tag 0
As our tracker goes down, R1 poisons the default-route and it will eventually timeout in our other routers.
R2#sh ip route 0.0.0.0 % Network not in table
Fun stuff, even RIP can be pretty tricky even though it’s such a basic protocol compared to the rest. 🙂