Cisco ASA 5500 – VPN Reverse Route Injection With OSPF


KB ID 0000982 Dtd 09/08/14


Reverse Route injection is the process that can be used on a Cisco ASA to take a route for an established VPN, and populate/inject that route into the routing table of other devices in it's routing group.

In the example below, on the main site, we have a Layer 3 switch that's routing all the 192.168.x.x networks, and we have an established site to site VPN to a remote site. To access the web server at the 192.168.x.x networks need a route to it.

Reverse Route Injection

I'm going to use OSPF, (the ASA also supports EIGRP and RIP.)


1. We already have a site to site VPN, let's find out what cryptomap it is using.

Main-GW# show run crypto map
crypto map CRYPTO-MAP 1 match address VPN-INTERESTING-TRAFIC
crypto map CRYPTO-MAP 1 set pfs
crypto map CRYPTO-MAP 1 set peer
crypto map CRYPTO-MAP 1 set ikev1 transform-set VPN-TRANSFORM
crypto map CRYPTO-MAP interface outside

2. So in this example it's called CRYPTO-MAP 1, let's enable reverse route injection for that crypto map.

Main-GW# configure terminal
Main-GW(config)# crypto map CRYPTO-MAP 1 set reverse-route

3. Now create a 'prefix-list' that contains our remote VPN subnet. (Note: if you had a lot of remote subnets you could add them separately or simply summarise them if they were contiguous networks).

Main-GW(config)# prefix-list PL-VPN-ROUTES description Route-Map For Injecting Remote VPN Routes
Main-GW(config)# prefix-list PL-VPN-ROUTES seq 5 permit

4. Now create a 'route-map' that uses our prefix-list.

Main-GW(config)# route-map RM-VPN-ROUTES permit 10
Main-GW(config-route-map)# match ip address prefix-list PL-VPN-ROUTE
Main-GW(config-route-map)# set metric 12000
Main-GW(config-route-map)# set metric-type type-1
Main-GW(config-route-map)# exit
Main-GW(config)# route-map RM-VPN-ROUTES deny 100 

5. To be honest you would already have OSPF setup at this point but, for completeness, let's run through the OSPF setup.

Main-GW(config)# router ospf 20
Main-GW(config-router)# router-id
Main-GW(config-router)# network area 0
Main-GW(config-router)# log-adj-changes

6. And while still in config-router mode, set it to redistribute our VPN network(s). And finally remember the ASA wont send out any networks, without a default-information originate command.

Main-GW(config-router)# redistribute static subnets route-map RM-VPN-ROUTES
Main-GW(config-router)# default-information originate

7. Save the changes.

Main-GW# write mem
Building configuration...
Cryptochecksum: 79745c0a 509726e5 b2c66028 021fdc7d

7424 bytes copied in 1.710 secs (7424 bytes/sec)

8. To make sure it has worked, on the core switch CORE-SW-01, look at the routing table.

Core-SW-01#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is to network

O E1 [110/12011] via, 00:00:04, GigabitEthernet0/0
C is directly connected, GigabitEthernet0/1
C is directly connected, GigabitEthernet0/2
C is directly connected, GigabitEthernet0/3
O*E2 [110/1] via, 00:00:03, GigabitEthernet0/0

Related Articles, References, Credits, or External Links

Cisco ASA 5500 - VPN Reverse Route Injection With EIGRP

Author: Migrated

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