LAN, Switching and Routing

Route leaking is a technique which is used together with summarization. It is used in the situations where we want to save memory by summarizing routes but still want some routes to be preffered over others for some reasons. Leak map reffernces an access-list and whatever network is permitted in the access-list will be leaked along summary route.

Topology and addressing scheme:

R1 has two point-to-point sub-interfaces going to frame-relay cloud connecting it to R2 and R3 respectively:

serial 1/0.12 : 10.1.12.1/24
serial 1/0.13 : 10.1.13.1/24

it also has three loopback interfaces:

loo 1 : 1.1.0.1/24
loo 2 : 1.1.1.1/24
loo 3 : 1.1.2.1/24

R2 has one point-to-point interface going to frame-relay cloud connecting it to R1

serial 1/0.21 : 10.1.12.2/24

R3 has one point-to-point interface going to frame-relay cloud connecting it to R1

serial 1/0.31 : 10.1.13.3/24

Eigrp has already been configured with summarization off on these routers. If you want to see the configuration it is attached with this blog.

Case 1: From R1, we want to advertise summary route and network attached to loopback 2 to R2:

Before we start, lets run a quick check. Lets have a look at Router 2 routing table:

After configuring summarization on R1

R1#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#int se 1/0.12
R1(config-subif)#ip summary-address eigrp 100 1.1.0.0 255.255.252.0
R1(config-subif)#
*Mar  1 00:17:48.083: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.12.2 (Serial1/0.12) is resync: summary configured

The routing table of R2 will look like this:

Now, if we want R2 to have the summary address plus the network for loopback 2

We can use leak-map and here is how we use it:

On R1, the advertising router:

We will first create an access-list matching network for loopback 2

R1(config)#access-list 1 permit 1.1.1.0 0.0.0.255

then we will create a route-map matching this access-list

R1(config)#route-map R1-2 permit
R1(config-route-map)#match ip add 1
R1(config-route-map)#exit

Now, remove the ip address under int se 1/0.12 connecting R1 to R2

R1(config)#int se 1/0.12
R1(config-subif)#no ip add

You will see the neigbors going down
*Mar  1 00:22:33.531: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.12.2 (Serial1/0.12) is down:

Now, we will create the virtual template interface

R1(config-subif)#int virtual-template 12
R1(config-if)#ip add 10.1.12.1 255.255.255.0

And, write the summary address command under it with leak map

R1(config-if)#ip summary-address eigrp 100 1.1.0.0 255.255.252.0 lea
R1(config-if)#$address eigrp 100 1.1.0.0 255.255.252.0 leak-map R1-2

Lastly, apply the virtual template interface to serial sub-interface

R1(config-if)#int se 1/0.12
R1(config-subif)#frame-relay interface-dlci 102 ppp virtual-template 12

This completes the configuration at R1

Now, lets go to R2 where we only need to configure a virtual-template interface:

R2(config)#int se 1/0.21
R2(config-subif)#no ip add
R2(config-subif)#int virtual-template 21
R2(config-if)#ip add 10.1.12.2 255.255.255.0
R2(config-if)#int se 1/0.21
R2(config-subif)#frame-relay interface-dlci 201 ppp virtual-template 21

At this point, you will see the adjaceny going up using virtual interface:

R2#sh ip eigrp neighbors
IP-EIGRP neighbors for process 100
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   10.1.12.1               Vi2               11 01:03:25  331  1986  0  15

R1#sh ip eigrp neighbors
IP-EIGRP neighbors for process 100
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
1   10.1.12.2               Vi2               11 01:04:43  325  1950  0  11
0   10.1.13.3               Se1/0.13          10 01:21:11  535  3210  0  5

Now, lets have a look at routing table of R2:

Case 2: From R1, we want to advertise summary route and all specific addresses to R3:

Lets configure R1 first:

R1(config)#route-map R1-3 permit 10

This time we have just created a route-map and not reffered any access-list. When this is done, then summary and all the
specific routes are advertised.

We will see how will it look like.

On R1, next we will create virtual template and associate it with the interface.

R1(config)#int se 1/0.13
R1(config-subif)#no ip add
R1(config-subif)#
*Mar  1 00:35:19.679: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.1.13.3 (Serial1/0.13) is down: interface down
R1(config-subif)#exit
R1(config)#int virtual-template 13
R1(config-if)#ip add 10.1.13.1 255.255.255.0
R1(config-if)#ip summary-address eigrp 1 1.1.0.0 255.255.252.0 leak-map R1-3
R1(config-if)#exit
R1(config)#int se 1/0.13
R1(config-subif)#frame-relay interface-dlci 103 ppp  virtual-template 13

Again, on R3 we will create virtual template just as we did on R2 in last example:

R3(config)#int se 1/0.31
R3(config-subif)#no ip add
R3(config-subif)#exit
R3(config)#int virtual-template 31
R3(config-if)#ip add 10.1.13.3 255.255.255.0
R3(config-if)#exit
R3(config)#int se 1/0.31
R3(config-subif)#frame-relay interface-dlci 301 ppp virtual-template 31

Lets now check if we achieved what we wanted:

And what we see is that we have the summary address and the specific routes as well.

So, all leaks are not harmful some help us as well..