have a group of address with the same major network and At a selected point in the
network you summarize the address using the subnet mask to define one singular
address that encompass all the relevant networks this is done to cut down the routing
table and also hides issue with internal networks. For example you have 3 address
150.10.7.0
150.10.8.0
150.10.9.0
so the major network 150.10.0.0/16
.7
.8
.9 in the third octet is what we will be looking at to make changes
128 64 32 16 8 4 2 0
.7 in binary is 0 0 0 0 0 1 1 1
.8 in binary is 0 0 0 0 1 0 0 0
.9 in binary is 0 0 0 0 1 0 0 1
so if you notice the address all have in common the 4 most left bits
so
for the network portion octet 1 is 8 bits octet 2 is 8 bits and third octet will be 4
bits.
/20 We are taking 4 bits for the netwokr porttion 2 power of 4 is 16 so we are going
up in 16
so first subnet 150.10.0.0/20
next subnet is 150.10.16.0/20
next subet is 150.10.32.0/20
The first subnet 150.10.0.0/20 is all we need to advertise to encompass all 3
networks 150.10.7.0,150.10.8.0 & 150.10.9.0
It is simple enough to do this on the interface level
ip summary-address rip "address"
The way that it hides issue with networks is for example if you send a summary the
summary address will keep sending and only be removed when all networks covered by
the summary are not in the routing table. So if you have an issue say with the
network 150.10.9.0 the summary route will still remain and be advertised. THe reason
you may want this is so that it does not register as a change on upstream routers and
cause the routers to recaluate there routing info.
One of the limitations of rip is you can not summarise past the major network
boundary for examp,le the networks
192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
192 is class c address so the mask /24
The summary for this would 192.168.0.0/22 this is completely different major network
boundary it is in the class b range of 16-24 other protocols support and allow this
rip does not so it is limitation
To get around it what you can do is create a static route of the network +
redistribute it in to the protocol
Other behaviours of RIP summarization
Autosummary will override the configured
summary-address feature on a given interface except when both of the following
are true:
•The configured
interface summary-address and the IP address of the configured interface share
the same major net (the classful, nonsubnetted portion of the IP address).
•Split horizon is
not enabled on the interface.
If split horizon is enabled, neither an autosummary
address nor the interface summary-address is advertised.
LAB
----
OK for the lab i will set r2 up with bunch of loopbacks 150.10.7.0 150.10.9.0
150.10.12.0 all /23 these will be advertised in rip. R2 connects to SW2. SW2 will
summarise these 150.10.0.0/20 and SW1 will recieve only the summary route for these
loopbacks cutting down it routing table size
I will just set everything up with rip v2 so all devices will run rip v2
if i do a sh ip route on s2
switch2(config)#do sh 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 not set
1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Loopback0
C 192.168.1.0/24 is directly connected, Port-channel2
C 192.168.2.0/24 is directly connected, FastEthernet0/2
150.10.0.0/22 is subnetted, 3 subnets
R 150.10.12.0 [120/1] via 192.168.2.2, 00:00:04, FastEthernet0/2
R 150.10.8.0 [120/1] via 192.168.2.2, 00:00:04, FastEthernet0/2
R 150.10.4.0 [120/1] via 192.168.2.2, 00:00:04, FastEthernet0/2
switch2(config)#
they come in as the subnet address so the subnet address of .7 for /23 is .4 for .9
its ,8 and for .12 it is .12
s1
--
Ok routing table for s1 pre summarisation is
switch1#sh 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 not set
1.0.0.0/24 is subnetted, 1 subnets
R 1.1.1.0 [120/1] via 192.168.1.1, 00:00:01, Port-channel2
C 192.168.1.0/24 is directly connected, Port-channel2
R 192.168.2.0/24 [120/1] via 192.168.1.1, 00:00:01, Port-channel2
150.10.0.0/22 is subnetted, 3 subnets
R 150.10.12.0 [120/2] via 192.168.1.1, 00:00:01, Port-channel2
R 150.10.8.0 [120/2] via 192.168.1.1, 00:00:01, Port-channel2
R 150.10.4.0 [120/2] via 192.168.1.1, 00:00:01, Port-channel2
switch1#
s2
===
switch2(config)#int po2
switch2(config-if)#ip summary-address rip 150.10.0.0 255.255.240.0
sw1
----
switch1#sh 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 not set
1.0.0.0/24 is subnetted, 1 subnets
R 1.1.1.0 [120/1] via 192.168.1.1, 00:00:25, Port-channel2
C 192.168.1.0/24 is directly connected, Port-channel2
R 192.168.2.0/24 [120/1] via 192.168.1.1, 00:00:25, Port-channel2
150.10.0.0/20 is subnetted, 1 subnets
R 150.10.0.0 [120/1] via 192.168.1.1, 00:00:25, Port-channel2
switch1#
so only recieving 150.10.0.0/20 not the more specfic routes
r2
---
on r2 i am just going to shut down the loopback int of 150.10.7.1
r2#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.2.2 YES manual up up
Serial0/0 unassigned YES NVRAM down down
Serial0/1 unassigned YES NVRAM down down
Loopback0 150.10.7.1 YES manual up up
Loopback1 150.10.9.1 YES manual up up
Loopback2 150.10.12.1 YES manual up up
r2#config t
Enter configuration commands, one per line. End with CNTL/Z.
r2(config)#int lo
% Incomplete command.
r2(config)#int lo-
^
% Invalid input detected at '^' marker.
r2(config)#int lo0
r2(config-if)#shut
r2(config-if)#exit
r2(config)#exit
r2#
*Mar 1 00:35:23.187: %LINK-5-CHANGED: Interface Loopback0, changed state to
administratively down
*Mar 1 00:35:23.847: %SYS-5-CONFIG_I: Configured from console by console
*Mar 1 00:35:24.187: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0,
changed state to down
S4
---
No affect on summary route on sw4
switch1#sh 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 not set
1.0.0.0/24 is subnetted, 1 subnets
R 1.1.1.0 [120/1] via 192.168.1.1, 00:00:03, Port-channel2
C 192.168.1.0/24 is directly connected, Port-channel2
R 192.168.2.0/24 [120/1] via 192.168.1.1, 00:00:03, Port-channel2
150.10.0.0/20 is subnetted, 1 subnets
R 150.10.0.0 [120/1] via 192.168.1.1, 00:00:03, Port-channel2
switch1#
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