Prefix list

Answered Question
Nov 19th, 2013
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hello l have a question about prefix list with 3 routers. l configure 3 router R-1 as the branch, R-2 as the WAN and R-3 the head office (with a lot of loopbacks).


l want to configure prefix list on the WAN(R-2) to block all address with /30 and some of the loopbacks from displaying on the branch office routing table.

it works fine but the problem is, l cannot ping from the branch to any of the loopback. the ICMP does't pass through WAN router.


it works fine with 2 routers as the configuration is done on the head office that will display and work fine.


---

Posted by WebUser Salis Mahama from Cisco Support Community App

Correct Answer by Karthick Murugan about 3 years 9 months ago

It looks like its an expected behavior as per your configuration. R3 doesn't have information about any of the prefixes of R1 as per the routing table entry. For R3 to reach R1, atleast we need a default-route which is not available.


I EIGRP advertising the default route as "network 0.0.0.0" won't work until you have a static route like ip route 0.0.0.0 0.0.0.0


Thanks & Regards,
Karthick Murugan
CCIE#39285

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cadet alain Tue, 11/19/2013 - 02:11
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Hi,

post topology diagram and the configs.


Regards


Alain



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fb_webuser Tue, 11/19/2013 - 07:34
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R1 -


........-R-2----


R3

f0/0 10.1.1.0/30 f0/0 f0/1 10.9.1.0/30 f0/0


R1 config

............

interface FastEthernet0/0

ip address 10.1.1.2 255.255.255.252

speed 100

full-duplex

!

router eigrp 100

network 0.0.0.0

no auto-summary

!


R2 config

.........

interface FastEthernet0/0

ip address 10.1.1.1 255.255.255.252

speed 100

full-duplex

!

interface Serial0/0

ip address 10.9.1.5 255.255.255.252

clock rate 2000000

!

interface FastEthernet0/1

ip address 10.9.1.1 255.255.255.252

speed 100

full-duplex

!

router eigrp 100

network 0.0.0.0

distribute-list prefix BLK30 out

no auto-summary

!

!

ip prefix-list BLK30 seq 5 deny 0.0.0.0/0 ge 30 le 30

ip prefix-list BLK30 seq 10 permit 0.0.0.0/0 le 32

!


R3 config

...........


interface Loopback1

ip address 17.1.1.1 255.255.0.0

!

interface Loopback2

ip address 17.2.2.2 255.255.128.0

!

interface Loopback3

ip address 17.3.3.3 255.255.192.0

!

interface Loopback4

ip address 17.4.4.4 255.255.224.0

!

interface Loopback5

ip address 17.5.5.5 255.255.240.0

!

interface FastEthernet0/0

ip address 10.9.1.2 255.255.255.252

speed 100

full-duplex

!

interface FastEthernet0/1

ip address 10.9.1.9 255.255.255.252

speed 100

full-duplex

!

interface FastEthernet1/0

ip address 10.9.1.13 255.255.255.252

duplex auto

speed auto

!

router eigrp 100

network 0.0.0.0

no auto-summary

!


this is the show ip route on R1


Branch1#show ip route eigrp

17.0.0.0/8 is variably subnetted, 5 subnets, 5 masks

D 17.1.0.0/16 via 10.1.1.1, 00:13:03, FastEthernet0/0

D 17.3.0.0/18 via 10.1.1.1, 00:13:03, FastEthernet0/0

D 17.2.0.0/17 via 10.1.1.1, 00:13:03, FastEthernet0/0

D 17.5.0.0/20 via 10.1.1.1, 00:13:03, FastEthernet0/0

D 17.4.0.0/19 via 10.1.1.1, 00:13:03, FastEthernet0/0

10.0.0.0/8 is variably subnetted, 5 subnets, 3 masks

D 10.16.2.0/25 via 10.1.1.1, 00:12:58, FastEthernet0/0

D 10.16.3.0/25 via 10.1.1.1, 00:12:58, FastEthernet0/0

D 10.16.1.0/24 via 10.1.1.1, 00:12:58, FastEthernet0/0

D 10.16.4.0/25 via 10.1.1.1, 00:12:58, FastEthernet0/0

Branch1#


from the config it block all the ip address with /30 and allow everything else but the problem is, after this configuration l cannnot ping from R1 to the loopbacks on R3.

thanks for your help



---

Posted by WebUser Salis Mahama from Cisco Support Community App

Correct Answer
Karthick Murugan Tue, 11/19/2013 - 09:41
User Badges:
  • Cisco Employee,

It looks like its an expected behavior as per your configuration. R3 doesn't have information about any of the prefixes of R1 as per the routing table entry. For R3 to reach R1, atleast we need a default-route which is not available.


I EIGRP advertising the default route as "network 0.0.0.0" won't work until you have a static route like ip route 0.0.0.0 0.0.0.0


Thanks & Regards,
Karthick Murugan
CCIE#39285

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