Solved: default route and routing protocols

sarahr202 · ‎08-04-2009

Hi every body.

I have few questions

Let say i configure the default route using ip route 0.0.0.0 0.0.0.0 199.199.199.1, on a router which is running a routing protocol.

1) will this route be advertised without the redistribution command if router is running:

rip

ospf

eigrp

is is

Thanks a lot.

srue · ‎08-04-2009

it will not be redistributed by default in any routing protocol. There are specific commands in each routing protocol to enable redistribution of a default route.

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Jerry Ye · ‎08-04-2009

Hi Sarah,

You might want to check out the following link

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094374.shtml#route0.0

HTH,

jerry

View solution in original post

srue · ‎08-04-2009

ospf:

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801ec9f0.shtml

rip:

http://www.cisco.com/en/US/docs/ios/12_1/iproute/command/reference/1rdrip.html#wp1017420

eigrp:

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#defaultrouting

View solution in original post

Peter Paluch · ‎08-04-2009

Hello,

The ip default-network command applies only to distance vector protocols, i.e. RIP, IGRP and EIGRP. It is not accepted by OSPF or IS-IS. Each protocol may send the default route differently. If you are running RIP, it will send the default route as 0.0.0.0/0. IGRP and EIGRP will just flag that route as a candidate for default route but they will send the route as is, not as 0.0.0.0/0. This creates a requirement that the network must first be included in the (E)IGRP process, only then it can be flagged as a candidate default route.

As to your scenario: if you configure the default network on R2, nothing will actually happend. The R2 will have a candidate default route pointing towards R3 but it will not generate a default route here because the network 10.0.0.0/8 was already discovered by the routing protocol in question. You can not take a route discovered by a particular routing protocol and having it further redistributed by that protocol as a candidate default route. Correctly, if you want to declare some route as a candidate default network using the ip default-network, it must first be present in your routing table from some other source (other routing protocol or a static entry).

Best regards,

Peter

View solution in original post

srue · ‎08-04-2009

it will not be redistributed by default in any routing protocol. There are specific commands in each routing protocol to enable redistribution of a default route.

Jerry Ye · ‎08-04-2009

Hi Sarah,

You might want to check out the following link

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094374.shtml#route0.0

HTH,

jerry

srue · ‎08-04-2009

ospf:

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801ec9f0.shtml

rip:

http://www.cisco.com/en/US/docs/ios/12_1/iproute/command/reference/1rdrip.html#wp1017420

eigrp:

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#defaultrouting

sarahr202 · ‎08-04-2009

Thanks Jerry and Srue.

How about default route configured using ip default-network x where x is classful network.?

Say R1-----s1R2s0----------R3

R2 has following routes:

R 10.0.0.0 255.0.0.0 via 199.199.199.2 s0

C 199.199.199.0/24 s0

if i configured ip default-network 10.0.0.0 on R2, will R2 advertise the default route (0.0.0.0 0.0.0.0) to R1 and R3? If R2 advertises this route to R3 it will cause routing loop with R3 sending the packet to R2, R2 then sending the packet back to R3 . Am i correct ?

How about if R2 were using ospf,eigrp,isis, instead of rip, will R2 advertise the default route in those cases ?

Thanks and have a nice day.

Peter Paluch · ‎08-04-2009

Hello,

The ip default-network command applies only to distance vector protocols, i.e. RIP, IGRP and EIGRP. It is not accepted by OSPF or IS-IS. Each protocol may send the default route differently. If you are running RIP, it will send the default route as 0.0.0.0/0. IGRP and EIGRP will just flag that route as a candidate for default route but they will send the route as is, not as 0.0.0.0/0. This creates a requirement that the network must first be included in the (E)IGRP process, only then it can be flagged as a candidate default route.

As to your scenario: if you configure the default network on R2, nothing will actually happend. The R2 will have a candidate default route pointing towards R3 but it will not generate a default route here because the network 10.0.0.0/8 was already discovered by the routing protocol in question. You can not take a route discovered by a particular routing protocol and having it further redistributed by that protocol as a candidate default route. Correctly, if you want to declare some route as a candidate default network using the ip default-network, it must first be present in your routing table from some other source (other routing protocol or a static entry).

Best regards,

Peter