Redistribution Confusion

[Ahmed Malik]

hi,

   I been going through some questions to clear concepts and this question about redistribution confused me. I think the answer is A the guide says its C. so correct me if i am wrong

plz look at the img

Regards,

Ali

[Giuseppe Larosa · Accepted solution]

Hello Ali,

this is a special case as default routes are involved.

answer C is wrong for sure

answer A shows an IP extended ACL that represents the default route 0.0.0.0/0.

The only doubt is if it is allowed/supported to redistribute a default route between two routing protocols.

Generally speaking it is not supported: for example from BGP to OSPF you need to use default-information originate on OSPF

EIGRP to EIGRP may be supported but I haven't tested it.

I would say that this setup should be tested.

For EIGRP and default routing see

http://www.informit.com/library/content.aspx?b=CCIE_Practical_Studies_I&seqNum=124

notice the D EX 0.0.0.0 in example 11-28

Hope to help

Giuseppe

[Peter Paluch · Accepted solution]

Hi Giuseppe,

Please allow me to join the discussion.

answer A shows an IP extended ACL that represents the default route 0.0.0.0/0.

Yes, I agree here. When using ACLs in route-maps for redistribution purposes, the "source IP/wildcard mask" portion of the ACL entry is used to match the redistributed network's address while the "destination IP/wildcard mask" portion is used to match the redistributed network's netmask. In this sense, the option 'A' is clearly the option selecting only the default route 0.0.0.0/0.

The only doubt is if it is allowed/supported to redistribute a default route between two routing protocols.

The question is really about whether it is allowed to redistribute a default route into an IGP routing protocol (regardless of what is the source of this default route).

Regarding RIP and EIGRP, it is always allowed to redistribute a default route into these protocols. In fact, it has been the common way for years - the default-information originate command in RIP does not work reliably and EIGRP does not support it at all. The only gotcha is to be careful about the seed metrics when redistributing into RIP or EIGRP - if redistributing from a different routing protocol, it is necessary to explicitly specify the seed metric, otherwise the redistributed route will not be advertised successfully.

With OSPF and IS-IS, it is not possible to redistribute the default route into these protocols (not a protocol limitation but rather a sanity check performed in Cisco's implementation). The only way to inject a default route (apart from the usual default route injection performed by ABRs towards non-regular areas in OSPF, or the special default route handling in IS-IS) into these protocols is to use the default-information originate command.

Best regards,

Peter