Default route and ad

[Manish.mac03]

For same destination, we have ospf route, eigrp route, and default route- which one to prefer ?

[paul driver]

Hello
I assume the igp routes are not defaulfs only the static is correct?
The all things being equal, igp prefixes all the same length, no manipulation to any administrative distance/interface/summarisation etc…l

Eigrp
OSPF
Static Default

[Manish.mac03]

Hi paul, really appreciate your response.

In order to reach 192.168.1.100 network we have 3 routes via 

ospf

eigrp

Default (0.0.0.0/0)

which route will be prefer first ??

[Jon Marshall]

It depends. 

Assuming the EIGRP and OSPF routes are the same prefix and mask and are more specific than the default route then - 

EIGRP Internal (AD 90)




OSPF (AD 120)




EIGRP External (AD 170)

Jon

[paul driver · Accepted solution]

Hello

---

@Manish.mac03 wrote:

Hi paul, really appreciate your response.

In order to reach 192.168.1.100 network we have 3 routes via 

ospf

eigrp

Default (0.0.0.0/0)

which route will be prefer first ??

---

It would be eigrp , ospf , then default static, but as i said this it could be different if those igp prefixes not equal in length and have some manipulation appended (distances/cost/delay/bw/redistrubtion,etc

Also
If they were all defaults and again nothing has been changed then the order would be

static default
eigrp
ospf

[Manish.mac03]

Thanks Paul,

This is what i was looking for.

Thanks 

Mac

[MHM Cisco World]

first the OSPF and EIGRP and static route have same route then the router will make decision and inject the lower AD into RIB.
after that you will see only the lower AD route in RIB and other save in DB and not show in RIB.
BUT 
if OSPF and EIGRP and static have different route then all will list in RIB. 
here if the router reactive the packet then the longest match will be used to forward the packet.
so it can OSPF or EIGRP or static and finally the default route which is /0 meaning all packet belong to this subnet.

[Richard Burts]

As other responses have pointed out there are several variables to be considered which would affect the decision of which route to use. And we have no details of the environment to help us understand what the behavior will be. Perhaps the original poster can provide some details to help us understand this better?

The first thing to consider is the prefix length of the route. If there are differences in prefix length the longer prefix is the one that will be preferred. Assuming that the OSPF and EIGRP routes are prefixes greater than zero then we can say with confidence that the default route will not be used. If one of the dynamic advertisements is greater length than the other then the longer prefix will be used. Since we do not know the prefix length of either the OSPF or EIGRP advertisements we can not know if one is the winner.

If the prefixes are the same length then the next thing to consider is Administrative Distance. If you have 2 advertisements (from 2 different protocols) that are the same prefix length then the advertisement with the lower AD will be used. Each dynamic protocol has a default AD (actually EIGRP has 2, one for internally learned and one for externally learned). But the AD can be manipulated. Since we do not know the AD of the OSPF or the EIGRP route we can not say which one will be used.

[paul driver · Accepted solution]

Hello

---

@Richard Burts wrote:

Since we do not know the AD of the OSPF or the EIGRP route we can not say which one will be used.

---

---

@MHM Cisco World @ wrote:

so it can OSPF or EIGRP or static and finally the default route which is /0 meaning all packet belong to this subnet.

---

Guys if the assumption is the igp are all running with default metrics/costing advertisng the same prefix/length etc.... no manipulation no summarisation/no redistribution etc, just basic advertisement of the same prefix, then we can confirm the order here for the OP

Again, the above assumption would apply to all default routes being advertised, otherwise there are to many variables that would produce differing results.

[Richard Burts]

Yes. If we make assumptions then we can say which route would be used. Without assumptions we do not have enough information to say which one will be used. We need details from the original poster if we are to be able to provide an answer.