OSPF configuration

Lammle says you have to remove IGRP and EIGRP because OSPF has an AD pf 110 while IGRP is 100 and EIGRP is 90. But why? Is it because a router can run only one routing protocol at a time?

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martell1000

if there are two routes to a destination, one with an AD of 90 and one withe an AD of 110 the router will use the route with the lower AD.
you can enable every protocol on a router you like, but if eigrp is converging in you network ospf wont stand a chance to get a "piece of the cake"

there are ways to tune around this but this is not a topic in the ccna and it can cause routing loops or black holes ...

thedude666

johnifanx98 wrote: »

Lammle says you have to remove IGRP and EIGRP because OSPF has an AD pf 110 while IGRP is 100 and EIGRP is 90. But why? Is it because a router can run only one routing protocol at a time?

A router can run multiple routing protocols at the same time. However your cpu usage will go up.

The route with the lowest AD(administrative distance) will be used. Note that a static route has 0 or 1 as its AD. RIP is 90 so that will have precedence over OSPF and EIGRP.
So if you use eigrp and ospf, it will receive routes from both routing protocols. However in the routing table (show ip route) only the one with the lowest AD and lowest metric will be used.

Excellent1

thedude666 wrote: »

RIP is 90 so that will have precedence over OSPF and EIGRP.

RIP is 120, OSPF is 110, and EIGRP is 90. Just tossing that out there to avoid confusion for someone.

jamesp1983

Do some research on Administrative Distance. You see it when you type "sh ip route". It can be thought of as the believabillity of the route.
It is the number to the left of the "/" in the brackets found after the destination. The number to the right of the "/" is the metric. Lower ADs are preferred. If there is a tie in AD (multiple routes to the same specific destination via the same protocol), then metric is used.

D 10.128.9.5/32 [90/153856] via 10.128.32.5, 4w4d, Vlan400

(That is an EIGRP route, AD of 90, with a metric of 153856)

Administrative Distances:
0=Directly connected
1=Static route
5=EIGRP summary
20=eBGP
90=Internal EIGRP
100=IGRP
110=OSPF
115=IS-IS
120=RIP
160=ODR
170=External EIGRP
200=iBGP
255=Unknown (not installed in the routing table)

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094195.shtml

Todd Burrell

I want to ask a follow up on this just for clarification. Let's say I have a static route of 192.168.0.0/16 and I have a RIP route of 192.168.1.0/24. Won't I used the RIP route to send to a 192.168.1.x address since it is the most specific? I'm basically asking if the order of matching is as follows:

Find all matching routes
Pick out the most specific match(es)
If you have more than one match, use the one with lowest AD

I think I'm right about this, but I want to clarify?

Thanks

martell1000

you are right todd.

192.168.0.0/16 and 192.168.1.0/24 both go to the routing table but are considered as different destinations. and the most specific destination will be taken - no matter how high the AD is.

jamesp1983

Todd Burrell wrote: »

I want to ask a follow up on this just for clarification. Let's say I have a static route of 192.168.0.0/16 and I have a RIP route of 192.168.1.0/24. Won't I used the RIP route to send to a 192.168.1.x address since it is the most specific? I'm basically asking if the order of matching is as follows:

Find all matching routes
Pick out the most specific match(es)
If you have more than one match, use the one with lowest AD

I think I'm right about this, but I want to clarify?

Thanks

Indeed, you are correct, that's why I mentioned "Specific destination" in my explanation. Longest match is the name of the game.

johnifanx98

thedude666 wrote: »

A router can run multiple routing protocols at the same time. However your cpu usage will go up.

The route with the lowest AD(administrative distance) will be used. Note that a static route has 0 or 1 as its AD. RIP is 90 so that will have precedence over OSPF and EIGRP.
So if you use eigrp and ospf, it will receive routes from both routing protocols. However in the routing table (show ip route) only the one with the lowest AD and lowest metric will be used.

Per the description of the Administrative distances, textbooks hint a routing table could contain routing entries of mixed administrative distances due to various routing protocols. Like one routing entry of AD 110 from RIP, and another routing entry of AD 90 from EIGRP, blahblah. And the router just picks the one with the smallest AD one. However, if each router practically just runs one protocol at a time, then is it possible to see mixed AD routing entries in a routing table?

Todd Burrell

You could still have connected, static and routes from your routing protocol all in your routing table. Also, remember that with the IP ROUTE command you can add a last parm of AD, so you could also have a static route with a higher AD than your routing protocol (say RIP). I have seen static routes added with an AD of 150 to make sure they are 'backup' static routes.

jamesp1983

look into floating static routes...

johnifanx98

Todd Burrell wrote: »

You could still have connected, static and routes from your routing protocol all in your routing table. Also, remember that with the IP ROUTE command you can add a last parm of AD, so you could also have a static route with a higher AD than your routing protocol (say RIP). I have seen static routes added with an AD of 150 to make sure they are 'backup' static routes.

Is it possible to see other combinations, like EIGRP and RIP entries in a same routing table?

Todd Burrell

I believe so. Say you have a core router that is connected to an OSPF area on one side and to an EIGRP network on the other side due to non-Cisco routers in the OSPF area. Someone can correct me if I'm wrong, but you could run OSPF and EIGRP on this router and receive routes from both 'networks'. You would still prefer the EIGRP routes due to the lower AD for EIGRP, but with this simple example there should be no issues.

jamesp1983

look into redistribution if this is something of interest to you... Allows you to advertise routes from one routing process into another. (as well as static and directly connected)