EIGRP question: partial update ? + redistributing via/advertised by with show ip route command

[Steph1963]

Hi Everybody,

Quesetion 1: Partial Update

I am presently studying for my Troute exam and there is one point that I just cannot uderstand with EIGRP. In every text book that I have seen, it is mention that EIGRP updates are partial and only send to afffected routers.

I am not sure that I understand this part correctly, it seems to me that when a new route is up on one EIGRP routers, it will sends an update to it`s neighbor,and the neighbor will also do the same with it`s own neighbor. It seems that this is the only way that every routers part of the AS will be aware of a new route.

Samething if one route is down, the router that knows that one route is down will query it`s neighbor, that might also query it`s neighbor if required.

I do not know why we can say that EIGRP updates are partial since every router must be aware of the update or the query, is it as opposed to a routing protocol like OSPF?

Question 2: redistributing via/advertised by with show ip route command

Show ip route

When we do a show ip route X.X.X.X for a route that was announced via EIGRP, there is always a line that shows redistibuting via ...and advertised by.... as shown in the next example:

R2 Configuration
router eigrp 1
redistribute ospf 101 metric 100000 100 0 1 1500
network 2.0.0.0
network 10.0.0.0
no auto-summary
!
router ospf 101
log-adjacency-changes
redistribute eigrp 1 subnets
network 2.2.2.2 0.0.0.0 area 0
network 192.168.1.0 0.0.0.3 area 0

R2#show ip route 1.1.1.1
Routing entry for 1.1.1.1/32
  Known via "ospf 101", distance 110, metric 11, type intra area
  Redistributing via eigrp 1
  Advertised by eigrp 1 metric 100000 100 0 1 1500

R2#show ip route 3.3.3.3
Routing entry for 3.0.0.0/8
  Known via "eigrp 1", distance 90, metric 409600, type internal
  Redistributing via eigrp 1, ospf 101
  Advertised by ospf 101 subnets

What does the Redistributing via eigrp 1, OSPF 1 means?
and
What does the Advertised by ospf 101 subnets means?

Thanks for your help

Stephane

[Richard Burts]

Stephane

Perhaps this example will help you understand the concept that EIGRP updates are partial. Think about routerA which has Ethernet0 with 172.16.1.1/24 and Ethernet1 with 172.16.12.1/24. routerA runs EIGRP on all interfaces. routerA connects to routerB which has Ethernet0 with 172.16.12.2/24 (connecting to routerA) and has Ethernet1 with 192.168.23.2/24 which connects to routerC. routerB run EIGRP on all interfaces and routerB performs summarization and advertises to routerC the 172.16.0.0/16.  routerC has Ethernet0 with 192.168.23.3/24 and has Ethernet1 wih 192.168.3.3/24.

Now what happens if Ethernet0 on routerA goes down. routerA sends an update to routerB because routerB knows about subnet 172,16,1,0/24. But routerB does not advertise to routerC because routerC does not know about the subnet 172.16.1.0 and only knows about network 172.16.0.0.

HTH

Rick

[Steph1963]

Hi Rick,

Your example makes a lot of sense, OSPF would behave differently in this situation. OSPF would just now allowed summarization of any interfaces that belongs to the backbone, if I am not mistaking. It would probably send 172.16.1.0/24 and 172.16.12.0/24 separetely.

Thanks for your help

Stephane

[Richard Burts · Accepted solution]

Stephane

The behavior of EIGRP and of OSPF are different when it comes to summarization.

With EIGRP there is default summarization at the boundary of classful networks. And there can be manual summarization at any point in the network. So the summarization behavior of EIGRP is quite flexible.

The summarization behavior of OSPF does not have any default summarization. So by default every individual network and subnet will be advertised over the entire OSPF routing domain. But OSPF does permit manual summarization at area borders. So within an area every subnet and every network will be advertised to routers within that area. But the area border router can configure summarization in advertising to other areas. And then the routers in other areas would not see the detailed individual routes but would see only the summarized routes.

HTH

Rick

[skarthic]

EIGRP updates are triggered when compared to RIP which does periodic updates. RIP periodically sends routing updates which is a lot of overheads.

EIGRP and OSPF does triggered updates.