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New Member

EIGRP Multiple Sucessors - Which topology entry is sent to neighbor peer

All,

If you have two EIGRP sucessors in your topology table (and selected in RT table, EIGRP has to update its peers with the update.

The topo table will have two entries in with its external data (assuming is external route from BGP). The external data may be different ie BGP MED may have been ported into the EIGRP external data. ie, one topo entry has a BGP MED of 5000 in the external portion of the EIGRP update and one has a BGP MED of 4000. (not that this matters cause nothing is routed based on this information)

Which topo entry is sent to the peer?

Is it based on the external router ID in the external data, as I always see the same external data throughout my AS1 and from only one of the routers that were imported from BGP.

If further info is needed, I can post a diagram :)

Kindest regards,

Ken

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Purple

Re: EIGRP Multiple Sucessors - Which topology entry is sent to n

Hello Ken,

Your observations and conclusions match exactly what I had described in my earlier post.

Some simple testing with your setup should confirm all of this too. Ain't EIGRP fun :-) ?

Hope that helps - pls remember to rate the posts.

Regards,

Paresh.

6 REPLIES
New Member

Re: EIGRP Multiple Sucessors - Which topology entry is sent to n

Pls see attched diagram :)

Many thx

Ken

Hall of Fame Super Silver

Re: EIGRP Multiple Sucessors - Which topology entry is sent to n

Ken

I have read your description which I find to be a bit confusing and have looked at the diagram you posted which (unfortunately) does not clarify much.

When you say that EIGRP has two successor routes am I correct in assuming that you are saying that EIGRP has two equal cost routes to some remote destination(s)?

If you do mean that EIGRP has two equal cost routes then EIGRP should advertise both routes to its neighbor.

If I have misunderstood something then please clarify. (posting appropriate output of show ip eigrp topology would be an excellent place to start)

HTH

Rick

Purple

Re: EIGRP Multiple Sucessors - Which topology entry is sent to n

Hi Ken,

Welcome back :-)

First of all, to respond to Rick's post: even if EIGRP has two equal cost routes to a destination, it will always advertise just the one route.

Now, to your observations.

This is how EIGRP works when it has two equal-cost routes and thus, two successors:

- the external metric is ignored

- the router ID is ignored. The only use of router ID in EIGRP is to detect self-originated external routes

- the route selected by EIGRP to advertise to its neighbors is the one that appears first in the output of 'sh ip eigrp topo'

- now, the thing to note that in the face of multiple equal cost routes, the route that appears first in the output of 'sh ip eigrp topo' is the one that was received last

However, there can be transient situations where different neighbors may contain different versions of the router. See my example below.

Now, consider the following case (assume that route N is an external route):

- RTA receives a route N from RTB.

- RTA does not have that route in its topology table so it installs it choosing RTB as the successor.

- RTA sends updates to all other neighbors regarding this new route

- now RTC comes online (a new neighbor for RTA)

- RTC sends route N to RTA and RTA determines that it is of equal cost to that received from RTB

- RTA will now insert the route into the first place in the topo table

- However, RTA will not send updates regarding this change to other neighbors (what would be the point ? The external info may be different but nothing else has really changed since it has the same cost)

- if you subsequently clear the EIGRP neighbor-ship between RTA and its neighbors, RTA will now send an advertisement for N which contains external attributes as received from RTC.

Does that help ?

Pls remember to rate posts.

Regards,

Paresh/.

New Member

Re: EIGRP Multiple Sucessors - Which topology entry is sent to n

Guys, Many thx indeed,

Paresh, just to clarify

I have the following router 2 connects to router 3 - router 3 has multiple sucessors to 133.133.133.0/24

Router 2 received the last entry (which may have been the only entry at a point in time) in router 3s topo table, as this is probably the entry that router 3 received first and then triggered an update to router 2 correct???? This is originating router 10.10.0.1)

So, in terms of multiple sucessors, the one you see in the LAST entry in the topo table (orig 10.10.0.1) MEANS that this was the topo entry first to get into router 3s topo DB and then router3 sent this to router2.

If for some reason, the link (or eigrp peer) between router 2 and router 3 went down, the first entry now in the topo table (the newest one originated from 10.10.0.3) would be sent to router 2.

Please can you confirm I have this correct and it makes sense.

Many many kind regards as always,

Ken

Router2>sh ip eigrp top 1 133.133.133.0 255.255.255.0

IP-EIGRP (AS 1): Topology entry for 133.133.133.0/24

State is Passive, Query origin flag is 1, 0 Successor(s), FD is 282112

Routing Descriptor Blocks:

10.11.1.1 (GigabitEthernet1/2), from 10.11.1.1, Send flag is 0x0

Composite metric is (282112/281856), Route is External

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 1020 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1500

Hop count is 2

External data:

Originating router is 10.10.0.1

AS number of route is 1000

External protocol is BGP, external metric is 5000

Administrator tag is 1000

Router2>

router1>sh ip eigrp topo 133.133.133.0 255.255.255.0

IP-EIGRP topology entry for 133.133.133.0/24

State is Passive, Query origin flag is 1, 2 Successor(s), FD is 281856

Routing Descriptor Blocks:

10.12.1.3 (Vlan41), from 10.12.1.3, Send flag is 0x0

Composite metric is (281856/281600), Route is External

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 1010 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1500

Hop count is 1

External data:

Originating router is 10.10.0.3

AS number of route is 1000

External protocol is BGP, external metric is 4000

Administrator tag is 1000

10.12.1.1 (Vlan41), from 10.12.1.1, Send flag is 0x0

Composite metric is (281856/281600), Route is External

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 1010 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1500

Hop count is 1

External data:

Originating router is 10.10.0.1

AS number of route is 1000

External protocol is BGP, external metric is 5000

Administrator tag is 1000

router1>

Purple

Re: EIGRP Multiple Sucessors - Which topology entry is sent to n

Hello Ken,

Your observations and conclusions match exactly what I had described in my earlier post.

Some simple testing with your setup should confirm all of this too. Ain't EIGRP fun :-) ?

Hope that helps - pls remember to rate the posts.

Regards,

Paresh.

New Member

Re: EIGRP Multiple Sucessors - Which topology entry is sent to n

Paresh, many thx for the help again.

Great stuff man :))

I kinda like EIGRP, just a shame there is not concept of a E1 E2 external route where you can have a set metric to a redistribution router (ASBR) not including the path to the ASBR for load balancing out over the AS - But thats another matter :)

Also, Thx to all else who posted.

Cheers,

Ken

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