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New Member

split horizon on vlan interface

I found some strange problem. Topo like this:

3550a-----trunk------3550b

|

10.0.30.251

|

vlan 230

|

10.0.30.249

|

7507---l0.0.64.0/23

3550b#sh ip eigrp to all

IP-EIGRP Topology Table for AS(10)/ID(10.0.97.2)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

r - reply Status, s - sia Status

P 10.0.64.0/23, 1 successors, FD is 130816, serno 68

via 10.0.30.249 (130816/128256), Vlan230

via 10.0.30.251 (131328/131072), Vlan230

I think 7507 send update to 3550a and 3550b, Then 3550a send same update to 3550b through trunk port. So split horizon is in affect on the layer2 port NOT layer 3 interface?

9 REPLIES

Re: split horizon on vlan interface

This is while EIGRP does not know about the physical topology.

To EIGRP the network looks like:

3550A -- vlan230 -- 3550B

! !

-- some other vlans -------

!

7507

So it is normal to see two possible routes to the 7507 from vlan230.

Goodluck

Re: split horizon on vlan interface

Sorry but I see that my diagram was truncated. What I mean is that both 3550's have connection to all other vlans. The exclamation marks were on the left and right ends. The spaces inbetween were appearently ingnored and now it looks a bit confusing.

New Member

Re: split horizon on vlan interface

thank for your reply.

My topo is this:

eigrp running on interface vlan230,vlan99 of 3550a and 3550b, int f4/0/0 (is vlan230) of 7507.

I guess the flowchart is like this:

1.7507 send 10.0.64.0/23 update to 3550a and 3550b

so 3550b has the route which next hop is 7507.

2.3550b send 10.0.64.0/23 update to 3550b through vlan 99

3.Then 3550a send 10.0.64.0/23 update to 3550b and 7507 through vlan230.

So my question is 3550a get the route from vlan 230,if split horizen is in affect,why 3550a send the same route to vlan230(although next hop is different).

When 3550b get the route from vlan 230,Does it send the route to vlan 99 again,and so on,so there is routing loop.Oh,I know this is not correct ,But I am confused with some base concept

3550a-----trunk(vlan230,vlan99)------3550b

|

10.0.30.251

|

vlan 230

|

10.0.30.249

|

7507---l0.0.64.0/23

New Member

Re: split horizon on vlan interface

I think split horizon rule is not applicable for the above case.

Because Split horizon rule states "A route canot be updated on an interface from which it is learned". In the above case 3550a is learning route via vlan99 from 3550b with different metric. The same would be advertised on VLAN230

The above is not a continuous process, because 3550b always advertises the route with best metric in its routing table via VLAN 99. Here DUAL algorithm takes care of loop prevention if you see topology table you can see only one successor because Advertised Distance of 3550a is more than Feasible Distance of the corresponding route.

I never used that command "sh ip eigrp topology all", if you use "sh ip eigrp topology", you can see only one entry in topology database.

Please correct me if I am wrong.

New Member

Re: split horizon on vlan interface

thank you!

Maybe I understand.

when I show route on 7507,I can see following:

Can I make 3550a not send this update to 7507 ? Does this display correct?

7507#sh ip eigrp to all

IP-EIGRP Topology Table for AS(10)/ID(10.0.28.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

r - reply Status, s - sia Status

P 10.0.64.0/23, 1 successors, FD is 128256, serno 79

via Summary (128256/0), Null0

via 10.0.30.251 (156672/131072), FastEthernet4/0/0

Re: split horizon on vlan interface

EIGRP updates are multicasts to all routers on the subnet (read vlan).

They are all aware of eachother. Combining all updates they build a topology of the network as it looks on layer 3. A trunk link corresponds to multiple segments when you look at it like this.

Hope this helps to clarify.

Regards,

Leo

New Member

Re: split horizon on vlan interface

thanks again,And another summary question :-D

when I do summary on interface,AD is 5.Why I show this route from 3550b,it display [90/130816] ?

Is the AD which set on ip summary-address in affect ?

7507#sh run int f4/0/0

interface FastEthernet4/0/0

ip address 10.0.30.249 255.255.255.0

ip summary-address eigrp 10 10.0.64.0 255.255.254.0 5

duplex auto

speed auto

bridge-group 4

bridge-group 4 spanning-disabled

end

3550b#sh ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area

* - candidate default, U - per-user static route, o - ODR

P - periodic downloaded static route

Gateway of last resort is 10.0.27.1 to network 0.0.0.0

10.0.0.0/8 is variably subnetted, 19 subnets, 5 masks

D 10.0.64.0/23 [90/130816] via 10.0.30.249, 03:02:33, Vlan230

New Member

Re: split horizon on vlan interface

Can you try this?

Please modify the mask for summery address from 23 to 18. Since your network number is 10.0.64.0 it does not harm you anyway.

Like this

7507#sh run int f4/0/0

interface FastEthernet4/0/0

ip address 10.0.30.249 255.255.255.0

ip summary-address eigrp 10 10.0.64.0 255.255.192.0 5

duplex auto

speed auto

bridge-group 4

bridge-group 4 spanning-disabled

end

Please try the above.

New Member

Re: split horizon on vlan interface

I tried it.But result is same. Even if I modify the AD from 5 to any,it doesn't have any in affect. Maybe IOS does not support this function?

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