Subject: [OSPF] OSPF multiple interfaces to the same subnet

From: Xi R Chen <[email-masked]>
Date: Thu, 9 Feb 2012 12:01:16 +0800

Hi

                  ------------------------------
                 |                                     |
                 |   eth1         eth2          |
                 -------------------------------
                       |                 |
                           |           |
                 -------------|-----------------
                |          eth3                    |
                |                                     |
                ------------------------------

Did anyone test this scenario before. Assuming both eth1 and eth2 connect
to eth3, eth1 eth2 and eth3 are in the same area and the same subnet (Let's
say 192.168.2.0/24). From the RFC, both eth1 and eth2 are considered
together to DR election cause they have same router ID. Then assuming eth2
is DR, eth3 is BDR.

My qustion is:

1. What is status of eth1?  Does eth1 have neighbors or adjacencies?

Some documents say, in this case,  all inbound traffic for the RTA will use
only one interface out of eth1 or eth2. the inbound traffic to RTA is not
load balanced across the interfaces.

2. The next question is, if one link fails (Let's say the link between eth2
and eth3 fails, but the interface is still active ), can the routers detect
that and switch the route to ETH1?

I tested it on some routers, it can not switch the route to eth1. It will
set eth2 and eth3  as two separate DR, after link fails. And the status of
eth1 is always backup. Which means, if I deactive eth2 (the primary
interface), eth1 will turn to primary interface, establish neighborship and
update the routing table. Otherwise, eth1 will always keep the backup
status and not have any neighbor or adjacencies.

Best Regards

Chen Xi (陈希)
Developer, TCP/IP on IBM i
IBM China System & Technology Lab in Beijing
Notes: Xi R Chen/China/IBM  E-mail: [email-masked]
Tel:(010)82450844
Address:28,Zhong Guan Cun Software Park,No.8 Dong Bei Wang West Road,
Haidian District Beijing P.R.China 100193
地址：北京市海淀区东北旺西路8号，中关村软件园28号楼 邮编：100193

---

From: Acee Lindem <[email-masked]>
Date: Thu, 9 Feb 2012 05:57:42 -0500

Chen,

The behavior for connecting multiple OSPF interfaces on the same router to the same subnet is not specified. Hence, the behavior is implementation dependent. Use a LAG (IEEE 802.3ad).

Thanks,
Acee

On Feb 8, 2012, at 11:01 PM, Xi R Chen wrote:

Hi

Did anyone test this scenario before. Assuming both eth1 and eth2 connect to eth3, eth1 eth2 and eth3 are in the same area and the same subnet (Let's say 192.168.2.0/24). From the RFC, both eth1 and eth2 are considered together to DR election cause they have same router ID. Then assuming eth2 is DR, eth3 is BDR.

My qustion is:

Some documents say, in this case,  all inbound traffic for the RTA will use only one interface out of eth1 or eth2. the inbound traffic to RTA is not load balanced across the interfaces.

2. The next question is, if one link fails (Let's say the link between eth2 and eth3 fails, but the interface is still active ), can the routers detect that and switch the route to ETH1?

I tested it on some routers, it can not switch the route to eth1. It will set eth2 and eth3  as two separate DR, after link fails. And the status of eth1 is always backup. Which means, if I deactive eth2 (the primary interface), eth1 will turn to primary interface, establish neighborship and update the routing table. Otherwise, eth1 will always keep the backup status and not have any neighbor or adjacencies.

Best Regards

Chen Xi (陈希)
Developer, TCP/IP on IBM i
IBM China System & Technology Lab in Beijing
Notes: Xi R Chen/China/IBM E-mail: [email-masked]<mailto:[email-masked]>
Tel:(010)82450844
Address:28,Zhong Guan Cun Software Park,No.8 Dong Bei Wang West Road, Haidian District Beijing P.R.China 100193
地址：北京市海淀区东北旺西路8号，中关村软件园28号楼 邮编：100193

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