[c-nsp] Strange behavior in ECMP of OSPF
Joe Shen
sj_hznm at yahoo.com.cn
Sun Aug 21 06:14:35 EDT 2005
Hi,
I met a problem which conflict with my understanding
of ECMP in OSPF. Two OSPF boder router form different
route table on six equal cost path to the same IP
address!
The following is my network configuration
Server6
\ (trunk carring Vlan3)
Cata6509-a ------------------- Cata6509-b
| \ /
Server5 \ /
\ /
\-------------------------/
| Cata4506 |
-------------------------
/ | | \
/ | | \
Server-1 Server-2 Server-3 Server-4
All Servers running OSPF daemon which announce address
10.101.172.35 beside its interface address
(10.101.172.128/28). All servers are put into Vlan 3,
while Catalyst6509-a and Catalyst6509-b have Vlan3
interface ( IP address is 10.101.172.129,
10.101.172.130). Catalyst6509-a, catalyst6509-b and
the six servers form OSPF NSSA area 10.101.172.128,
Catalyst6509-a & catalyst6509-b are border routers
which belong to area0 too.
Both Catalyst6509 is configured as:
=============================================
router ospf 18181
router-id 10.71.101.1
log-adjacency-changes
auto-cost reference-bandwidth 10000
area 10.101.172.128 authentication message-digest
area 10.101.172.128 nssa
default-information-originate no-summary
redistribute connected metric-type 1 subnets
redistribute static metric-type 1 subnets
network 10.101.172.128 0.0.0.15 area 10.101.172.128
maximum-paths 8
distribute-list 18 in Vlan3
6509-a#sh ip access-lists 18
Standard IP access list 18
permit 10.101.172.35 (626 matches)
permit 10.101.172.47 (626 matches) check=160
permit 10.101.172.128, wildcard bits 0.0.0.31 (58
matches) check=21160
deny any (21160 matches)
6509-a#
================================================
And, Vlan 3 on both Catalyst6509 is configure like:
========================================
6509-a#sh run int vlan 3
Building configuration...
Current configuration : 291 bytes
!
interface Vlan3
description Server-Cluster-1
ip address 10.101.172.130 255.255.255.240
no ip redirects
no ip unreachables
ip route-cache flow
ip ospf message-digest-key 19 md5 7 02070A4208071C35
ip ospf priority 10
end
=====================================================
After setting up 6509-a , 6509-b and the servers, I
noticed all of them establish ospf neighbor
relationship successfully, but DR is not on 6509 but
on one of the servers! On the other hand, there are
six equal cost routing path on Catalyst6509-a to
address 10.101.172.35, but there is ONLY ONE route to
that destination on Catalyst6509-b!!!
================================================
6509-a#sh ip route 10.101.172.35
Routing entry for 10.101.172.35/32
Known via "ospf 18181", distance 110, metric 20,
type NSSA extern 2, forward metric 10
Last update from 10.101.172.137 on Vlan3, 00:42:24
ago
Routing Descriptor Blocks:
10.101.172.133, from 10.101.172.133, 00:42:24 ago,
via Vlan3
Route metric is 20, traffic share count is 1
10.101.172.134, from 10.101.172.134, 00:42:24 ago,
via Vlan3
Route metric is 20, traffic share count is 1
10.101.172.135, from 10.101.172.135, 00:42:24 ago,
via Vlan3
Route metric is 20, traffic share count is 1
10.101.172.136, from 10.101.172.136, 00:42:24 ago,
via Vlan3
Route metric is 20, traffic share count is 1
10.101.172.137, from 10.101.172.137, 00:42:24 ago,
via Vlan3
Route metric is 20, traffic share count is 1
* 10.101.172.138, from 10.101.172.138, 00:42:24 ago,
via Vlan3
Route metric is 20, traffic share count is 1
6509-a#
6509-b#sh ip route 10.101.172.35
Routing entry for 10.101.172.35/32
Known via "ospf 100", distance 110, metric 20, type
NSSA extern 2, forward metric 10
Last update from 10.101.172.136 on Vlan3, 00:42:24
ago
Routing Descriptor Blocks:
10.101.172.136, from 10.101.172.136, 00:42:24 ago,
via Vlan3
Route metric is 20, traffic share count is 1
6509-b#
==================================================
If I withdraw 10.101.172.35 from 10.101.172.136, route
on 6509-b will move to another server, never will it
see all the servers at one time!
Browsing ospf database on 6509-b, it shows the
database stores all LSA announcing 10.101.172.35 from
the six servers.
Why? Is there any configuration problem?
I run quagga0.98.3 on those servers, the 6509 IOS
version is:
============================================
6509-a:
6509-a# sh ver
Cisco Internetwork Operating System Software
IOS (tm) c6sup2_rp Software (c6sup2_rp-PSV-M), Version
12.1(13)E15, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2004 by cisco Systems, Inc.
Compiled Wed 11-Aug-04 05:15 by pwade
Image text-base: 0x40008C00, data-base: 0x417A8000
ROM: System Bootstrap, Version 12.1(11r)E1, RELEASE
SOFTWARE (fc1)
BOOTLDR: c6sup2_rp Software (c6sup2_rp-PSV-M), Version
12.1(13)E15, RELEASE SOFTWARE (fc1)
6509-b#sh ver
Cisco Internetwork Operating System Software
IOS (tm) MSFC2 Software (C6MSFC2-PSV-M), Version
12.1(13)E15, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2004 by cisco Systems, Inc.
Compiled Wed 11-Aug-04 09:27 by pwade
Image text-base: 0x40008C00, data-base: 0x41884000
ROM: System Bootstrap, Version 12.1(4r)E, RELEASE
SOFTWARE (fc1)
BOOTLDR: MSFC2 Software (C6MSFC2-PSV-M), Version
12.1(13)E15, RELEASE SOFTWARE (fc1)
======================================
Each word will be highly appreciated!
Joe
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