With Peter Palúch
Welcome to the Cisco Support Community Ask the Expert conversation. This is an opportunity to get expert insights and learn about Unicast IP routing protocols with Cisco Designated VIP Peter Palúch. Peter Palúch is an assistant professor and a Cisco Networking Academy instructor at the University of Zilina, Slovakia. His fields of interest include routing, switching, and MPLS technologies. He is a seasoned teacher who has cooperated in various educational activities in Slovakia and abroad, focusing particularly on networking and Linux-based network server systems. Peter holds a doctoral degree in the area of VoIP quality degradation factors; he also holds CCIP certification and CCIE certification (#23527) in Routing & Switching.
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Hello Jiri,
Nice to hear from you again!
To be completely exact about the prefix suppression technique, I will be quoting its description from the following URL here:
The OSPF mechanism to exclude connected IP prefixes from LSAs allows network administrators to control what IP prefixes are installed into LSAs. This functionality is implemented for router and network LSAs in the following manner:
- For the router LSA, to exclude prefixes, the feature excludes link type 3 (stub link).
- For the network LSA, the OSPF Designated Router (DR) generates LSAs with a special /32 network mask (0xFFFFFFFF).
Subsequently, the guide indicates:
Prefixes that are associated with loopbacks, secondary IP addresses, and passive interfaces are excluded because typical network designs require those to remain reachable.
In this particular statement, the word "excluded" actually means excluded from the prefix suppression feature, i.e. being advertised even with prefix suppression activated.
In other words, after configuring the prefix suppression globally in the OSPF configuration, the router will advertise only the following directly connected prefixes:
- Loobpacks
- Secondary IP addresses
- Passive interfaces
of course assuming that these interfaces are assigned to the particular OSPF process.
I have confirmed this in Dynamips running 2691 12.4(15)T13 Advanced IP Services IOS - the behavior aligned perfectly with the description.
all OSPF networks was gone from the routing table. What am I doing wrong? At least stub networks and loopbacks should remain.
This surprises me. The loopbacks should not have disappeared from your routing tables if they were advertised before configuring the prefix suppression. Physical directly-connected networks are supposed to disappear - you have to manually declare the interfaces as passive in order to have them advertised if the prefix suppression is active. The router will not automatically advertise an interface's network just because it is considered a stub network (i.e. no other OSPF neighbors detected).
What is the IOS version you were using when performing these experiments? Also, would you mind posting your entire configuration so I can try to reproduce the problem? Thank you!
Best regards,
Peter
Hi Peter,
Today is the last day of your "Ask the Expert" so I thought i will ask you one last question here. Hope you dont mind
I am doing some testing on mpls tunnels on a 7200 image on GNS3.. Now, it's a simple set up. I am uploadig my topo here.
I have 2 tunnels.
1. dynami tunnel from PE31-P1-P32 which comes up fine.
2. explicit path going via PE31-PE-MGT-PE32.
Now the second tunnel always remains down and shows me the following error.
Name: ** MPLS-TE via PE-MGT (Tunnel1) Destination: 192.168.3.33
Status:
Admin: up Oper: down Path: not valid Signalling: Down
path option 1, type explicit PE31-PE_MGT-PE32
Config Parameters:
Bandwidth: 100 kbps (Global) Priority: 2 2 Affinity: 0x0/0xFFFF
Metric Type: TE (default)
AutoRoute: enabled LockDown: disabled Loadshare: 100 bw-based
auto-bw: disabled
History:
Tunnel:
Time since created: 8 minutes, 18 seconds
Path Option 1:
Last Error: PCALC:: Explicit path has unknown address, 192.168.5.50
PE31#sh ip route 192.168.5.50
Routing entry for 192.168.5.48/30
Known via "connected", distance 0, metric 0 (connected, via interface)
Routing Descriptor Blocks:
* directly connected, via Ethernet1/3
Route metric is 0, traffic share count is 1
PE31#sh ip explicit-paths
PATH PE31-PE_MGT-PE32 (strict source route, path complete, generation 5)
1: next-address 192.168.5.50
3: next-address 192.168.4.1
4: next-address 192.168.3.33 <<< destination of the tunnel.
PE31#
Why is it showing me unknown address although its on the directly connected subnet
PE31#show mpls traffic-eng topology | inc 192.168.5.50 <<< in the topology
link[1 ]:DR Intf Address: 192.168.5.50, gen:2
IGP Id: 192.168.5.50, Network Nod
Regards,
Kishore
