Ospf selection

Hi Experts.
i have some confusion in ospf path selection process. I have a core switch which connects to two router(RA&RB) these are running ospf. Core switch is receving E2 routes for x.x.x.x/24 from both routers. How is it possible to make one router say RA as primary & RB as secondary. I can not manipulate this routes other than E2 so in this case path cost is going to work??
Pls help me here.

Hi,
the main characteristic of E2 routes is that the external part of the metric takes precedence over the internal part.
That means, the cost of the Type-5 LSA's 'metric' fields are always evaluated first, the LSA with the lower cost will be installed in the routing table, regardless of the internal cost, which is the cost of the path to the border router [1].
If, like in your case, the costs of the LSAs are equal, the paths to the ASBRs (in terms of costs) [1] are evaluated. The lower-cost path wins; in case of equality both routes are installed (ECLB).
You can see the two parts of the metric with a "show ip route <prefix>":
R1#show ip route 169.254.0.0
Routing entry for 169.254.0.0/16
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 100
R1#show ip ospf border-routers
i 2.2.2.2 [100] via 172.16.12.2, Serial0/0, ASBR, Area 0, SPF 9
i 3.3.3.3 [64] via 172.16.13.3, Serial0/1, ASBR, Area 0, SPF 9
This route has been installed although there is a "cheaper" ASBR, but it obviously injects the LSA for this prefix with a metric greater than 20:
R1#show ip ospf database external 169.254.0.0
LS Type: AS External Link
Link State ID: 169.254.0.0 (External Network Number )
Advertising Router: 3.3.3.3
Network Mask: /16
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 30
So yes, you can achieve a primary/secondary path when both LSAs have the same metrics by changing a interface's cost, but remember that this may affect the whole topology.
HTH
Rolf
EDIT:
[1]: More precisely it's the path to the 'forwarding address', which is another field of external LSAs and used to avoid unnecessary extra-hops. If the FA is 0.0.0.0, the path to the advertising ASBR is used instead.

Similar Messages

OSPF Equal Cost Path Selection

This is a nerdy enough qeury in reality.
We have a single area - area 0.0.0.32. All intra-area routes. We have 2 switches in the core of the network, and 10 switches at the edge. All of these switches are connected via layer 3 OSPF routed links.
The cost for all links is 20 - which is based on bandwidth between the boxes - which is 2Gbps.
Have a look at the enclosed jpeg to get an idea.
Very simple.
Query revolves around the path selection available to OSPF.
The path from Core 2 to the 10.32.51.0 network is easy - straight across the link between the core switches for a cost of 20.
Question :- if the link between the cores fail, which path will be chosen by OSPF and why?
It can go through ANY of the other edge switches for a total cost of 40, but it will choose a particular one. What criteria does OSPF use to select this path?
Remember, the path costs are equal, they are all intra-area.
I tried messing with Router ID, but this doesn't seem to be it.
I tried highest interface IP addresses, but it doesn't seem to be this.
It is not a random act, the algorithm chooses the same one every time.
There must be some parameter in the LSDB that is the defining one when it comes to path selection.
Can you help me out please. I need someone who understands the OSPF algorithm better than I do (which might not be hard!).
Appreciate any comments for debate.

Disclaimer
The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.
Liability Disclaimer
In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.
Posting
My guess (as I haven't re-read the RFC), selection of an ECMP to retain in a routing table (assuming all possible ECMP are not retained) and/or exactly how packets or flows are ECMP routed is implementation dependent.
I recall years ago bumping into a situation where I had 6 (OSPF) ECMP on a Cisco router which had the (then) default allowance of 4 ECMP in the route table. I don't recall exactly what the issue was, but whatever it was doing I considered it a bug. The "fix" was to allow the router to use all 6 ECMP. (Again, don't remember the specifics, but the issue I saw was more involved than 2 of the 6 ECMPs weren't retained.)

OSPF Improper Path Selection

This is related to the way the FA is set (to zero or non-zero).
Have a look here: http://www.costiser.ro/2013/04/04/quiz-12/

Disclaimer
The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.
Liability Disclaimer
In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.
Posting
My guess (as I haven't re-read the RFC), selection of an ECMP to retain in a routing table (assuming all possible ECMP are not retained) and/or exactly how packets or flows are ECMP routed is implementation dependent.
I recall years ago bumping into a situation where I had 6 (OSPF) ECMP on a Cisco router which had the (then) default allowance of 4 ECMP in the route table. I don't recall exactly what the issue was, but whatever it was doing I considered it a bug. The "fix" was to allow the router to use all 6 ECMP. (Again, don't remember the specifics, but the issue I saw was more involved than 2 of the 6 ECMPs weren't retained.)

Lesson BGP & OSPF path selection in VSS routing environment

Hi, I would like a lesson on how traffic is passed in the following environment:
One 3945 router with interfaces connected to a pair of 4500X switches configured as VSS pair. One link into each of the 4500 running as routed interfaces using separate IP subnets meaning there are two equal cost paths between the router and the 4500X.
We are running a single OSPF area and iBGP between the devices.
I would like to find out, in normal circumstances where both equal cost links are operating normally, how the 4500 selects the path to send a packet to the router. We would be trying to avoid traffic passing through the VSL but want to know if the system is smart enough to do that.
Is there somebody out there who can tell me if the VSS process will select the path directly to the router or if it cannot be guaranteed to do so.
I also would like to get opinions on whether it is best to create two iBGP neighbour relationships on the link addresses or one relationship between the loopback addresses.
Thanks
LP

Hi,
The OSPF traffic would not pass through the VSL link. The path would directly go from each 4500 to the 3945 (Equal cost load balancing). I think, the 3900 series supports Etherchannel, if this is the case you can also create a L-3 Portchannel between the VSS and 3945 router. This way you use one /30 instead of 2 and you still have redundancy. For BGP, I would do one peering with Loopbacks.
HTH

ASA: OSPF Path Selection RFC1583 or 2328?

Hi,
I've been searching for this information but didn't find anything so far.
Among other things, with RFC 2328 the way to determine the prefered path has been changed.
However, per default Cisco IOS platforms still use the RFC 1583 mechanisms for compatibility reasons.
NX-OS, in contrast, use the RCF 2338 specifications per default.
The recommendation is to use the same method within an OSPF domain. Otherwise, at the worst, routing loops could occur.
I know that our routers and multilayer switches run RFC 1583 compatible mode but I'm not sure about our ASAs.
Can anybody confirm (if possible a reference would be desirable)?
Thanks,
Rolf

RFC 1583 mode is the default on ASA. You can switch to RFC 2328 with the command "no compatible rfc1583".
Command Reference
Configuration Example
The configuration example states it most clearly - see the notes under the advanced properties figure.

OSPF external route selection problem

Hello. I have a situation where I got two paths to get to a destination. Router A can get to subnet C either through my Telco's onsite router (Router A->telco router->Router C)or through a secondary link that travels from Router A-> Router B->diffenent Telco router->Router C.
The link between Router A and the telco router is area 0. The link between Router A->B->Telco router is also area 0.
Normally I want traffic to go directly through the onsite telco router...and only go through the longer route if the onsite router goes down.
For some reason I'm going through the suboptimal path. Here's what the OSPF database is telling me:
Routing Bit Set on this LSA
LS age: 1267
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 192.168.33.0 (External Network Number )
Advertising Router: 192.168.50.14
LS Seq Number: 80000084
Checksum: 0x4B8B
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 100
Forward Address: 0.0.0.0
External Route Tag: 66
LS age: 262
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 192.168.33.0 (External Network Number )
Advertising Router: 192.168.50.94
LS Seq Number: 800003B8
Checksum: 0xF757
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 100
Forward Address: 0.0.0.0
External Route Tag: 66
Both telco routers advertise this route as an extern type 1 with a metric of 100. Can anyone shed some light as to why my router is picking the path via 50.14 instead of 50.94? 50.94 is a locally connected network. 50.14 is a couple of hops away.
Thanks,
Mike

No problem.
Here's the output...
show ip ospf border-routers
OSPF Process 2 internal Routing Table
Codes: i - Intra-area route, I - Inter-area route
OSPF Process 1 internal Routing Table
Codes: i - Intra-area route, I - Inter-area route
i 10.0.22.3 [2] via 192.168.19.14, Vlan168, ABR/ASBR, Area 0, SPF 42
i 10.0.22.2 [2] via 192.168.19.2, Vlan168, ABR/ASBR, Area 0, SPF 42
i 192.168.30.5 [501] via 192.168.19.15, Vlan168, ASBR, Area 0, SPF 42
i 192.168.30.5 [501] via 192.168.19.14, Vlan168, ASBR, Area 0, SPF 42
i 172.29.50.1 [1] via 192.168.19.11, Vlan168, ASBR, Area 0, SPF 42
i 172.18.1.2 [3] via 192.168.19.15, Vlan168, ABR/ASBR, Area 0, SPF 42
i 172.18.1.3 [2] via 192.168.19.15, Vlan168, ABR/ASBR, Area 0, SPF 42
i 192.168.1.5 [1] via 192.168.19.8, Vlan168, ASBR, Area 0, SPF 42
i 10.0.32.2 [2] via 192.168.19.15, Vlan168, ABR/ASBR, Area 0, SPF 42
i 10.0.32.3 [2] via 192.168.19.3, Vlan168, ABR/ASBR, Area 0, SPF 42
i 172.27.95.1 [2] via 192.168.19.16, Vlan168, ABR/ASBR, Area 0, SPF 42
i 172.19.1.3 [1] via 192.168.19.15, Vlan168, ABR/ASBR, Area 0, SPF 42
i 172.19.1.2 [1] via 192.168.19.14, Vlan168, ABR/ASBR, Area 0, SPF 42
i 10.0.24.2 [1] via 192.168.19.22, Vlan168, ABR/ASBR, Area 0, SPF 42
i 10.0.24.3 [1] via 192.168.19.23, Vlan168, ABR/ASBR, Area 0, SPF 42
i 192.168.19.4 [1] via 192.168.19.4, Vlan168, ASBR, Area 0, SPF 42
i 172.27.87.1 [1] via 192.168.19.12, Vlan168, ASBR, Area 0, SPF 42
i 192.168.19.19 [1] via 192.168.19.19, Vlan168, ASBR, Area 0, SPF 42
i 172.20.1.2 [3] via 192.168.19.3, Vlan168, ASBR, Area 0, SPF 42
i 172.20.1.3 [2] via 192.168.19.3, Vlan168, ASBR, Area 0, SPF 42
I 10.0.16.11 [2] via 192.168.19.3, Vlan168, ASBR, Area 0, SPF 42
I 10.0.16.11 [3] via 192.168.19.2, Vlan168, ASBR, Area 0, SPF 42
I 10.0.16.10 [2] via 192.168.19.2, Vlan168, ASBR, Area 0, SPF 42
I 192.168.50.14 [2] via 192.168.19.14, Vlan168, ASBR, Area 0, SPF 42
i 192.168.50.94 [100] via 192.168.50.94, Vlan162, ASBR, Area 0, SPF 42
i 172.21.1.2 [1] via 192.168.19.16, Vlan168, ASBR, Area 0, SPF 42
i 10.0.229.2 [1] via 192.168.19.24, Vlan168, ABR/ASBR, Area 0, SPF 42
i 10.0.17.2 [2] via 192.168.19.2, Vlan168, ABR/ASBR, Area 0, SPF 42
i 10.0.17.3 [2] via 192.168.19.3, Vlan168, ABR/ASBR, Area 0, SPF 42
i 192.168.8.1 [1] via 192.168.19.28, Vlan168, ASBR, Area 0, SPF 42
i 192.168.16.173 [565] via 192.168.19.14, Vlan168, ABR/ASBR, Area 0, SPF 42
i 192.168.16.173 [565] via 192.168.19.15, Vlan168, ABR/ASBR, Area 0, SPF 42
i 172.27.193.2 [1] via 192.168.19.20, Vlan168, ASBR, Area 0, SPF 42
i 172.16.1.2 [1] via 192.168.19.2, Vlan168, ABR/ASBR, Area 0, SPF 42
i 172.16.1.3 [1] via 192.168.19.3, Vlan168, ABR/ASBR, Area 0, SPF 42

Path Selection between 10 gig fiber and microwave

Hello everyone,
my network is running OSPF as an IGP, i have a 10 gig Ethernet fiber connected between two sites and a microwave link as a redundant connection.
since ospf metric is cost ( or bandwidth ), the 10 gig ethernet connection is always preferred. however, sometimes the 10 gig link is flapping or the bit error rate is bad, is there anyway to change the path selection to go through the microwave when the bit error rate in the 10 gig link is bad or the link flaps ?
basically can we make the path selection based on anything than the speed or cost ?

Disclaimer
The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.
Liability Disclaimer
In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.
Posting
Bandwidth can be a metric to OER/PfR. Much else can be used by OER/PfR.
The intent of this technology is sort of described by the names, Optimized Edge Routing (v1) and Performance Routing (v2).
Both can account for path bandwidth and/or analyze performance.
Understand typical dynamic routing protocols keep track of paths between source and destination and some have a way to "weight" paths (for example, OSPF link cost [which by RFC, hasn't nothing to do with bandwidth, but is often based on that]).
OER/PfR, for example, can run their own SLA tests.
Years ago, I set up OER in large dual MPLS/VPN environment. Our initial "problem", after activation, our WAN performance monitoring tools (and our users!) no longer "saw" any WAN performance issues. They were still happening, but OER "saw" them first, and worked around them before the monitoring tools saw them.

Question about network statement in OSPF and BGP

The network statements in OSPF and BGP can be used to advertise networks. But I'm not clear under what circumstances would make more sense to use network statements to advertise a network than by using other methods to have the network learned by other routers.
Here is an example: assume I'm running BGP on router A. I want to advertise network 10.1.1.0/24 to other BGP peers. I have a OSPF route for this network. I can do 2 things: one is to use "network 10.1.1.0 mask 255.255.255.0", the other is to do "redistribute OSPF ... route-map OSPF-INTO-BGP", and create a prefix list to permit 10.1.1.0/24.
Both would work to have this network learned by other BGP peers. But which is better for what purpose?
Thanks a lot
Gary

Hi Gary,
There is one little difference between the use of the two approaches - the route injected into BGP by using a network statement will carry an Origin attribute of IGP, whereas the route injected using redistribution will have an Origin attribute of Incomplete. Now, that is not a huge issue since you can always change that whatever value you desire both with the use of the network statement and redistribution. The important thing, however, is that in the BGP best path selection process, the Origin attribute comparison is fairly high up and will prefer a route with the attribute of IGP.
Apart from that, there is absolutely no difference between using the network statement and using redistribution with a route-map that matches exactly on the same route that you would have specified with the network statement.
I guess one advantage of using the redistribute approach is that it does not clutter up the BGP config. If you wish to add more routes, you simply add them to the prefix list so that you don't really touch the BGP config portion at all..
Hope that helps - pls do remember to rate posts that help.
Paresh

Load Balancing with OSPF and maximum-paths command

Hello,
Just a quick query really, we have a disribution layer 3 switch, in its routing table it has 3 default routes all with the same metric from the core router, this is because the core router is setup with the comamnd "default-information originate always metric 50" which obviously proagates the default route around the area and the metric never changes from 50.
So i have a routing table that looks like this:
O*E2 0.0.0.0/0 [110/50] via 77.95.176.9, 06:44:51, GigabitEthernet4/9
[110/50] via 77.95.176.17, 06:44:51, Vlan903
[110/50] via 91.203.72.5, 06:44:51, Vlan262
Three default routes with the same metric, does this mean that the router IOS will load balance traffic over all three routes evenly? I mean i have been reading up on it and appartemtly i dont have the command "maximum-paths 3" under my ospf process?
I have been doing some traceroutes from this switch to the internet (various sites) and all the traffic seems to be going out over the first route in the table that next hop is 77.95.176.9
My question is how can i verify that load balancing is taking place, or if its not then i need to add this "maximum-paths 3" command to the ospf on the local switch? I would say load balancing is not taking place but im sure i have seen traffic from one customer being routes over all 3 paths due to matching spikes on the SNMP sensors?
Many Thanks.
Matt

Disclaimer
The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.
Liability Disclaimer
In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.
Posting
Yes, your traffic should use all three paths, as Rick notes, OSPF, on Cisco, normally defaults to using up to 4 equal cost paths.
As Rick also notes mentioning CEF, how actual traffic is forwarded across ECMP can vary. Often, the device will keep all traffic for the same flow on the same egress port, and attributes selected for actual egress port selection might be deterministic. I.e. it's possible same traffic flow will always be sent to the same egress port. (This means even with ECMP, you may not see an equal load distribution.)

Controlling path selection in multihomed network

Hi All,
I *think* I've worked out the answer to this from reading the docs and from other similar posts here, but I would appreciate a sanity-check and any constructive criticism from the experts here.
The customer has an MPLS VPN with two links into HQ and a bunch of singly-connected remote sites. He wants to load-balance across the two HQ links and (the crucial bit) wants to control which of the two HQ links is used by each remote site.
For the sake of discussion, lets divide the remote sites into a "Red" group and a "Green" group. I figure if I make one HQ link part of the "Red" group and the other HQ link part of the "Green" group and then configure all of the PEs (the HQ ones and the remote ones) as follows:-
"Red" sites
===========
ip vrf Customer
rd x:y
route-target export 1000:1
route-target import 1000:1
route-target import 1000:2
import-map Prefer-Red
route-map Prefer-Red permit 10
match community 1000:1
set local-preference 100
route-map Prefer-Red permit 20
match community 1000:2
set local-preference 10
set metric +10
"Green" sites
=============
ip vrf Customer
rd x:z
route-target export 1000:2
route-target import 1000:1
route-target import 1000:2
import-map Prefer-Green
route-map Prefer-Green permit 10
match community 1000:2
set local-preference 100
route-map Prefer-Green permit 20
match community 1000:1
set local-preference 10
set metric +10
Does this look like a sensible approach ? Does anyone have any better suggestions for accomplishing the desired result ?
Thanks,
Eamonn
(This is my first post here...I hope it is appropriate).

Hello Eamonn,
Another option for path selection over the MPLS VPN backbone is to create an additional OSPF intra-area (logical) link between ingress and egress VRFs on the relevant PE routers, also known as "Sham-Link"
For further details...
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122t/122t8/ospfshmk.htm
Let us know if this helps !!!
sultan

Specific path selection in E-BGP

I have two routers ASR 9K platform with the image file is "disk0:asr9k-os-mbi-4.3.4.sp4-1.0.0/0x100305/mbiasr9k-rsp3.vm"
Primary link b/w Router 1 and 2 :
Router 1 ------> Directly connected with 30G to Router 2 on a bundle ( Neither ISIS nor BGP running on this link )
Router 2 ------> Directly connected with 30G to Router 1 on a bundle ( Neither ISIS nor BGP running on this link )
Secondary link b/w Router 1 and 2 :
Router 1 and 2 is connected on TenGig /0/0/0/4.451 and I am using secondary link for both Internet and Private (VPN) traffic as this link is running ISIS as my IGP and IPv4/v6 Unicast and IPv4/v6 Labeeled Unicast Peering using this interface
My query is how can I seggregate my IPv4/v6 Unicast Traffic in secondary link and IPv4/v6 Labelled Unicast traffic in my Primary link
Please suggest how can I do in BGP to select one path for Internet Prefixes and another for Private Prefixes
Many thanks in advance
Sankar.

Hi,
The OSPF traffic would not pass through the VSL link. The path would directly go from each 4500 to the 3945 (Equal cost load balancing). I think, the 3900 series supports Etherchannel, if this is the case you can also create a L-3 Portchannel between the VSS and 3945 router. This way you use one /30 instead of 2 and you still have redundancy. For BGP, I would do one peering with Loopbacks.
HTH

BGP Selection .. Why Path #2 is better ! ?

I have a question please. For below output why 2nd output is considered best
According to my topology this is the right behavior however I am just curious why it is considered the best
Many Thanks
||||||||||||||
R9#show ip bgp vpnv4 vrf ABC 172.9.0.5
BGP routing table entry for 1009:9:172.9.0.5/32, version 142
Paths: (2 available, best #2, table ABC, RIB-failure(17) - next-hop mismatch)
Not advertised to any peer
Local
    172.9.195.15 from 0.0.0.0 (9.9.0.9)
      Origin incomplete, metric 2560077056, localpref 100, weight 32768, valid, sourced
      Extended Community: SoO:109:109 RT:1009:9
        Cost:pre-bestpath:129:2560077056 (default+412593409) 0x8800:0:9
        0x8801:100:77056 0x8802:259:2560000000 0x8803:257:1
        0x8804:1009:2886270986 0x8805:9:0
      mpls labels in/out 31/nolabel
9, imported path from 9:9:172.9.0.5/32
    9.9.0.8 (metric 30) from 9.9.0.8 (9.9.0.8)
      Origin IGP, metric 4125934090, localpref 100, valid, internal, best
      Extended Community: RT:9:9 OSPF DOMAIN ID:0x0005:0x0000006D0200
        OSPF RT:0.0.0.0:2:0 OSPF ROUTER ID:172.9.0.5:36783
      mpls labels in/out 31/62
R9#

The BGP cost community modifies the BGP path selection process.
the "pre-bestpath" point of insertion was introduced in the BGP Cost Community feature. This POI is applied automatically to EIGRP routes that are redistributed into BGP and carries the EIGRP route type and metric. This POI influences the best path calculation process by influencing BGP to consider this POI before any other comparison step effectivefly forcing BGP to use IGP-like selection rules.
https://supportforums.cisco.com/document/58226/dual-ce-pe-connection-and-eigrp
http://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/s_bgpcc.html
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html#custom
Best Regards,
Bheem

Interface OSPF cost issue

Image link (the image above looks distorted in the post preview): http://fodder.s3.amazonaws.com/ospf-cost-diagram.jpg
In the test network shown in the diagram I have the following issue that I can't seem to figure out:
Without any modification to ospf interface cost traffic from R8 to 172.16.88.1 (R5) takes R3 --> R2 --> R5 (total metric 67).
This is not ideal because of the f1/0 interface on R2. The path via R3 --> R4 --> R6 --> R5 (total metric 68) is actually preferable but is not selected because of OSPF's baseline inability to distinguish between FastEthernet and greater link speeds.
I am fully aware that I can use the auto reference-bandwidth command on all the routers to fix things but I am testing behaviors of the ip ospf cost command at the moment.
Setting the ospf cost of the f1/0 interface on R2 to 10 changes the path for traffic coming from R8 on its way to 172.16.88.1 (R5). This is desired result. However, the return traffic from R5 continues to use the path through R2 (R2 --> R3 --> R8). This is what is illustrated in the dotted arrows in the included image. R2's interface cost is not being taken into account by R5, which I am assuming is because R5 respects his own interface cost on the link over R2's cost. However, this is producing a sub-optimal return path.
My question is this: Aside from using a route map is there an ospf configuration that I am overlooking that will cause R5 to stop thinking that the path through R2 is preferred?

Hi Colin,
The only way I could solve this was to change the cost on the return path of R5 to R8. It isn't tidy, but setting the ip ospf cost on R2's G0/0 port to 10, changes the cumulative path from R5 to R8.
I changed the reference bandwidth to 1000 in the topology to see how this would changes things. I just learnt that a 'no ip ospf cost' on both int g0/0 and f1/0 on R2, does not change the cost by itself, so had to clear the ospf process in order to affect the cost. Not sure if this is normal behaviour...
With no ip ospf cost configured on R2, and the ref bw changed to 1000, the path from R8 to R5 now takes the optimal path. However traffic R5 to R8 will still take the path through R2. Checking on the cost of G0/0 on R5 the cost is 1.
Great scenario... to get the optimal return path,R5 through R6 to R8, R2's G0/0 port needed manual adjusting to 10, so R5 would not have R2 as the best path to reach R8.
Taking R2 off the shared link of R5 and R6 and having it's own Fastethernet connection to R5 would be the simplest solution? Thanks for sharing Colin.
Cheers, Jeremy.

Why OSPF is not used in MPLS, though it is very efficient protocol

Dear all, though OSPF is a very efficient protocol, with a number of features to handle much larger networks,it is not used in MPLS. Can u plz let me know, what special features BGP posses over OSPF, for its selection in MPLS.

For core MPLS switching, any routing protocol can be used, including static. Service providers have preferred to use link state protocol like OSPF and ISIS and they still do.
However, different MPLS applications have their routing protocol requirements. MPLS Traffic Engineering for example requires a link state protocol. Hence, if you want to do Traffic Engineering, your IGP must be either OSPF or ISIS. This is because for proper operation of MPLS TE, the routers should have a complete topology view of the network, so as to be able to route packet accordingly.
For MPLS VPN, there is a requirement to deploy iBGP on the edge routers. The main requirement stems from label allocation procedure of MPLS. While MPLS routers assigns a label for every route in the FIB, routes learned via BGP are exempted. Rather, for BGP learnt routes, the label for the BGP next-hop of the route is used. This is critical to the operation of the MPLS VPN, because it effectively creates a tunnel and ensures that every router on the network does not have to carry all the VPN routes.
I hope the explanation is clear

Cisco ASA - BGP or OSPF support on Multicontext Firewall?

Hello Forum,
I would like to know why is the limitation of Cisco ASA in multicontext mode that it is not able to run routing protocols like OSPF, BGP?
if I see SRX firewall, you can cut that virtually and can configure BGP, OSPF routing instances with virtual firewall.
is there any possibility in ASA product to run OSPF, BGP in multicontext mode?
comments are welcome...
Thanks
Dave

To answer your question, Cisco wants its customers (or at least used to want its customers) to use the ASA as a firewall and not a router. So you would have one device that is your firewall and one device that is your router. I suppose they started to realize that customers are looking for an all in one device, so they started adding routing features to the ASA, and firewall features to the routers, yet the firewall still doesn't have all the routing capabilities of a router and the router doesn't have all the firewalling capabilities of the ASA.
You can speculate that this is a marketing ploy so you are required to purchase more devices, or you could look at it in such a way that it is best practice to seperate all functionality in the instance that a device does get hacked.
Please remember to select a correct answer and rate

Ospf selection

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