EIGRP- Equal Cost Paths Selection

Hello,
This might be a simple question; using EIGRP and having 2 equal cost paths to the destination; how does the router/L3 switch route select the path from those two equal cost paths? Is there a way to force one path over anthoer?
Thanks in advance.
Best, ~sK

To answer your question we need to be clear that there are 2 parts to the decision about which path will be used to forward traffic. The first part is the identification of paths and their insertion into the routing table. This is the part where EIGRP plays a role. If EIGRP determines that there are two equal cost viable paths then both of the paths will be put into the routing table. This ends EIGRP involvement in the process. The part of the process that takes a particular packet and determines which path to use is handled by CEF. By default CEF looks at the number of available paths toward a destination and when there are multiple paths CEF will use a process that calculates based on source and destination address to choose a particular path. For the same source address and same destination address CEF should choose the same path every time.
HTH
Rick

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.

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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.
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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.)

Nexus 7010 OSPF Equal Cost Paths

Hello,
I currently have two physical links connecting one data center to another. These are both 10Gb links and I have manually set the cost to the primary link to '1' and the secondary link to '10'. My question is, if I set the secondary link to '1' they would have equal cost routes. What is the selection process at this point? Will equal cost load balancing automatically kick in and use both links?
Thank you,
Scott

Scott
Haven't used Nexus switches but generally yes it should do depending on the routing protocol ie. statics, EIGRP, OSPF etc. will use equal cost paths if they are in the routing table.
BGP is different in that without further configuration it picks just one path so there is only one entry in the routing table.
By default it will use per destination load sharing and the default on Nexus is destination IP address and port number to choose which link to use.
"sh ip load-sharing"
will show you the current method it is using.
Jon

IS-IS and IPv6 in equal cost path topology

Network of 7609s running 12.2(18)SXE3. Two
equal cost path links between each 7609. IS-IS Level 2 routers only. When pinging an off campus IPv6 host every other packet is lost. It seems like the equal cost paths prevent an alternative path off campus but that path isn't working.
We cannot figure out where to look next.
Any thoughts ? Is this possibly an IS-IS
issue ?
Thanks

OK. I'll try. We have 5-7609s running 12.2(18)SXE3.
Each 7609 has a variety of 100/1000Base Ethernet
interfaces. They each also have one WS-X6704-10GE out
of which connections to our primary and secondary
backbones originate. This is how the routers are
interconnected.
Three of the five core routers are currently running
IS-IS. If we ssh into any router and ping
www.netbsd.org every other packet is lost.
At the end of the network are two 6506 switches each
with a 10GE module and a 1GE module.
Attached to the 1GE module are fiber links two
two different Juniper M10s which also run IS-IS
and through which we get upstream connectivity
to our provider.
All of our routers are level 2 routers in the same area.
Does this help? If not please let me know what else
you would need.
Thanks,Steve

OSPF equal cost path

Hi all,
I have
The network 192.168.0.0 is directly connected to Router A and B. Router A and B are redistributing the network in the ospf area 0.
Router A,B,C,D belong to the same OSPF Area.
Router C is DR. The cost of the link is indicated in the draw.
Question:
Router D (and router C) is using as next hop the Router A for reaching the network 192.168.0.0.
Why the router D is not using the router B as next hop due to the fact that all the possible paths are equal cost?
Thanks.

Hi,
At the end the information provided by Rolf fit in my case.
Playing with the cost to reach the ASBR and the metric of the external route I was able to figure out why the router A is the50
preferred exit point: during the redistribution router A is modifying the metric value, in this way C and D are using A as
Thanks all!!!

MPLS and dual equal-cost paths

If a site has two outbound WAN routers (for redundancy) that are configured as PE routers with the WAN interfaces configured with MPLS, will two LSPs be created using LDP (one from each PE) allowing the WAN routers to loadbalance traffic across the MPLS backbone?

Load balancing is very much possible in MPLS environment. Following are the general rules for load balancing labeled packets
1. If the MPLS payload is an IPV4 packet, the load balancing is done by hashing the source and destination IP address of the IPv4 header
2.IF the MPLS payload is an IPv6 packet, the load balancing is done by hashing the source and destination IP address of the IPv6 header.
3.If the MPLS payload is not an IPv4 packet, the load balancing is done by looking at the value of the bottom label
Label stack does not have protocol identifier field so to know what is the MPLS payload is, router can look first nibble following the MPLS label stack, if it is 4 then router considers this an IPv4 packet and performs IPv4 CEF hashing. if it is 6 , MPLS payload is considered IPv6, and the router performs IPv6 CEF hashing
For more information you can refer book "MPLS fundamentals" by Luc De Ghein.

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

ECMP - equal cost multi path

Hi,
What is the concept behind ECMP (equal cost multi path) ? Is it different for EIGRP , OSPF , ISIS etc ?
thanks

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
The concept behind ECMP is to actively and concurrently take advantage of multiple link/path bandwidth.
Oh, and to just add to what Jon has already noted, ECMP usually doesn't track actual load. So, "seeing" unequal path/link loading, especially short term, isn't unusual.
PS:
There's also unequal cost multi-path routing too - EIGRP supports that.

RIP/EIGRP maximum number of Equal cost routes

I am looking for documentation on:
If a router has 20 equal cost routing paths and only 4 show up in the routing table, what is the determining factor that chooses which four and where are the other 16 put in case the 4 routes become unusable.
Most documentation will tell you the maximum amount and how to change it but I have not found any documentation on how EIGRP or RIP chooses the routes.

You can have up to 4 by default, but you can configure the routing table to accept up to 6 on older code, and up to 8 on newer code, using the command "maximum-paths" under router eigrp. EIGRP will attempt to install all of the available paths, but the routing table will only allow it to install the first x that it installs.
The routes which are not installed by EIGRP are placed in a "backup table." If one route fails, each routing protocol running on the router is notified, and will attempt to install any routes (which match the destination, of course) back into the routing table. Again, the frst x paths would win.
Be careful with this much redundancy in eigrp--you're playing with fire if you have 15 or 20 links between a pair of routers.Russ

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

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.

BGP Path Selection

With reference to cisco's document on BGP Best Path Selection Algorithm (http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html).
Out of given 9 paths why 6th has been selected even though AS_PATH for 8th route is better.
Can anyone explains here, as this document has not considered the AS-PATH during path selection and used lowest ROUTER ID only.
Thanks in advance and expect technical explanation here.

Hey Buddy
The AS_PATH for both is only 1, don't get confused by (AS_SET) which only counts as 1 no matter how many AS are in the set. Refer to section "How the Best Path Algorithm Works"
4.Prefer the path with the shortest AS_PATH.
Note: Be aware of these items:
◦An AS_SET counts as 1, no matter how many ASs are in the set.
So bearing the above in mind
Example: BGP Best Path Selection
Path6
(64955 65003) 65089 --- this equals 1
    172.16.254.226 (metric 20645) from 10.57.255.11 (10.57.255.11)
      Origin IGP, metric 0, localpref 100, valid, confed-external, best
      Extended Community: RT:1100:1001
      mpls labels in/out nolabel/362
!--- BGP selects this as the Best Path on comparing
!--- with all the other routes and selected based on lower router ID.
Path8
(65003) 65089 --- this equals 1
    172.16.254.226 (metric 20645) from 172.16.254.234 (172.16.254.234)
      Origin IGP, metric 0, localpref 100, valid, confed-external
      Extended Community: RT:1100:1001
      mpls labels in/out nolabel/362
Comparing path 6 with path 8:
Both paths have reachable next hops
Both paths have a WEIGHT of 0
Both paths have a LOCAL_PREF of 100
Both paths are learned
Both paths have AS_PATH length 1 --- because the (AS_SET) always equals 1
Both paths are of origin IGP
Both paths have the same neighbor AS, 65089, so comparing MED.
Both paths have a MED of 0
Both paths are confed-external
Both paths have an IGP metric to the NEXT_HOP of 20645
Path 6 is better than path 8 because it has a lower Router-ID.
Hope it helps (:

Path Selection – Leaving blank holes

Hello,
I'm missing a very important feature since I switched from CS4 to CS 5.5: In Photoshop CS4, when you created a path in another and selected the path, the inner path was left blank (Creating an "o" and selecting it got you the selection in shape of an o).
Now, as I'm using CS5.5, Photoshop totally ignores those inner pathes and makes one big selection (example: http://dl.dropbox.com/u/5024014/path.png). This is very bad for me, because I have to put big industrial machines on transparent background, and now I have to create dozends of paths for every litte hole and then must put them together manually, which takes a lot of time.
So, if anyone knows how to solve this problem and getting my path selection back to how it worked in CS4, I would be very glad!
Greetings, Andreas

Path Selection Tool issues

After reading up on some of the problems people are having with the path/shape changes in CC 2014, I haven't seen an exact mention of the problem I am having so here I am.
2 part problem really:
1. When using the Path Selection Tool or Direct Selection Tool, sometimes I am unable to switch to any other tool using a keyboard shortcut, regardless of isolation mode or Active/All layers toggles. I press Z or B or whatever, and nothing happens. I have to click the tool icon to get it to work. This seems to happen randomly.
2. When using the Path Selection or Direct Selection Tool, after a few operations of moving points around, when I move to another layer to work on another shape the shape outline and the handles are not visible. When in All Layers mode, the layer is switched to the new shape as expected and the Properties panel shows details of the shape path but I can't see the outline. The only way I have found to 'fix' this is to draw a new shape or reopen the document or Photoshop itself.
Are these 'problems' just me missing some new way of working brought into the shape tools or is it as borked as it seems?
Thanks

No I have been using CC for about 8 months or so. I updated to CC 2014 a couple of days ago. I have been using the shapes with no issues up till now. The changes to the shape system seemed to bring it closer to illustrator, so it was logical. A change from the past, but I thought I was up to speed with it.
As for keystrokes to reproduce the problems I am having - that is part of the problem it seems kind of random as to when it happens.
When working on a shape, pressing return will turn the outline on and off as expected. Pressing esc will turn it off as expected, double clicking will take it into isolation mode as expected but then sometimes, the whole outline becomes unresponsive and it isn't visible. The shape layer is selected and the layer selection changes as expected when I have All Layers mode on and I click different shapes. The path is also selected in the Path panel. It shows up in the properties window too. Clicking/double clicking the shape, return and esc make no difference. I don't get it its all happening randomly though
Then I have the 'unable to switch tools' problem where shortcuts for different tools stop working. I have to click the tool icon to switch. Drawing a new shape brings it all back As I said in my first post, it could just be me missing something but i'm stumped. At this point i'm going to try a reinstall but I don't think it'll make much difference with something like this.
Thanks for the reply though.

Path Selection Thumbnails

Hello Can Someone Help Me Im Having Problems with my paths panel it seems the path selection thumbnail has a circle and slash through it.. when i try and load it as a selection i cant.

Heres the screen shot the circle with the slash isnt highlighted because im hitting the prt sc button but all of the thumbnails in the path panel has that circle and slash

EIGRP- Equal Cost Paths Selection

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