EIGRP advertisement of duplicate routes?

We know that EIGRP load-balances across equal links (up to 4) by default. Can someone tell me how the EIGRP load-balancing router decides which one of these routes to advertise to other neighbors?

 

For example, consider this simple scenario (all links are equal with default EIGRP metrics): 

[10.1.1.0/24]                    BGP Domain

 |            |

 A           B        +++++++++++++++++++++++++++

         /                        EIGRP DOMAIN

       /

       C _ _ _ D

 

Router A's Router ID is 12.12.12.1 and Router B's Router ID is 12.12.12.2

Router C has two routes for 10.1.1.0/24 in it's topology table. 1 route originated by Router A with External Originating ID of 12.12.12.1 and second originated by Router B with
 External Originating ID of 12.12.12.2. Which one shows up in router D's topology table and why?

Comments

  • Hi ajCCIE2b,

    Since router D has only a single connection, it will contain only one route for the 10.1.1.0/24 network. This is going be an external EIGRP route generated by router A, in this case.

    HTH

    Good luck!

  • Hi!

    Recently there was a discussion about EIGRP load balancing. Maybe you can find some information for you out of it:

     

    http://ieoc.com/forums/t/18814.aspx

     

    Regards!

    Markus

  • Sorry, I thought this was an easy question, but after reading it again I understood it better. When looking into Router D's topology table you are seeing the originating router as 12.12.12.1 and even if you change the router-id to a higher value on router A you still get the same route installed. I guess this has to do with the uplink port from router C to router A, if you change the IP address of that link to a higher value as compared to the link between router C to router B, you are going to see that router D has 12.12.12.2 as the originator of that route. I just labbed it up and it behaves like this.

    Very interesting scenario :)

    Good luck!

  • C has two equal metric routes in its topology table so in its routing table. In order to avoid confusion of which route to send to D in its eigrp updates, C decides to choose only route perhaps from the lowest source address (it does not send both routes to save bandwidth resources on the link) though it does not matter at all for D whichever is the originating router for the receiving route. The actual data traffic forwarding will still depends on the cef table at C.

  • Hi ajCCIE2b,

    Based on your scenario, Router D forwards all packets to Router C for 10.1.1.0/24 network because there is no other links for this network.

    Now look at Router C, how it does the load-balancing, this really depends on CEF (by default CEF load balancing is per-destination the source and destination IP address in the incoming IP packet are hashed into a 4-bit value that selects the outgoing path in the CEF has table.)

    See here:

    Network 1.1.1.1 and 11.11.11.11 are from same router but using different paths (this is because of CEF destination load-balancing)

    R4#show ip cef exact-route 4.4.4.4 1.1.1.1
    4.4.4.4         -> 1.1.1.1        : FastEthernet1/0 (next hop 34.34.34.3)
    R4#show ip cef exact-route 4.4.4.4 11.11.11.11
    4.4.4.4         -> 11.11.11.11    : FastEthernet0/0 (next hop 24.24.24.2)
    !

    R4#show ip route eigrp
         1.0.0.0/32 is subnetted, 1 subnets
    D       1.1.1.1 [90/158720] via 34.34.34.3, 00:19:17, FastEthernet1/0
                    [90/158720] via 24.24.24.2, 00:19:17, FastEthernet0/0

         2.0.0.0/32 is subnetted, 1 subnets
    D       2.2.2.2 [90/156160] via 24.24.24.2, 00:19:17, FastEthernet0/0
         3.0.0.0/32 is subnetted, 1 subnets
    D       3.3.3.3 [90/156160] via 34.34.34.3, 00:19:17, FastEthernet1/0
         5.0.0.0/32 is subnetted, 1 subnets
    D       5.5.5.5 [90/156160] via 45.45.45.5, 00:19:20, FastEthernet2/0
         11.0.0.0/32 is subnetted, 1 subnets
    D       11.11.11.11 [90/158720] via 34.34.34.3, 00:02:55, FastEthernet1/0
                        [90/158720] via 24.24.24.2, 00:02:55, FastEthernet0/0

    D    192.168.0.0/24 [90/30720] via 34.34.34.3, 00:19:17, FastEthernet1/0
                        [90/30720] via 24.24.24.2, 00:19:17, FastEthernet0/0

    R4#show ip eigrp topology
    IP-EIGRP Topology Table for AS(100)/ID(4.4.4.4)
    Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
           r - reply Status, s - sia Status

    P 11.11.11.11/32, 2 successors, FD is 158720
            via 24.24.24.2 (158720/156160), FastEthernet0/0
            via 34.34.34.3 (158720/156160), FastEthernet1/0
    P 3.3.3.3/32, 1 successors, FD is 156160
            via 34.34.34.3 (156160/128256), FastEthernet1/0
    P 1.1.1.1/32, 2 successors, FD is 158720
            via 24.24.24.2 (158720/156160), FastEthernet0/0
            via 34.34.34.3 (158720/156160), FastEthernet1/0
    R5#show ip eigrp topology
    IP-EIGRP Topology Table for AS(100)/ID(5.5.5.5)
    Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
           r - reply Status, s - sia Status

    P 11.11.11.11/32, 1 successors, FD is 161280
            via 45.45.45.4 (161280/158720), FastEthernet0/0
    P 3.3.3.3/32, 1 successors, FD is 158720
            via 45.45.45.4 (158720/156160), FastEthernet0/0
    P 1.1.1.1/32, 1 successors, FD is 161280
            via 45.45.45.4 (161280/158720), FastEthernet0/0

    Where R4 and R5 are in Router C and Router D position.

  • Hi Narayan

    Nice exhibit but he was trying to ask; which route router C will forward to D, is it originated by router A or router B i.e in the control plane. Your exhibit is saying more about data plane forwarding.

  • Hi Qqabdal,

    Where you found originating router as 12.12.12.1? This is the output from Router D:

    R5#show ip route 1.1.1.1
    Routing entry for 1.1.1.1/32
      Known via "eigrp 100", distance 90, metric 161280, type internal
      Redistributing via eigrp 100
      Last update from 45.45.45.4 on FastEthernet0/0, 00:04:30 ago
      Routing Descriptor Blocks:
      * 45.45.45.4, from 45.45.45.4, 00:04:30 ago, via FastEthernet0/0
          Route metric is 161280, traffic share count is 1
          Total delay is 5300 microseconds, minimum bandwidth is 100000 Kbit
          Reliability 255/255, minimum MTU 1500 bytes
          Loading 1/255, Hops 3
    !

    R5#show ip eigrp topology detail-links
    IP-EIGRP Topology Table for AS(100)/ID(5.5.5.5)
    Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
           r - reply Status, s - sia Status

    P 11.11.11.11/32, 1 successors, FD is 161280, serno 27
            via 45.45.45.4 (161280/158720), FastEthernet0/0
    P 3.3.3.3/32, 1 successors, FD is 158720, serno 28
            via 45.45.45.4 (158720/156160), FastEthernet0/0
    P 1.1.1.1/32, 1 successors, FD is 161280, serno 25
            via 45.45.45.4 (161280/158720), FastEthernet0/0

    !

    R5#show ip eigrp topology 1.1.1.1/32
    IP-EIGRP (AS 100): Topology entry for 1.1.1.1/32
      State is Passive, Query origin flag is 1, 1 Successor(s), FD is 161280
      Routing Descriptor Blocks:
      45.45.45.4 (FastEthernet0/0), from 45.45.45.4, Send flag is 0x0
          Composite metric is (161280/158720), Route is Internal
          Vector metric:
            Minimum bandwidth is 100000 Kbit
            Total delay is 5300 microseconds
            Reliability is 255/255
            Load is 1/255
            Minimum MTU is 1500
            Hop count is 3

  • narayan ,

    1.1.1.1 should be an external route.

    i would expect to see extra information in the "show ip eigrp topo" , like below

    sh ip eigrp topo 1.1.1.0/26
    EIGRP-IPv4 (AS 1): Topology default(0) entry for 1.1.1.0/24
      State is Passive, Query origin flag is 1, 2 Successor(s), FD is 282624
      Routing Descriptor Blocks:
      45.45.45.45 (GigabitEthernet1/1), from fa0/0, Send flag is 0x0
          Composite metric is (282624/282368), Route is External
          Vector metric:
            Minimum bandwidth is 100000 Kbit
            Total delay is 10040 microseconds
            Reliability is 255/255
            Load is 8/255
            Minimum MTU is 1500
            Hop count is 4
          External data:
            Originating router is 12.12.12.X
            AS number of route is 2
            External protocol is BGP, external metric is 10
            Administrator tag is 0 (0x00000000)

    and i beleive that was Aj`s point , which one originator should it be ???

    HTH

  • i would also agree that it doesnt matter from D `s perspective which router originated it ,

    D only cares that it has the route , not where it came from as it only has a single link.

     but it could be an interesting Lab question ..

    "make sure D see all external routes coming from 12.12.12.1 only config this on C"

    hth

  • I am aware that Router D doesn't care what the external RID is. I'm just trying to understand the logic that C uses to determine which route it advertises to D. From the post above, it appears that C only compares the IP address of the originator. I am going to lab this up myself and see if my results confirm this.

  • Hi Qqabdal,

    Where you found originating router as 12.12.12.1? This is the output from Router D:

    Hi Narayan,

    You can find it using sh ip eigrp topo 10.1.1.0 255.255.255.0. It will show you the RID of the originator.

    HTH

  • I am aware that Router D doesn't care what the external RID is. I'm just trying to understand the logic that C uses to determine which route it advertises to D. From the post above, it appears that C only compares the IP address of the originator. I am going to lab this up myself and see if my results confirm this.


    Hi ajCCIE2b,

     

    Here is what you can do to test it out and what I did on a quick lab:

     

    [10.1.1.0/24]                    BGP Domain

     

     |            |

     

     A           B        +++++++++++++++++++++++++++

     

             /                        EIGRP DOMAIN

     

           /

     

           C _ _ _ D

     

     

    First test:

    A<->C: Use the network 10.10.10.0/30

    B<->C: Use the network 10.10.10.4/30

     

    You will see that the RID of the originator on router D is going to be router A's RID - 12.12.12.1

     

    Second test:

    A<->C: Use the network 10.10.10.8/30

    B<->C: Use the network 10.10.10.4/30

     

    You will see that the RID of the originator on router D is going to be router B's RID - 12.12.12.2

     

    So bottom line is that router C will advertise the route of its highest IP Address for the next-hop.

     

    HTH

    Let us know the test results

    Good luck!

  • So bottom line is that router C will advertise the route of its highest IP Address for the next-hop.

     

    I think you meant to saw "lowest." I just tested it out similarily to what you proposed and received the same results. When C looks at the next hop IP address of A and sees that it is higher than than of B, router C will begin advertising router B's route to router D! Lowest sender IP wins (sort of like STP Bridge ID when these is a cost tie). Cool, thanks for everyone's input.

  • Hi,


    Correct; I meant lowest! Sorry for that and thanks for confirming.

    Good luck!

  • You can find it using sh ip eigrp topo 10.1.1.0 255.255.255.0. It will show you the RID of the originato

    In case of EIGRP external route, yes we can see originator but I thought it is EIGRP internal routes, thanks qqabdal.

  • No problem Narayan, I realized that after posting. Yes, you should have a D EX route to see that. Glad you sorted it out.

    Take care!

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