CCIE R&S v5 ATC - EIGRP Traffic Engineering with Summarization - Q&A section

Hi community,

At the end of the EIGRP Traffic Engineering with Summarization ATC, we had a good question in the Q&A dedicated section. Specifically this was : "Can we increase the metric of the summary route to null 0 that is installed in the routing table ?"

CASE I - EIGRP MULTI-AF MODE

EIGRP summary-address configured under interface configuration mode used to support setting the AD directly to the summary prefix, to support floating summaries.

Let's take a look at the command line and see what's the behavior in 15.x code. R6 is defining a summary-address for its loopback as a /31 : 

 

R6(config-subif)#ip summary-address eigrp 1 150.1.6.6/32 ?

leak-map  Allow dynamic prefixes based on the leak-map

  <cr>

R6(config-subif)#ip summary-address eigrp 1 150.1.6.6/31

 

On R4, nothing changes, the command has no effect.

R4#sh ip rou 150.1.6.6

Routing entry for 150.1.6.6/32

  Known via "eigrp 1", distance 90, metric 1024640, type internal

  Redistributing via eigrp 1

  Last update from 155.1.146.6 on Ethernet0/1.146, 00:02:22 ago

  Routing Descriptor Blocks:

  * 155.1.146.6, from 155.1.146.6, 00:02:22 ago, via Ethernet0/1.146

      Route metric is 1024640, traffic share count is 1

      Total delay is 1002 microseconds, minimum bandwidth is 10000 Kbit

      Reliability 255/255, minimum MTU 1500 bytes

      Loading 1/255, Hops 1

 

Even though the <AD> option is not present in the interface summary-address command configuration mode, I see that it's actually working to set a value, but it still doesn't do anything : 

R6(config-subif)# ip summary-address eigrp 1 150.1.6.6/31 100

%EIGRP: summary accepted, admin distance value ignored, use summary-metric command for distance

The action is logged, but nothing really happens.

 

Configuration should be placed under af-interface mode :

 

R6(config-subif)#router eigrp EIGRP

R6(config-router)#ad ipv4 as 1

R6(config-router-af)#af-interface e0/1.146

R6(config-router-af-interface)#summary-address 150.1.6.6/31 ?

  leak-map  Allow dynamic prefixes based on the leak-map

  <cr>

 

No option here for <AD> either, but actually you can set it here. The router will do all the work for you behind the scenes and translate the <AD> configuration where it's supposed to be set :

 

R6(config-router-af-interface)#summary-address 150.1.6.6/31 100      

%EIGRP: summary-address accepted but distance option deprecated; use summary-metric command for distance.

R6(config-router-af-interface)#

%DUAL-5-NBRCHANGE: EIGRP-IPv4 1: Neighbor 155.1.146.4 (Ethernet0/1.146) is resync: summary configured

%DUAL-5-NBRCHANGE: EIGRP-IPv4 1: Neighbor 155.1.146.1 (Ethernet0/1.146) is resync: summary configured 

 

Now we have a neighbor resync and actually the summary is created : 

R4#sh ip rou 150.1.6.6 255.255.255.254

Routing entry for 150.1.6.6/31

  Known via "eigrp 1", distance 90, metric 1024640, type internal

  Redistributing via eigrp 1

  Last update from 155.1.146.6 on Ethernet0/1.146, 00:00:44 ago

  Routing Descriptor Blocks:

  * 155.1.146.6, from 155.1.146.6, 00:00:44 ago, via Ethernet0/1.146

      Route metric is 1024640, traffic share count is 1

      Total delay is 1002 microseconds, minimum bandwidth is 10000 Kbit

      Reliability 255/255, minimum MTU 1500 bytes

      Loading 1/255, Hops 1

 

Here's how the final configuration looks like on R6 : 

router eigrp EIGRP

 !

 address-family ipv4 unicast autonomous-system 1

  !

  af-interface Ethernet0/1.146

   summary-address 150.1.6.6 255.255.255.254

  exit-af-interface

  !

  topology base

   summary-metric 150.1.6.6/31 distance 100

  exit-af-topology

  network 150.1.0.0

  network 155.1.0.0

 exit-address-family

 

And let's check the result of this configuration : 

R6#sh ip rou 150.1.6.6 255.255.255.254

Routing entry for 150.1.6.6/31

  Known via "eigrp 1", distance 100, metric 1280, type internal

  Redistributing via eigrp 1

  Routing Descriptor Blocks:

  * directly connected, via Null0

      Route metric is 1280, traffic share count is 1

      Total delay is 2 microseconds, minimum bandwidth is 8000000 Kbit

      Reliability 255/255, minimum MTU 1514 bytes

      Loading 1/255, Hops 0

 

CASE II - CLASSIC MODE

Let's see what happens when we run EIGRP in classic mode. Again, R6 defines a summary-address for its loopback, the configuration being applied under interface configuration mode, as there is no other option in classic mode :

 

R6(config-subif)#ip summary-address eigrp 1 150.1.6.6/31 ?

  leak-map  Allow dynamic prefixes based on the leak-map

  <cr>

 

No option for <AD> here ... let's just type it and see what happens : 

 

R6(config-subif)#ip summary-address eigrp 1 150.1.6.6/31 100

%EIGRP: summary accepted, admin distance value ignored, use summary-metric command for distance

R6(config-subif)#

%DUAL-5-NBRCHANGE: EIGRP-IPv4 1: Neighbor 155.1.146.1 (Ethernet0/1.146) is resync: summary configured

%DUAL-5-NBRCHANGE: EIGRP-IPv4 1: Neighbor 155.1.146.4 (Ethernet0/1.146) is resync: summary configured

R6(config-subif)#end 

 

Well, it seems that command was accepted, although a log message has been generated. Any results ? It looks like the command accomplished what we thought it would : 

R6#sh ip rou 150.1.6.6 255.255.255.254

Routing entry for 150.1.6.6/31

  Known via "eigrp 1", distance 100, metric 128256, type internal

  Redistributing via eigrp 1

  Routing Descriptor Blocks:

  * directly connected, via Null0

      Route metric is 128256, traffic share count is 1

      Total delay is 5000 microseconds, minimum bandwidth is 8000000 Kbit

      Reliability 255/255, minimum MTU 1514 bytes

      Loading 1/255, Hops 0

 

Here's the final config for R6 in EIGRP classic mode. Summary-metric command has been automatically created, under EIGRP process level, in this case also : 

 

router eigrp 1

 summary-metric 150.1.6.6/31 distance 100

 network 150.1.0.0

 network 155.1.0.0

 

Thanks for reading.

Comments

  • Hi,

        With IOS codes supporting both EIGPR named and classic mode, you can actually configure the metric for the summary using the "summary-metric" command. Question is why would you want to do it? ANother question is what's the default summary metric if you don't specify it?

    Regards,

    Cristian.

  • Hi Cristian,

    The default AD of the summary routes is 5. The reason you would want to change the AD would be to create floating summary-addresses, as backup, for existing routes in the RIB.

    PS: I know you already know the answer to "your own questions", thanks for stopping by !! [:D]

  • Metric does NOT equal administrative distance, see my question again.

  • Hi,

    Good question. The default metric, if not statically set, would be extracted and copied from the prefix with the best metric, encompassed in the summary. Therefore, if the best prefix from which the metric is copied is withdrawn at one moment, then the metric of the summary would have to be recalculated and the summary to be readvertised with the new metric. This causes network instability. If the best metric (prefix) is not the one that flapped, EIGRP woould still have to do all the calculations, adding overhead to the process. 

    All this problems can be avoided by manually setting the summary-address metric.

  • And the best metric is the lowest. Can you change that to be the highest? Does it make sense and if so what would be the benefit?

  • Hi Cristian,

    At one point I will be running out of answers and that's the moment I'll be asking for your expertize. [:D] This is the moment I'm talking about. [:D]

    This is fun talk !

    Ok now, so the metric can be set to the highest possible value 4294967295 and I think this can be used for traffic engineering when we have multiple points of summarization.

    How's that ?

  • Note that in 15.5(2)XB that I'm using when you enter the command:

    summary-address ::/0 111

    Under interface mode you get the error and the parser converts the config for you to include the summary-metric statement:

    router eigrp LAB
    !
    address-family ipv6 unicast autonomous-system 100
    !
    af-interface Tunnel0
    summary-address ::/0
    no split-horizon
    exit-af-interface
    !
    af-interface GigabitEthernet0/0.100
    shutdown
    exit-af-interface
    !
    topology base
    distribute-list prefix-list R1 in
    summary-metric ::/0 distance 111

    But removing it with:

    no summary-address ::/0 111

    Does not remove the new statement from the config or notify you of the acceptance or otherwise of the change but resets the neighbours:

    router eigrp LAB
    !
    address-family ipv6 unicast autonomous-system 100
    !
    af-interface Tunnel0
    no split-horizon
    exit-af-interface
    !
    af-interface GigabitEthernet0/0.100
    shutdown
    exit-af-interface
    !
    topology base
    distribute-list prefix-list R1 in
    ** summary-metric ::/0 distance 111**

    If you then attempt to modify the AD via the original method you get the same error as before:

    R5(config-router-af-interface)#**summary-address ::/0 113 **
    %EIGRP: summary accepted, admin distance value ignored, use summary-metric command for distance

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