Saturday, July 22, 2017

EIGRP – summarization – part 3

This is 3rd part from the EIGRP summariztion thread. Please read the previous two as well. I would like to test EIGRP stub feature to see what is the impact on EIGRP behavior.



Ever network behind R4, R5 and R6 we may consider as a stub network. If we lose access to one of them, there is no need to query other routers as any of them is a transit network.

I modify the configuration on R4-R6 routers:


R4#sh run | s eigrp
router eigrp TEST1
 !
 address-family ipv4 unicast autonomous-system 100
 !
 topology base
 exit-af-topology
 network 10.2.0.0 0.0.0.255
 network 10.2.1.0 0.0.0.255
 network 10.2.2.0 0.0.0.255
 network 10.2.3.0 0.0.0.255
 network 10.2.4.0 0.0.0.255
 network 10.2.5.0 0.0.0.255
 eigrp stub connected summary
 exit-address-family
R4#
 
 
Now it’s time to shutdown a loopback5 (network 10.3.5.0/24) and check what we see on R4:
 
 
*Jul 21 18:26:47.874: EIGRP: Received UPDATE on Fa0/0 - paklen 44 nbr 10.2.0.2
*Jul 21 18:26:47.874: AS 100, Flags 0x0:(NULL), Seq 401/92 interfaceQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/0
*Jul 21 18:26:47.878: {type = 602, length = 44}
*Jul 21 18:26:47.878: {vector = {afi = 1, tid = 0}
*Jul 21 18:26:47.878: {routerid = 10.3.5.5
*Jul 2
R4#1 18:26:47.878: {offset = 0, priority = 0, reliability = 255, load = 1,
*Jul 21 18:26:47.878: mtu = {1500:[00, 05, DC]), hopcount = 2,
*Jul 21 18:26:47.878: delay = 281474976710655, bw = 100000,
*Jul 21 18:26:47.878: reserved = 00, opaque_flags = 00}
*Jul 21 18:26:47.878: {nh:00000000}
*Jul 21 18:26:47.878: {180A0305}
*Jul 21 18:26:47.878: }
 
 
We don’t have summarization in place and R4 kept 10.3.5.0/24 subnet in its routing table. Once we shutdown it, R4 received update that the network is not reachable. What I can see R4 didn’t receive any query for that missing prefix. It sent own query asking for a backup path:
 
 
*Jul 21 18:26:47.898: EIGRP: Sending QUERY on Fa0/0 - paklen 44 tid 0
*Jul 21 18:26:47.902: A
R4#S 100, Flags 0x0:(NULL), Seq 94/0 interfaceQ 0/0 iidbQ un/rely 0/0 serno 269-269
*Jul 21 18:26:47.906: {type = 602, length = 44}
*Jul 21 18:26:47.906: {vector = {afi = 1, tid = 0}
*Jul 21 18:26:47.906: {routerid = 10.3.5.5
*Jul 21 18:26:47.910: {offset = 0, priority = 0, reliability = 255, load = 1,
*Jul 21 18:26:47.910: mtu = {1500:[00, 05, DC]), hopcount = 2,
*Jul 21 18:26:47.910: delay = 281474976710655, bw = 100000,
*Jul 21 18:26:47.914: reserved = 00, opaque_flags = 04}
*Jul 21 18:26:47.914: {nh:00000000}
*Jul 21 18:26:47.922: {180A0305}
*Jul 21 18:26:47.926: }
 
 
By enabling stub feature we limited the scope of the EIGRP query (without summarization). Now I re-enable summarization, do the same test and compare the output.
 
 
 
 
 
 
I repeated the test once again:
 
R5#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R5(config)#int loop5
R5(config-if)#sh
R5(config-if)#
*Jul 21 18:49:42.930: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback5, changed state to down
*Jul 21 18:49:42.934: %LINK-5-CHANGED: Interface Loopback5, changed state to administratively down
R5(config-if)#
 
 
 
and on R4 I can’t see any EIGRP message. So R4 wasn’t queried about the missing subnet and wasn’t notified about 10.3.5.0/25 outage as R5 summarize all networks and presents in one prefix:
 
 
D 10.3.0.0/16 [90/204800] via 10.2.0.2, 00:05:48, FastEthernet0/0
 
  
What is the difference then between scenario presented in part 2 (no stub but summarization) and here(stub and summarization)? In both cases we don’t see any EIGRP messages. I think you have better control with the stub feature. In some cases you may not have summarization in place due to limited number of prefixes or from different reasons and the stub feature helps you keep all unnecessary EIGRP messages away.
 
 
 

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