Monthly Archives: September 2012

Hello, is there anybody in there…..(or EIGRP basics)


Hello, is there anybody in there?  Just nod if you can hear my EIGRP hello packets being multicast to….


Apologies to any Pink Floyd fans out there, but the opening line to Comfortably Numb describes perfectly what happens when an interface starts participating in an EIGRP routing process (well, not really).

As we know from CCNA study, the config to set up basic EIGRP is very simple.  We just define the AS (Autonomous System) number, and which networks to advertise.  We’ll use the following network as an example to illustrate the process:




Let’s get EIGRP configured on R1 and get it advertising it’s directly attached networks.  We’ll use all zero’s wildcard masks for the loopback interfaces, to make them as specific as possible, and turn off auto summarization (EIGRP is a classless routing protocol, but it will still summarize networks using classful boundaries by default):

R1(config)#router eigrp 1000
R1(config-router)#no auto-summary


Next, lets check that the network statements we’ve used above have enabled EIGRP on the correct interfaces:


R1#show ip eigrp interfaces
IP-EIGRP interfaces for process 1000
                Xmit Queue  Mean Pacing Time Multicast  Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Se0/0       0       0/0        0    10/10         0     0
Lo1         0       0/0        0    0/10          0     0
Lo11        0       0/0        0    0/10          0     0


Remember that enabling EIGRP on an interface has two effects:

  • EIGRP Hello packets will start being sent out of the interface to multicast  address  This means that neighbour relationships (adjacencies) can be formed with other routers directly connected to the same subnet and using the same EIGRP AS number
  • the network that the interface is part of will be advertised by the EIGRP routing process


To confirm that Hello packets are being sent, we’ll enable debugging of EIGRP packets:

R1#debug eigrp packets
EIGRP Packets debugging is on
*Mar 1 00:28:51.867: EIGRP: Sending HELLO on Serial0/0
*Mar 1 00:28:51.867: AS 1000, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
*Mar 1 00:28:52.931: EIGRP: Sending HELLO on Loopback1
*Mar 1 00:28:52.931: AS 1000, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
*Mar 1 00:28:52.935: EIGRP: Received HELLO on Loopback1 nbr
*Mar 1 00:28:52.935: AS 1000, Flags 0x0, Seq 0/0 idbQ 0/0
*Mar 1 00:28:52.935: EIGRP: Packet from ourselves ignored
*Mar 1 00:28:56.031: EIGRP: Sending HELLO on Loopback11
*Mar 1 00:28:56.031: AS 1000, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
*Mar 1 00:28:56.035: EIGRP: Received HELLO on Loopback11 nbr
*Mar 1 00:28:56.035: AS 1000, Flags 0x0, Seq 0/0 idbQ 0/0
*Mar 1 00:28:56.035: EIGRP: Packet from ourselves ignored


This output shows that Hello packets are being sent out of the S0/0, Lo1 and Lo11 interfaces as we’d expect.  Note that the router ignores hello packets that it receives on in it’s loopback interfaces as it realises that these have been sent from itself.


We’ll also use the show ip protocols command to confirm we are routing for the correct networks:


R1#show ip protocols
Routing Protocol is "eigrp 1000"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
  EIGRP maximum hopcount 100
  EIGRP maximum metric variance 1
  Redistributing: eigrp 1000
  EIGRP NSF-aware route hold timer is 240s
  Automatic network summarization is not in effect
  Maximum path: 4
  Routing for Networks:
  Routing Information Sources:
    Gateway         Distance      Last Update
    (this router)         90      00:15:51
  Distance: internal 90 external 170



So at this point R1’s config is done, but it hasn’t yet formed an adjacency with R2.   In order for two routers to form an adjacency, the following conditions must be met:


  • the two routers must have the same EIGRP AS number configured
  • the interfaces through which the two routers are exchanging EIGRP packets must be connected to the same IP subnet
  • K values (used in EIGRP metric calculation) must match on both routers


Next we’ll configure EIGRP on R2:

R2(config)#router eigrp 1000
R2(config-router)#no auto-summary


As soon as we enter the network statement for the network on R2 we see the R1 <–> R2 adjacency come up:


R2(config-router)# *Mar  1 00:20:34.679: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1000:
Neighbor (Serial0/0) is up: new adjacency
*Mar  1 00:20:24.475: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1000: Neighbor
(Serial0/0) is up: new adjacency


This is because R1 is listening for EIGRP hello packets on interface S0/0 and has now received one from R2 that contains a matching AS number and K values – therefore the adjacency will form.  We’ll use some extra show commands on R2 to verify the config:


R2#show ip eigrp neighbors
IP-EIGRP neighbors for process 1000
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0           Se0/0            11 00:01:47   12   200   0   4

R2#show ip eigrp topology
IP-EIGRP Topology Table for AS(1000)/ID(

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status 

P, 1 successors, FD is 2297856
        via (2297856/128256), Serial0/0
P, 1 successors, FD is 128256
        via Connected, Loopback2
P, 1 successors, FD is 2297856
        via (2297856/128256), Serial0/0
P, 1 successors, FD is 2169856
        via Connected, Serial0/0
P, 1 successors, FD is 128256
        via Connected, Loopback22


Finally, let’s check the routing tables of each router to confirm that each one has learned about the other’s attached networks via EIGRP:


R1#show ip route eigrp is subnetted, 4 subnets
 D [90/2297856] via, 00:14:28, Serial0/0
 D [90/2297856] via, 00:14:28, Serial0/0


R2#show ip route eigrp is subnetted, 4 subnets
 D [90/2297856] via, 00:14:00, Serial0/0
 D [90/2297856] via, 00:14:00, Serial0/0


All looking good!  In the next post I’ll cover some more EIGRP theory and config for CCNP study.




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What’s The Point Of Static Routes?

With clever, dynamic routing protocols (RIP, EIGRP, OSPF et al) being able to learn routes all by themselves, and even adapt to network problems by re-routing traffic when a link goes down, why would you even bother using boring old Static routes to get your packets to their destination?

Well, in defence of Static routes here are few reasons why you might use them:

  • It’s all gone a bit runny – your super-duper dynamic routing protocols have let you down, you can’t figure it out quickly and are in danger of being fired, unless…….you whack in a quick static route and bingo – all is good again and you can figure out the issue while your heart rate drops back to normal.
  • You might sleep better at night – if you’re a control freak and can’t bear the thought of anything happening without your say so on your precious network, then Static routes can help.
  • To give your router a break – you may have an old, creaking router that can barely send a packet on it’s way, never mind run a fancy dynamic routing protocol with all it’s processing overhead.
  • Keep it simple – you simply don’t have that much routing going on in your network, so perhaps dynamic routing is overkill.
  • You need a backup – dynamic routing with all it’s built in resilience not enough for you?  Bung in a Floating Static route for good measure.  With it’s AD set to higher than that of any dynamic protocols, it’ll only kick in if your dynamically learned routes drop out of the routing table.
  • There’s nowhere else to go – if your router has no idea how to get a packet to it’s next hop, then rather than just drop the packet it will give it a fighting chance and send it on to the router’s “gateway of last resort” AKA a Default Static route.

Configuring Static Routes

A Static route to network network via a next hop IP address
Router(config)#ip route

A Static route to network network via local interface fastEthernet 0/1:
Router(config)#ip route fastEthernet 0/1

A Floating Static route to network via next hop IP address with an AD of 200:
Router(config)#ip route 200
A Default Static route via next hop IP address
Router(config)#ip route


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CCNP ROUTE Studies – IP Routing Basics


Before getting into the detail of the various Routing Protocols, lets go over some of the fundamental concepts and principles of IP Routing, starting with an explanation of Distance Vector routing protocols…..


Distance Vector Routing Protocols

  • examples: RIP, IGRP, EIGRP
  • a router using a DV protocol knows 2 things:
    • Distance to final destination
    • Vector (or direction) traffic should sent
  • used in smaller environments where routes do not change often, such as a LAN’s
  • do not scale well, so not suitable for WAN’s
  • each DV router sends some or all of it’s routing table in updates, but only to it’s neighbours
  • DV routers only know about their neighbours
  • built in features for preventing routing loops:
    • Split Horizon – a routing protocol will never advertise a  route back out of the interface that the route was learnt on
    • Poison Reverse – if a network becomes unavailable then a router will advertise that network with a metric of 16 (max is 15) , which allows other routers to quickly learn that a network is unavailable


Distance Vector Routing Protocol Comparison Table:


algorithm? Bellman-Ford Bellman-Ford DUAL
update method? broadcast multicast to multicast to
update frequency? every 30 Seconds every 30 Seconds when there’s a change
update contains? whole routing table whole routing table only route changes
max routes per update? 25 25 n/a
classless? no yes yes
equal cost load balancing? yes, by default yes, by default yes, by default
un-equal cost load balancing? no no yes, with “variance” command
max hop count? 15 15 255 (100 is default)
authentication? no clear-text & MD5 MD5

*note – EIGRP is not a true DV protocol but has some features of DV and is included here for comparison.


How does a Router decide which route to use?

When a router knows of multiple routes to get packets to a particular destination network, it has to decide which is the best route to use.

Here is the logic that the outer uses to make the forwarding decision:

  1. multiple routes  =  route with longest Prefix wins
  2. multiple routes + same prefix length  =  route with lowest Administrative Distance wins
  3. multiple routes + same prefix length + same AD  =  route lowest Metric wins
  4. multiple routes + same prefix length + same AD  + same Metric  =  router will Load Balance


Administrative Distances


Type Of Route Administrative Distance
Directly Connected 0
Static (with exit interface) 0
Static (with next hop IP) 1
EIGRP Summary 5
External BGP 20
EIGRP (Internal) 90
OSPF 110
RIP 120
EIGRP (External) 170
Internal BGP 200
Unknown Network 255


So that’s a  bunch of the basics covered off, next post up is Static Routing…….



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CCNP ROUTE Studies – Back to the Books!

So…..I passed my SWITCH (642-813: Implementing Cisco IP Switched Networks) exam back in March of this year, and after a lull of 5 months I’ve decided to get back on track with my certification goals.

One of my goals at the start of 2012 was to achieve CCNP certification in Routing & Switching by the end of the year.  In order to gain CCNP status a candidate is required to pass three exams, so with September now upon us my chances of achieving this goal are looking distinctly slim!!  However, I’m going to make it my goal to pass the ROUTE exam by the end of this year, leaving just TSHOOT left to take in 2013.

There are a number of factors I can put this down to, such as being extremely busy at work, trying to learn JunOS for a new project etc, but at the end of the day these are just excuses.

So, what’s given me this new found enthusiasm and motivation to get back on track?  Well, in a nutshell it’s the always excellent @packetpushers podcasts and in particular show 114, which featured CBT nuggets video instructor Jeremy Cioara.  This was a great show and listening to it you can’t help but get enthusiastic (in a geeky kind of way) about Cisco certifications.

Anyway, that’s enough waffle.  I’ll be posting again soon with my first lot of study notes for the ROUTE exam.



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