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BSCI – PASS

March 13, 2010 8 comments

I’ve  scheduled my exam for Monday but last night around 11pm I told my wifi that I can’t look at my study guide\notes\labs anymore. I just had enough after 6 weeks of intense studies and played with the idea of rescheduling for Saturday morning and having the rest of the weekend to relax.
I didn’t know if it is possible to reschedule in such short notice, 12 hours before taking it but few minutes later it was done and the clock started ticking.

As with my previous exams, I used the same center (and the same pre-exam coffee at Financier Patisserie). Since it is Saturday I took some extra time incase the subway will have problems and getting there 30 minutes ahead of time was great because I could start earlier and get it done.

The exam was fair, most of it as I expected but few point (remember NDA) worth mentioning:

  • IS-IS had a major presence in the exam. I did not expect it to get so much attention as Cisco removed it from the new CCNP track
  • None of my labs (can’t say how many but more than expected) had the copy run start enabled. Usually Cisco consider saving the configuration as part of the lab requirement but though the IOS showed this command (and completed it when I used the TAB button) the Enter button produced a “this command is not implemented” message
  • Another problem with the labs was the ping command. In more than one lab I had to confirm the configuration by running ping to a certain IP address but it was disabled!
  • BGP did not get as much attention as I expected from such a major topic (both in the books and videos). The same apply to redistribution.

Now I’m ONT short of my CCNP and after few days of really needed rest (and well deserved if I may say :)) I’ll start working toward the last piece of my Cisco puzzle.

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BSCI – IS-IS review

March 10, 2010 2 comments

Buried under dozens of definitions, values and parameters I try to wrap up my studies for the exam. I have to admit that it is as hard as they say and I wonder why am I doing this horrible thing to myself 🙂

IS-IS is one of the topics that I could avoid by taking the ROUTE exam but it is a fascinating protocol that take link-state to another level and open your mind to other ideas. You can think of it as trying a new browser – you know it is working like your Firefox (which 54% of you use) but it has a different feeling, some other features or just a different look. You’ll keep using your favorite fox but it is nice to know what other options are available out there.

Other than my study notes and my study guide, I found Aragoen Celtdra’s BSCI: IS-IS Configuration and even more his BSCI: IS-IS Concepts I BSCI: IS-IS Concepts II posts great reading.

Here are few important points:
IS-IS has 3 types of routers:

  • L1 – intra-area, uses LSP
  • L2 – inter-area, the backbone of IS-IS network
  • L1/L2 – connectors between L1 and L2 areas

IS-IS has 4 OSI routing levels:

  • level 0 routing – ES-IS protocol, same subnet
  • level 1 routing – ISs in the same area
  • level2 routing – L1 or L2 routers find L1/L2 router
  • level 3 routing – L1/L2 pass traffic to other AS

IS-IS must be configured both at the router level and interface configuration mode.

The comparison between IS-IS to OSPF is a major topic as we try to understand which one is better for our network.
Here are some of IS-IS benefits over OSPF:

  • supports more routers in an area
  • produces fewer link state advertisements for a given network
  • supports network layer protocols other than IP
  • forms adjacencies with all neighbors

There are some problems with IS-IS and CLNS:

  • CLNS adjacency can have IP addresses on different subnets at different end of the connection. That would make troubleshooting much harder
  • IS-IS neighbor relationships are established over CLNS and again, the fact it is not using IP make it more complicated

One way to bypass this problem is using only IP routing. In order to make it happen each IS-IS router require NET address and all network devices must use CLNS.

Read about CLNS, NSEL, NET address and DIS, they all show on the exam and Aragoen cover them well on concepts part II.

You can find another great IS-IS document here and a Cisco presentation on IS-IS. Detailed command guide from Cisco is available here.

ip helper-address

March 9, 2010 1 comment

I’m in the last stage of my studies, preparing for the BSCI exam. The big picture is clear but the amount of details…

I was playing with ip helper-address commands and want to review this simple yet useful command, that BSCI present when DHCP is in question but has other uses. I was reading the relevant parts from the Advanced Routing Companion Guide book by CiscoPress.

In short, broadcast is blocked on routers (remember the broadcast domain from your CCNA exam?) but sometimes we still need to forward this traffic. While BSCI focus on forwarding DHCP traffic across routers, DNS, TFTP or BOOTP are commonly used protocols that require the same solution. Let’s say you have a time-server at the HQ office and you want your branch office to use it or maybe a DNS server at HQ that branch offices use to resolve IPs.
ip helper-address present a solution to those scenarios.

The command syntax is simple:

ip helper-address [vrf name | global] address [redundancy vrg-name]

The important option is address = Destination broadcast or host address to be used when forwarding UDP broadcasts. There can be more than one helper address per interface.

By default the helper option is disabled.
When enabled the following UDP services will be available: 37, 49, 53, 67-69, 137, 138.
The default services can be locked using the global no ip forward-protocol command.

The ip forward-protocol command (syntax), used in global mode to specify which protocols and ports the router forwards when forwarding broadcast packets.

ip forward-protocol {udp [port] | nd | sdns}

SPF algorithm

March 6, 2010 Leave a comment

As I mentioned on my EIGRP Metric Lab, this post is dedicated to SPF algorithm and the cool  Dijkstra’s algorithm. I do not want to get too techie in this post, just to get the general concept. I’ll point you at the end of the post to some good in-depth reads.

Dijkstra's algorithm

OSPF and IS-IS are link-state protocols, they use Shortest Path First (SPF) to calculate distance between the routers and create the routing table.

For the SPF algorithm to work, it requires all routers in the OSPF\IS-IS network to know about links and all the other routers in the same network.

OSPF encode its link-state information in Link State Advertisements (LSAs) and floods it. IS-IS encode its information in a Link State Packet (LSP).

When the initial data collection process is completed, OSPF \ IS-IS process runs the Dijkstra Shortest Path First algorithm to find the shortest path from itself to all the other routers in the network. The same process happen on each router in the network. When the algorithm processing is completed, all the routers have a similar table and consistent routing can start.

How does it work?
Dijkstra algorithm put the router as the root of a tree and calculate the shortest path to each destination. While the overall picture on all routers is similar (they all have the same routers and links), each router look differently at the result as the point of view is personal. It is just like in life – you share a room with 3 other people, each one stand in a different corner. When you are asked to describe an object you describe the exact same object but it does look a bit different from different angels.

When any change is noticed (link state change), SPF start the calculation all over and re-build the map. OSPF ability to use many areas is a way to reduce these frequent updates as it has less routers per area. This is a major consideration when using a link-state protocol.

Two recommended reads that actually describe the shortest path calculation step by step are:
1. Example & descriptive explanation to how does SPF algorithm work in OSPF and IS-IS.
2. RFC 2328 is also a good read to get better understanding on the OSPF protocol.

EIGRP Metric Lab

March 5, 2010 Leave a comment

I spent the last week reading my notes, answering questions and digging in topics that I didn’t feel complete about. I start seeing the big picture and get closer to the exam. This post will cover EIGRP metric and the next one will cover the SPF algorithm, two subjects I found both interesting and major exam topics.

EIGRP uses a relatively complicated metric system as it has five parameters:
Bandwidth, Load, Delay, Reliability and MTU.

By default only Bandwidth and Load are being used (and we’ll see why later) but in more complex scenarios the composite metric is calculated using the K values of each variable.
Bandwidth – calculated as 10^7/BW (using Kbit), the lowest along the path is chosen
Delay – cumulative along the path, the sum of all the delays in tens of microseconds (ms).
This is tricky and important for exam configurations – for 1000 ms we type 100.

The formula to calculate the metric is

Metric=[K1*bandwidth+(K2*bandwidth)/(256-load)+K3* delay]* [K5 /(reliability+K4)]

Values K1 through K5 can be changed with the metric weights command.
The default values are:
K1 & K3 = 1
K2, K4 & K5 = 0
The zero value of K2, K4 & K5 explain why we only use Bandwidth and Delay by default as the formula zero shorten the formula:

Metric = [256 * (10^7/BW) + 256 * (delay)]

In order to see the metric output I’ve built a small lab with 3 full-mesh routers and some Loopback interfaces on one of them.
One way to see the metric value is the command show interfaces s0/0:

Serial0/0 is up, line protocol is up
Hardware is M4T
Internet address is 10.2.2.2/30
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255

Another way is the show ip eigrp topology 10.0.0.0/8 command:

IP-EIGRP (AS 100): Topology entry for 10.0.0.0/8
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2169856
Routing Descriptor Blocks:
0.0.0.0 (Null0), from 0.0.0.0, Send flag is 0x0
Composite metric is (2169856/0), Route is Internal
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 20000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 0
172.16.20.2 (Serial0/1), from 172.16.20.2, Send flag is 0x0
Composite metric is (2681856/2169856), Route is Internal
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 40000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1

I broke the lab with the shutdown command on one of the Interfaces.
Using the command show ip eigrp topology I was able to catch the Active state

P 10.0.0.0/8, 1 successors, FD is 2169856
via Summary (2169856/0), Null0
A 10.0.1.0/30, 0 successors, FD is 2681856, Q
1 replies, active 00:00:00, query-origin: Local origin
Remaining replies:
via 172.16.20.2, r, Serial0/1

Another lab breaking step was changing the metric values on one side only. Using the command metric weights 0 3 2 4 1 2 resulted in:

*Mar  1 00:35:16.935: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.2.2.1 (Ser
ial0/0) is down: K-value mismatch
*Mar  1 00:35:21.399: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.2.2.1 (Ser
ial0/0) is down: Interface Goodbye received

Normal show ip eigrp topology show another mysterious buzz word – serno

P 172.22.71.208/29, 2 successors, FD is 46163456
via 172.30.1.42 (46163456/45651456), Serial0.2, serno 7539273
via 172.30.2.49 (46163456/45651456), Serial2.6, serno 7539266

This is Cisco’s explanation: serno stand for serial number

Some other definitions that this post will not be complete without:
Feasible Distance (FD) – the best metric along a path to a destination network, including the metric to the neighbor advertising that path
Reported Distance (RD) – the total metric along a path to a destination network as advertised by an upstream neighbor
Feasible Successor – a path whose RD <  FD (current best path)

Finally, I want to recommend reading a great post on EIGRP with non-default K values. It is well written and have good examples.

Find more labs here

OSPFv2 versus OSPFv3

March 2, 2010 Leave a comment

Jeremy Stretch of PacketLife.net wrote an amazing article about OSPFv2 vs OSPFv3
(available as pdf).
This is a must read (for life and the exam) and I won’t waste any more of your time,
go to his post and have fun!

OSPF default route

March 1, 2010 Leave a comment

OSPF default-information originated is an important command. It allows the administrator to inject a default route into OSPF routing domain.

There are 4 optional keywords (to be added after the command):

  • always – advertises the default route regardless of whether the software has one.
    This default route will be kept under any condition no matter which changes happen.
  • metric – the default metric value is 10 and this keyword can change it.
  • metric-type – this keyword has two optional values
    E1 (metric-type 1), where metric will be changed across the network
    E2 (metric-type 2), where the metric is the same throughout the OSPF domain (this is the default)
  • route-map – if the routing map conditions are matched the setting will take effect.
    This is the only way to override the always keyword.

It is working great in my lab. If you study for either BSCI or ROUTE exams I highly recommend playing with those options and get yourself familiarized with both syntax and capabilities. Make sure you understand how to recognize the different show outputs and the commands that generated them.