CCIE Pursuit Blog

January 1, 2008

Internetwork Expert Volume III: Lab 3 – Section 5

5 Exterior Gateway Routing

5.1 BGP Peerings

This task started out looking very straight-forward, then took an ugly turn.  😦

“All devices running BGP should have reachability to all BGP learned prefixes.”
“Don’t use tunnelling to accomplish this.”

This seemed really simple.  I set up my peerings and all of the BGP learned routes were valid best routes.

I had problems with the routes learned from the backbone routers:

r3#p 113.0.0.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 113.0.0.1, timeout is 2 seconds:
U.U.U
Success rate is 0 percent (0/5)
r3#trace 113.0.0.1

Type escape sequence to abort.
Tracing the route to 113.0.0.1

  1 190.1.135.532 msec 32 msec 28 msec
  2 190.1.135.5 !H  *  !H

I didn’t read enough into this question as “all devices running BGP” means ALL devices not just my devices.  Since the backbones are running BGP, I need to ensure that they have full reachability as well.

I threw in the towel here.  I reallyneed to review BGP.  Luckily, the answer guide has a breakdown for this task (the only breakdown in the guide).  It had a lot of good information, but I am still lacking in my BGP skills.

5.2 BGP Summarization

You are asked to summarize 112.0.0.0/8 – 119.0.0.0/8 into a single advertisement without overlapping any other networks.

This is a case where you can quickly whip this up by utilizing the Windows Calculator in scientific mode. [You are allowed to use the Windows calculator in the CCIE lab.]

1) Open the Microsoft calculator (you can search for it in Programs or just type ‘calc’ in Start->Run).
2) Click ‘View’.  Make sure that that calculator is in ‘scientific’ mode.
3) Dec (decimal) should be selected.  Key in 112.
4) Now select Bin (binary).  You should get 1110000
5) Switch back to Dec and do the same for 119.  You should get 1110111

You want to summarize at the point where the bits no longer match

112  – 01110|000
119  – 01110|111

So your summarization will be 112.0.0.0/5 or 112.0.0.0 with a mask of 248.0.0.0.  Now it’s just a simple matter of using “aggregate-address” to advertise the summary:

aggregate-address  

router bgp 100
 aggregate-address 112.0.0.0 248.0.0.0

The IE answer key introduced me to a cool new verification command:

show ip bgp cidr-only

r6#sh ip bgp cidr-only
BGP table version is 30, local router ID is 150.11.6.6
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
              r RIB-failure, S Stale
Origin codes: i – IGP, e – EGP, ? – incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 28.119.16.0/24   54.11.1.254                            0 54 i
*> 28.119.17.0/24   54.11.1.254                            0 54 i
*> 112.0.0.0/5      0.0.0.0                            32768 i
s> 190.11.0.0/24    0.0.0.0                  0         32768 i

2 Comments »

  1. I wanted to discuss with you some issues regarding this section.
    First I have a synchronization problem on R1 although I have the IGP route learned.
    Normally i disable synchronization by default but I wanted to learn more about it this time.

    I have posted about it in the forum.
    http://forum.internetworkexpert.com/ubbthreads.php?ubb=showflat&Number=23588#Post23588

    As per below on R1 BGP shows this as not synchronized even when there is an OSPF IGP route to same destination.
    R2 however does not have this issue
    For me this is really strange and I might need to read more about BGP synchronization

    Rack1R1#sh ip bgp 28.119.16.0/24
    BGP routing table entry for 28.119.16.0/24, version 0
    Paths: (1 available, no best path)
    Not advertised to any peer
    54
    54.1.1.254 (metric 20) from 150.1.6.6 (150.1.6.6)
    Origin IGP, metric 0, localpref 100, valid, internal, not synchronized
    Rack1R1#sh ip route 28.119.16.0
    Routing entry for 28.119.16.0/24
    Known via “ospf 1”, distance 110, metric 20
    Tag 54, type extern 2, forward metric 64
    Last update from 190.1.135.5 on Serial1/0, 00:01:50 ago
    Routing Descriptor Blocks:
    * 190.1.135.5, from 150.1.5.5, 00:01:50 ago, via Serial1/0
    Route metric is 20, traffic share count is 1

    Rack1R2#sh ip bgp 28.119.16.0/24
    BGP routing table entry for 28.119.16.0/24, version 12
    Paths: (1 available, best #1, table Default-IP-Routing-Table, RIB-failure(17))
    Not advertised to any peer
    54
    54.1.1.254 (metric 2172416) from 150.1.6.6 (150.1.6.6)
    Origin IGP, metric 0, localpref 100, valid, internal, synchronized, best

    Comment by Aly — April 24, 2008 @ 3:03 am | Reply

  2. Another Question irritating me

    I understand why we advertised these networks from R6->BB1 so BB3 can learn router to 190.1.0.0/16
    But why didn’t we have to configure the same from R3->BB2 ???

    Rack1R6#sh run | b router bgp
    router bgp 100
    synchronization
    network 190.1.0.0 mask 255.255.255.0
    aggregate-address 190.1.0.0 255.255.0.0 summary-only

    Comment by Aly — April 24, 2008 @ 3:08 am | Reply


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