From: Pavel Bykov (slidersv@gmail.com)
Date: Wed Dec 10 2008 - 12:57:06 ARST
1) BB1 originates the prefixes using network command. You don't say what is
the source IP address of ping that R5 is using to ping BB1. Either look for
the interface that R5 is using to send pings, or "debug ip icmp" on BB1 and
BB3 and see what source address of the request is there.
Then do "show ip route" for that address, and see where the information is
coming from.
2) BB1 originates prefixes, as I already mentioned, so BB3 has iBGP routes
to those prefixes. R6 changes the NEXT_HOP to SELF, since it's EBGP, maybe
that's why BB1 can reach R5 and BB3 cannot. In either case, 12.0.0.1 has to
reach BB1, because that is where the IP address is located. Again, look for
lost replies - do "show ip bgp x.x.x.x" and show ip route x.x.x.x for source
address of pings
Other questions that you asked:
i.) Not L2 but L3. L2 is Spanning Tree. Also, CONNECTED and STATIC are also
considered routing protocols, so you should not forget them as well.
ii.) Again, look for source address of ping packets, and see what are
BB1/BB3 are using.
3). Most likely - yes. That is why you have to watch for NEXT_HOP_SELF and
Route Reflectors.
You should look at technological papers from either CCIE vendors or other
training providers. BGP is a complex area that needs special attention.
On Wed, Dec 10, 2008 at 11:44 AM, Nitro Drops <nitrodrops@hotmail.com>wrote:
> Hi All,
>
> Need some technical expertise to clear some doubts and concepts. Have been
> reading tonnes of notes, maybe i am slow 8(
>
> Pardon me for this long scenario, and thank you for your patience.
>
> R5 > R4 > R6 > BB1 (AS54)
> R5 > R3 > BB3 (AS54)
>
> R3, R4, R5, R6 all in AS100.
>
> R5 has IGP reachability to BB1 (54.1.1.254) and BB3 (204.12.1.254)
>
> Rack1R5#sh ip route 204.12.1.254
> Routing entry for 204.12.1.0/24
> Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric
> 64
> Redistributing via eigrp 100
> Advertised by eigrp 100 metric 1 1 1 1 1
> Last update from 183.1.0.3 on Serial1/0, 00:09:02 ago
> Routing Descriptor Blocks:
> * 183.1.0.3, from 150.1.3.3, 00:09:02 ago, via Serial1/0
> Route metric is 20, traffic share count is 1
>
> Rack1R5#ping 204.12.1.254
> !!!!!
>
> Rack1R5#sh ip route 54.1.1.254
> Routing entry for 54.1.1.0/24
> Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric
> 110
> Redistributing via eigrp 100
> Advertised by eigrp 100 metric 1 1 1 1 1
> Last update from 183.1.45.4 on Ethernet0/1, 00:15:33 ago
> Routing Descriptor Blocks:
> * 183.1.45.4, from 150.1.6.6, 00:15:33 ago, via Ethernet0/1
> Route metric is 20, traffic share count is 1
>
> Rack1R5#ping 54.1.1.254
> !!!!!
>
> Rack1R5#sh ip bgp
> BGP table version is 346, local router ID is 150.1.5.5
> 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
> *>i28.119.16.0/24 204.12.1.254 0 100 0 54 i
> * i 54.1.1.254 0 100 0 54 i
> *>i28.119.17.0/24 204.12.1.254 0 100 0 54 i
> * i 54.1.1.254 0 100 0 54 i
> *>i112.0.0.0 204.12.1.254 0 100 0 54 50 60 i
> * i 54.1.1.254 0 100 0 54 50 60 i
> *>i113.0.0.0 204.12.1.254 0 100 0 54 50 60 i
> * i 54.1.1.254 0 100 0 54 50 60 i
> *>i114.0.0.0 204.12.1.254 0 100 0 54 i
> * i 54.1.1.254 0 100 0 54 i
> *>i115.0.0.0 204.12.1.254 0 100 0 54 i
> * i 54.1.1.254 0 100 0 54 i
> *>i116.0.0.0 204.12.1.254 0 100 0 54 i
> * i 54.1.1.254 0 100 0 54 i
> *>i117.0.0.0 204.12.1.254 0 100 0 54 i
> * i 54.1.1.254 0 100 0 54 i
> *>i118.0.0.0 204.12.1.254 0 100 0 54 i
> Network Next Hop Metric LocPrf Weight Path
> * i 54.1.1.254 0 100 0 54 i
> *>i119.0.0.0 204.12.1.254 0 100 0 54 i
> * i 54.1.1.254 0 100 0 54 i
> * i150.1.11.0/24 183.1.123.1 0 100 0 200 i
> *> 183.1.105.10 0 200 i
> * i205.90.31.0 183.1.123.1 0 100 0 200 254 ?
> *> 183.1.105.10 0 200 254 ?
> * i220.20.3.0 183.1.123.1 0 100 0 200 254 ?
> *> 183.1.105.10 0 200 254 ?
> * i222.22.2.0 183.1.123.1 0 100 0 200 254 ?
> *> 183.1.105.10 0 200 254 ?
> Rack1R5#
>
> When connection between R3 & BB3 is down. BGP routes from Backbone are
> learnt
> from BB1
>
> Rack1R5#sh ip bgp
> BGP table version is 356, local router ID is 150.1.5.5
> 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
> *>i28.119.16.0/24 54.1.1.254 0 100 0 54 i
> *>i28.119.17.0/24 54.1.1.254 0 100 0 54 i
> *>i112.0.0.0 54.1.1.254 0 100 0 54 50 60 i
> *>i113.0.0.0 54.1.1.254 0 100 0 54 50 60 i
> *>i114.0.0.0 54.1.1.254 0 100 0 54 i
> *>i115.0.0.0 54.1.1.254 0 100 0 54 i
> *>i116.0.0.0 54.1.1.254 0 100 0 54 i
> *>i117.0.0.0 54.1.1.254 0 100 0 54 i
> *>i118.0.0.0 54.1.1.254 0 100 0 54 i
> *>i119.0.0.0 54.1.1.254 0 100 0 54 i
>
>
>
> When connection between R6 & BB1 is down. BGP routes from Backbone are
> learnt
> from BB3
>
> Rack1R5#sh ip bgp
> BGP table version is 406, local router ID is 150.1.5.5
> 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
> *>i28.119.16.0/24 204.12.1.254 0 100 0 54 i
> *>i28.119.17.0/24 204.12.1.254 0 100 0 54 i
> *>i112.0.0.0 204.12.1.254 0 100 0 54 50 60 i
> *>i113.0.0.0 204.12.1.254 0 100 0 54 50 60 i
> *>i114.0.0.0 204.12.1.254 0 100 0 54 i
> *>i115.0.0.0 204.12.1.254 0 100 0 54 i
> *>i116.0.0.0 204.12.1.254 0 100 0 54 i
> *>i117.0.0.0 204.12.1.254 0 100 0 54 i
> *>i118.0.0.0 204.12.1.254 0 100 0 54 i
> *>i119.0.0.0 204.12.1.254 0 100 0 54 i
>
> In both BB1 and BB3, there are no aggregate routes back to the internal
> network
>
> BB3(config-router)#do sh ip bgp
> BGP table version is 135, local router ID is 31.3.0.1
> 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 0.0.0.0 0 32768 i
> *> 28.119.17.0/24 0.0.0.0 0 32768 i
> *>i112.0.0.0 172.16.4.1 0 100 0 i
> *>i113.0.0.0 172.16.4.1 0 100 0 i
> *>i114.0.0.0 172.16.4.1 0 100 0 i
> *>i115.0.0.0 172.16.4.1 0 100 0 i
> *>i116.0.0.0 172.16.4.1 0 100 0 i
> *>i117.0.0.0 172.16.4.1 0 100 0 i
> *>i118.0.0.0 172.16.4.1 0 100 0 i
> *>i119.0.0.0 172.16.4.1 0 100 0 i
> *> 150.1.11.0/24 204.12.1.3 0 100 200 i
> * i 172.16.4.1 0 100 0 100 200 i
> *> 205.90.31.0 204.12.1.3 0 100 200 254 ?
> * i 172.16.4.1 0 100 0 100 200 254 ?
> *> 220.20.3.0 204.12.1.3 0 100 200 254 ?
> * i 172.16.4.1 0 100 0 100 200 254 ?
> *> 222.22.2.0 204.12.1.3 0 100 200 254 ?
> Network Next Hop Metric LocPrf Weight Path
> * i 172.16.4.1 0 100 0 100 200 254 ?
> BB3(config-router)#
>
> BB1#sh ip bgp
> BGP table version is 111, local router ID is 212.18.3.1
> 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
> *>i28.119.16.0/24 172.16.4.3 0 100 0 i
> *>i28.119.17.0/24 172.16.4.3 0 100 0 i
> *> 112.0.0.0 0.0.0.0 0 32768 i
> *> 113.0.0.0 0.0.0.0 0 32768 i
> *> 114.0.0.0 0.0.0.0 0 32768 i
> *> 115.0.0.0 0.0.0.0 0 32768 i
> *> 116.0.0.0 0.0.0.0 0 32768 i
> *> 117.0.0.0 0.0.0.0 0 32768 i
> *> 118.0.0.0 0.0.0.0 0 32768 i
> *> 119.0.0.0 0.0.0.0 0 32768 i
> *> 150.1.11.0/24 54.1.1.6 0 100 200 i
> * i 172.16.4.3 0 100 0 100 200 i
> *> 205.90.31.0 54.1.1.6 0 100 200 254 ?
> * i 172.16.4.3 0 100 0 100 200 254 ?
> *> 220.20.3.0 54.1.1.6 0 100 200 254 ?
> * i 172.16.4.3 0 100 0 100 200 254 ?
> *> 222.22.2.0 54.1.1.6 0 100 200 254 ?
> Network Next Hop Metric LocPrf Weight Path
> * i 172.16.4.3 0 100 0 100 200 254 ?
> BB1#
>
>
> Qns 1.) How come R5 is able to ping to the BGP prefixs (112.0.0.1) learned
> from BB1 even though BB1 does not have any any BGP aggregates/prefixes back
> to
> the internal network or back to R5?
>
> <Connection between R3 & BB3 is down>
>
> Rack1R5#ping 112.0.0.1
>
> Type escape sequence to abort.
> Sending 5, 100-byte ICMP Echos to 112.0.0.1, timeout is 2 seconds:
> !!!!!
> Success rate is 100 percent (5/5), round-trip min/avg/max = 36/133/292 ms
>
> Rack1R5#sh ip route 112.0.0.1
> Routing entry for 112.0.0.0/8
> Known via "bgp 100", distance 200, metric 0
> Tag 54, type internal
> Last update from 54.1.1.254 00:02:38 ago
> Routing Descriptor Blocks:
> * 54.1.1.254, from 150.1.6.6, 00:02:38 ago
> Route metric is 0, traffic share count is 1
> AS Hops 3
> Route tag 54
>
> Rack1R5#sh ip bgp 112.0.0.1
> BGP routing table entry for 112.0.0.0/8, version 423
> Paths: (1 available, best #1, table Default-IP-Routing-Table)
> Flag: 0x820
> Advertised to update-groups:
> 1 2
> 54 50 60, (Received from a RR-client)
> 54.1.1.254 (metric 20) from 150.1.6.6 (150.1.6.6)
> Origin IGP, metric 0, localpref 100, valid, internal, best
> Rack1R5#
>
>
> Qns 2.) How come R5 is NOT able to ping to the BGP prefixs (112.0.0.1)
> learned
> from
> BB3? Whereas it is able to from BB1?
>
> Rack1R5#ping 112.0.0.1
>
> Type escape sequence to abort.
> Sending 5, 100-byte ICMP Echos to 112.0.0.1, timeout is 2 seconds:
> .....
> Success rate is 0 percent (0/5)
>
> Rack1R5#sh ip bgp 112.0.0.1
> BGP routing table entry for 112.0.0.0/8, version 438
> Paths: (1 available, best #1, table Default-IP-Routing-Table)
> Flag: 0x820
> Advertised to update-groups:
> 1 2
> 54 50 60, (Received from a RR-client)
> 204.12.1.254 (metric 20) from 183.1.0.3 (150.1.3.3)
> Origin IGP, metric 0, localpref 100, valid, internal, best
> Rack1R5#sh ip bgp 112.0.0.1
> BGP routing table entry for 112.0.0.0/8, version 438
> Paths: (1 available, best #1, table Default-IP-Routing-Table)
> Flag: 0x820
> Advertised to update-groups:
> 1 2
> 54 50 60, (Received from a RR-client)
> 204.12.1.254 (metric 20) from 183.1.0.3 (150.1.3.3)
> Origin IGP, metric 0, localpref 100, valid, internal, best
> Rack1R5#
>
>
> I am not able to figure out the results of Qns1. My understanding is for
> BGP
>
> i.) there must be an underlying L2 IGP protocol for BGP to learn best
> routes
> ii.) Routing is a bi-directional process, for R5 to ping to Backbone route
> 112.0.0.1, R5 must have a BGP route to it and vice versa meaning the BB1
> and
> BB3 must have a BGP route back to R5.
>
> In the above scenario, both BB1 and BB3 (i) dont have an BGP aggregate
> routes
> back to the internal network nor (ii) have a BGP route to R5. But how come
> R5
> is able to ping Backbone route 112.0.0.1 via through BB1 and not BB3.
>
> To make R5 able to ping 112.0.0.1 via through BB3, i advertise an internal
> subnet onto R5 via BGP, and R5 is able to ping to 112.0.0.1 from the
> source
> which is advertised into BGP.
>
> Qns3.) So in the lab exam, does full reachability includes reachabillity to
> BGP prefixes learnt from BB routers?
>
> Quoted from Brian McGahan
>
> It is always an implicit
> requirement that you have to have an IGP route to the next-hop of a BGP
> learned prefix. This means that either you need an IGP route to the
> link between the EBGP neighbors or you need to modify the next-hop
> value with the next-hop-self command or manually with a route-map and
> the "set ip next-hop" command.
>
> Does it mean we just need to emphasize on IGP reachability to the next-hop
> ONLY of a BGP learned prefix?
>
> Pretty weak in BGP, just want to get all my concepts right. Have been
> trying
> this for the last 4 hours.
>
> Thank you for any kind replies.
>
> Cheers
> Nit
>
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