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Beginner BGP question....

For a lab, I am trying to use the attached topo and ping from R1 to 200.1.1.1 (on R5). On R1 my BGP routes are best and valid, but I can not ping Lo on R5.  The traceroute dies at R3, but on R3, when I do sh ip route, it shows valid paths, what gives?  The lab is CBT Nuggest Implementing Basic BGP (yes, stuck on a lab with the word Basic in it...lol)..in the lab he never pinged, but I thought we should be able to.

Thank you,

2 ACCEPTED SOLUTIONS

Accepted Solutions

Re: Beginner BGP question....

OK, based on the info you provided, I get the impression that routers R2 and R3 have no route to the loopback networks on R5.

R1 learns them via iBGP but R2 & R3 only know what they learn via ospf. This is a classical problem when combining iBGP with an IGP in a network where not all routers are BGP speakers.

You can fairly easy troubleshoout this by checking the routing tables on R2, R3. If they have no routes to 200.1.1.0 etc, there's your problem.

Also, you may check if it works partially when adding a static route to one of the loopback networks (ip route 200.1.1.1 255.255.255.0 4.4.4.4)

It looks like there are two equal cost paths to R4 so you may get one hit, one miss when you add the static to only one router.

As long as the eBGP routes learned from R5 (output from 'R5#sh ip bgp neighbors 10.1.45.1 advertised-routes' ) are not in any way redistributed into ospf, there is no way to get this working. There are about three solutions:

1: add static routes on R2, R3.

2: advertise a default route from R4.

3: make R2, R3 iBGP speakers.

for solution 2, you could do something like this:

R4#conf t
router ospf 1
default-information originate

Good luck with the lab! Hope this info is sufficient to let you find a working solution.

As it's a lab, feel free to try them all!

That's still one of the best ways I know to get to grips with this kind of problems.

regards,

Leo

Re: Beginner BGP question....

Thanks for the offer, I have no short term plans for visiting Vegas but one never knows!

Indeed your remark to the solution is correct, routes are needed in both directions. However, you have found the root cause and solved it yourself.

Such learning experiences are not easily forgotten so you now know how to deal with this type of problem in the future. Bravo!

Answering your question: A packet will not really "choose" a routing protocol for it's transport.

This is fundamental. Forwarding occurs based on the routing table.

Basically a packet is always sent to the next hop with the best metric, regardless of whether this was learned through ospf, eigrp or bgp.

regards,

Leo

5 REPLIES

Re: Beginner BGP question....

The configs show that R1 is missing a command which R4 does have:

neighbor 1.1.1.1 update-source Loopback4

Please apply the equivalent to R1 to begin with.

You may also be missing the wildcard masks on your route map FILTER on R5.

If it doesn't work then, you may post some routing info like the output from
sh ip bgp nei brief

and

sh ip bgp

also nice to know:

sh ip bgp nei advertised routes

goodluck with your lab!

Leo

Re: Beginner BGP question....

I did havve that line, but readded it just now, and ran clear ip bgp *, same issue.

The only thing the route map on R5 is supposed to do is limit the networks being advertised.  R5 had Lo0-6, but we (for purpose of lab) only wanted to advertise Lo0-3.

Here are the requested commands from R1

R1#sh ip rou
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

B    200.1.4.0/24 [200/0] via 4.4.4.4, 00:05:46
     1.0.0.0/32 is subnetted, 1 subnets
C       1.1.1.1 is directly connected, Loopback1
     50.0.0.0/24 is subnetted, 1 subnets
B       50.1.1.0 [200/0] via 4.4.4.4, 00:05:46
     4.0.0.0/32 is subnetted, 1 subnets
O       4.4.4.4 [110/129] via 10.1.13.2, 00:12:02, Serial0/0
B    200.1.1.0/24 [200/0] via 4.4.4.4, 00:05:46
B    200.1.2.0/24 [200/0] via 4.4.4.4, 00:05:46
B    200.1.3.0/24 [200/0] via 4.4.4.4, 00:05:46
     10.0.0.0/24 is subnetted, 4 subnets
C       10.1.13.0 is directly connected, Serial0/0
C       10.1.12.0 is directly connected, Serial0/1
O       10.1.24.0 [110/128] via 10.1.12.2, 00:12:03, Serial0/1
O       10.1.34.0 [110/128] via 10.1.13.2, 00:12:03, Serial0/0
R1#ping 200.1.2.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 200.1.2.1, timeout is 2 seconds:
U.U.U
Success rate is 0 percent (0/5)
R1#sh ip bgp
BGP table version is 6, local router ID is 1.1.1.1
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
*>i50.1.1.0/24      4.4.4.4                  0    100      0 6500 i
*>i200.1.1.0        4.4.4.4                  0    100      0 6500 ?
*>i200.1.2.0        4.4.4.4                  0    100      0 6500 ?
*>i200.1.3.0        4.4.4.4                  0    100      0 6500 ?
*>i200.1.4.0        4.4.4.4                  0    100      0 6500 ?

R1#sh ip bgp sum
BGP router identifier 1.1.1.1, local AS number 5500
BGP table version is 6, main routing table version 6
5 network entries using 585 bytes of memory
5 path entries using 260 bytes of memory
3/2 BGP path/bestpath attribute entries using 372 bytes of memory
1 BGP AS-PATH entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 1241 total bytes of memory
BGP activity 10/5 prefixes, 10/5 paths, scan interval 60 secs

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
4.4.4.4         4  5500      23      19        6    0    0 00:06:12        5

R1#sh ip bgp neighbors 4.4.4.4 advertised-routes
Total number of prefixes 0

Re: Beginner BGP question....

OK, based on the info you provided, I get the impression that routers R2 and R3 have no route to the loopback networks on R5.

R1 learns them via iBGP but R2 & R3 only know what they learn via ospf. This is a classical problem when combining iBGP with an IGP in a network where not all routers are BGP speakers.

You can fairly easy troubleshoout this by checking the routing tables on R2, R3. If they have no routes to 200.1.1.0 etc, there's your problem.

Also, you may check if it works partially when adding a static route to one of the loopback networks (ip route 200.1.1.1 255.255.255.0 4.4.4.4)

It looks like there are two equal cost paths to R4 so you may get one hit, one miss when you add the static to only one router.

As long as the eBGP routes learned from R5 (output from 'R5#sh ip bgp neighbors 10.1.45.1 advertised-routes' ) are not in any way redistributed into ospf, there is no way to get this working. There are about three solutions:

1: add static routes on R2, R3.

2: advertise a default route from R4.

3: make R2, R3 iBGP speakers.

for solution 2, you could do something like this:

R4#conf t
router ospf 1
default-information originate

Good luck with the lab! Hope this info is sufficient to let you find a working solution.

As it's a lab, feel free to try them all!

That's still one of the best ways I know to get to grips with this kind of problems.

regards,

Leo

Re: Beginner BGP question....

You are the man....ever in Vegas, I owe you one.

I was trying something, that as you said, cant work (unless other measures are performed).

I added the command on R4,

default information-orginaite

When I did this, the pings got R5, but had no way back,

*Mar  1 00:15:17.467: ICMP: echo reply sent, src 200.1.1.1, dst 10.1.13.1

R5#sh ip route 10.1.13.1
% Subnet not in table

I had to create a static route R5 for this. 

Question,

So the packet is using BGP to get from R1 to R4, from R4 it is using the default route to R5, from R5 to R4 a static route, and from R4 to R1 BGP?

Re: Beginner BGP question....

Thanks for the offer, I have no short term plans for visiting Vegas but one never knows!

Indeed your remark to the solution is correct, routes are needed in both directions. However, you have found the root cause and solved it yourself.

Such learning experiences are not easily forgotten so you now know how to deal with this type of problem in the future. Bravo!

Answering your question: A packet will not really "choose" a routing protocol for it's transport.

This is fundamental. Forwarding occurs based on the routing table.

Basically a packet is always sent to the next hop with the best metric, regardless of whether this was learned through ospf, eigrp or bgp.

regards,

Leo

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