BGP issue with /31 point to point links

OK so in a lab I set up 4 routers, in a ring topology with the links between each router with a point to point subnet mask (255.255.255.254)

all device can ping the neighbor so all good.

R1 ----- R2
 |        |
 |        |
R3 ----- R4

Then each is running BGP in the same AS 9900

I then gave each router a third link to separate sub-nets (/24's) to give each a network to advertise, (used a router on the end of each to bring up the link and give a pingaable end point.

Lastly on each router I shared out the two local /31 networks and their /24 network.

Now this is where it got strange.

From and of the BGP routers I could see all the /31 networks and ping all the "internal" assigned IP address of the AS. However if I use R2 as an example I can see the /24 "external" networks of R1 and R4 but not R3?? And if I go on R4 I can see the /24 network connected to R3 and R2 but not R1?

So basically I can't see the diagonally opposite connected /24 network from and of the routers? Doing a #show bgp does not show up the network as I might have expected, and I cant see any updates for the networks being sent.

I can post the configs but Can any one tell me whats going on??

Cheers

Find more posts tagged with

Comments

NetworkVeteran

The configs would really help.

Since all devices are in the same AS, do you have a full mesh of iBGP connections? In other words, R4 needs an iBGP session to R1, R2, and R3. If you only created an iBGP session from R4 to R2/R3, of course it won't know much about R1. If you've brought RRs of Confederations into the fray, that is also of course acceptable.

DevilWAH

NetworkVeteran wrote: »

If you've brought RRs of Confederations into the fray, that is also of course acceptable.

Err it must be late cause that line has escaped me for meaning

Hold on, so you are saying that If R1 has a neighbor relationship to R2 and R2 has a neighbor relation ship to R3. then even if R1 can see R3 via its IP address, and is advertising a network to R2. R2 will not forward this advertisement to R3. I would need to also create neighbor relationships between R1 and R3, and R2 and R4 so all 4 routers have direct relationships.

This sounds like if a large BGP AS you could end up with a hell of a lot of neighbor relationships.. I understood if the intermediate routers run BGP then network advertisements would propagate through a iBGP network.

DevilWAH

Config for R1, neighbors are R2 and R3 + the External AS router
networks advertised are the 2 connected /31 networks and the connected 10.1.1.0 /24 network

R2 and R3 get this route in iBGP, but R4 does not see it. the same pattern is repeated on the other 3 routers

router bgp 9900
no synchronization
bgp log-neighbor-changes
network 10.1.1.0 mask 255.255.255.0
network 192.168.11.0 mask 255.255.255.254
network 192.168.11.6 mask 255.255.255.254
neighbor 10.1.1.2 remote-as 5500
neighbor 192.168.11.1 remote-as 9900
neighbor 192.168.11.6 remote-as 9900
no auto-summary

Danielh22185

NetworkVeteran wrote: »

The configs would really help.

Since all devices are in the same AS, do you have a full mesh of iBGP connections? In other words, R4 needs an iBGP session to R1, R2, and R3. If you only created an iBGP session from R4 to R2/R3, of course it won't know much about R1. If you've brought RRs of Confederations into the fray, that is also of course acceptable.

^^ This. I can't know for sure until I see the configs but you must have a full mesh of iBGP to share out all injected networks. Also the use of route reflectors will accomplish this but you can potentially create SPOFs this way.

Also why are you using a /31 for the point to point links? Wouldn't that only give 2 total IP addresses? /30 would give you a proper 2 usable hosts per subnet and a broadcast / network address (4 total).

Danielh22185

DevilWAH wrote: »

Config for R1, neighbors are R2 and R3 + the External AS router
networks advertised are the 2 connected /31 networks and the connected 10.1.1.0 /24 network

R2 and R3 get this route in iBGP, but R4 does not see it. the same pattern is repeated on the other 3 routers

You don't have a full iBGP mesh. To do so all 4 routers must have neighbor peering. The use of a RR would also accomplish this.

DevilWAH

/31 is fine for point to point links. saves ip address and is a standard method to use

NetworkVeteran

DevilWAH wrote: »

Hold on, so you are saying that If R1 has a neighbor relationship to R2 and R2 has a neighbor relation ship to R3... R2 will not forward this advertisement to R3.

Precisely! An iBGP-speaker will never forward a route it received from another iBGP speaker.

I would need to also create neighbor relationships between R1 and R3, and R2 and R4 so all 4 routers have direct relationships.

Bingo, if they are all in the same Autonomous System.

This sounds like if a large BGP AS you could end up with a hell of a lot of neighbor relationships.

That's why RRs, Confederations, and MPLS were invented. They all address this in some way.

FloOz

When a BGP speaker receives an UPDATE message from an internal peer, the receiving BGP speaker shall not re-distribute the routing information contained in that UPDATE message to other internal peers. This is split horizon rule use within AS to prevent loops.

NetworkVeteran

DevilWAH wrote: »

/31 is fine for point to point links. saves ip address and is a standard method to use

Yup, the /31 is fine.

DevilWAH

cheers guys you have answered my question

never played with AS apart from my network with only the edge routers (x2) running BGP to the ISP.

Thanks for that most informative

Danielh22185

DevilWAH wrote: »

/31 is fine for point to point links. saves ip address and is a standard method to use

You're right on that, sorry brain fart.

Danielh22185

DevilWAH wrote: »

cheers guys you have answered my question

never played with AS apart from my network with only the edge routers (x2) running BGP to the ISP.

Thanks for that most informative

This is where BGP can EASILY become confusing because of the manual items you have to input. You can inject a network subnet but the peering are the important piece that interconnects them. The Free Route lab book has a great lab on this where you build Route Reflectors, its pretty neat to see the operational workings of those.