CISCO EXPERT – CCIE#23373

Ricardo Martins

Cisco Expert – MPLS and BGP – Part 1

Posted by Ricardo Martins on March 7, 2009

When I decided to start studying for CCIE SP, I was a lilttle bit nervous because I thought MPLS would be very difficult and complex. I am not saying that it is easy but if you understand the basic concept, you problably can solve more complex scenarios.

In simple words, MPLS was designed to avoid running BGP everywhere in a network. These days, the internet is composed by over 250k routes, so it would not be very scalable to run BGP in everywhere in your AS. Basically, MPLS will provide transport end-to-end for BGP routes.

In R&S world there are 3 ways you can do this:

1 – Run BGP everywhere
2 – Redistribute BGP into IGP
3 – Run a GRE tunnel from PE to PE

Obviously, none of these solutions are very scaleable for any big network, so the option here would be

4 – Run a MPLS free BGP core

First of all, we are going to enable OSPF in the links and advertise the loopbacks into OSPF and then enable BGP on R1 and R3 and advertise the LAN’s into BGP. Assume that all loopbacks are in the format 1.1.1.1, 2.2.2.2, etc. At this point, forget MPLS. We just want to build our network and test ip reachability. Just as a side note, if you are using OSPF in the network, you must always use a single area.

The network:

mpls

After we have enabled OSPF we should have reachability from R3 loopback to R1 loopback, let’s test it

R3#ping 1.1.1.1 source lo0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
Packet sent with a source address of 3.3.3.3
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

Now, we can safely enable BGP on R1 and R3 and advertise the LAN’s

R1
router bgp 10
no synchronization
bgp log-neighbor-changes
network 10.10.1.0 mask 255.255.255.0
neighbor 3.3.3.3 remote-as 10
neighbor 3.3.3.3 update-source Loopback0
no auto-summary

R1#sh ip bgp
BGP table version is 3, local router ID is 10.10.1.1
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
*> 10.10.1.0/24 0.0.0.0 0 32768 i
*>i10.10.3.0/24 3.3.3.3 0 100 0 i

R3
router bgp 10
no synchronization
bgp log-neighbor-changes
network 10.10.3.0 mask 255.255.255.0
neighbor 1.1.1.1 remote-as 10
neighbor 1.1.1.1 update-source Loopback0
no auto-summary

R3#sh ip bgp
BGP table version is 3, local router ID is 10.10.3.3
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
*>i10.10.1.0/24 1.1.1.1 0 100 0 i
*> 10.10.3.0/24 0.0.0.0 0 32768 i

Now the problem that we run into here, is that for example R3 will not have ip reachability to R1’s LAN 10.10.1.0/24 (Remember that we need to source traffic from R3’s LAN, otherwise R1 will not have a route back)

Rack1R3#ping 10.10.1.1 source lo1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.1.1, timeout is 2 seconds:
Packet sent with a source address of 10.10.3.3
…..
Success rate is 0 percent (0/5)

The reason we cannot ping is that, R2 will not have a route installed for R1 and R3’s LAN, let’s check

R2#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/2] via 172.16.12.1, 00:13:51, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/2] via 172.16.23.3, 00:13:51, FastEthernet0/1

R2 only provides transport for R1 and R3 to be able to peer via BGP

In part 2, we will see how can we solve this problem implementing the option “4-Run a MPLS free BGP core”, being R2 the core of the network. We will also take a closer look of the MPLS labeling process.

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12 Responses to “Cisco Expert – MPLS and BGP – Part 1”

  1. Smail said

    Cool. I am just learning MPLS, I will do this lab to.

  2. Ricardo Martins said

    If possible, put another router on the core of the network, either between R1 and R2 or R2 abd R3, so you can see the SWAP label process.

  3. Smail said

    Ok. Thank you for the tip.

    What about labs with ATM? Do you have an ATM switch?

  4. Smail said

    Ok. Thank you for the tip.

    What about labs where you need an ATM switch, how do you gonna lab this? Rack rental or GNS?

  5. Ricardo Martins said

    I dont have an ATM switch, but the amount of Cell mode you need to know for the lab it is not worth the cost. I only use a back to back ATM connection. All vendors focus mostly in frame mode, so Im guessong cell mode isnt much of a deal.

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    […]Cisco Expert – MPLS and BGP – Part 1 « CISCO EXPERT – CCIE#23373[…]…

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  9. Richard said

    Hey this is so great – I am working with an ISP and studying for my CCNP.

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