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In almost every book you will read about MPLS, the author will state that one of the MPLS benefits is having a BGP free core network; sometimes they explain it sometimes not. However, to really understand this statement I encourage you to imagine removing MPLS from your core network and see what adjustments you need to do in order to get the transit traffic to its destination. Do this  just in your mind’s eye please

If you are not willing to do this I will try in this post to bring the picture closer to you.

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Whenever you have a little IGP chit chat you’ll hit this endless story. I’ve tried to reach a final solid conclusion my self but IMHO its all about personal preference and taste. It is something like a Ferrari vs Lamborghini story, they offer comparable performance, but totally different feeling. It is all about a good design, that contains a balanced mixture of scalability, convergence, flexibility, extensibility, resources consumption, configuration, troubleshooting, etc.

In this series of posts I’ll try to contrast their likes and differences (not the Ferrari vs Lamborghini of course!), however I am not going to try to influence your opinion (vote for IS-IS ), rather I’ll try to share with you a deep enough in-depth knowledge to feel both protocols.
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In my previous post I explained what MPLS is and how it works from a high level perspective. In this post I will explain MPLS label operations and how labeled packets are processed in MPLS networks.

When a labeled packet is received the label value at the top of the stack is examined to determine two things:

1. The next hop and the exit interface to which the packet is to be forwarded.
2. The operation to be performed by the LSR on the label stack before forwarding the packet.

Listed below are the operations performed by the LSR on the label stack of the MPLS packet:

Push operation: adds a new label to the IP packet or to the label stack of the MPLS packet. The push operation is commonly done by the ingress router except in some traffic engineering scenarios.

Swap operation: the top most label is swapped by another one before switching the packet to the next downstream LSR. This is commonly done by intermediate LSRs in the provider network.

POP operation:  removes the top most label from the label stack to prepare that packet for its final destination. This is commonly done by the egress router or by the router preceding the egress router as Penultimate Hop Popping or PHP in brief.

Penultimate hop popping is an operation performed by a certain LSR in the MPLS network before sending the packet to the Label Edge Router (LER). The process is done by removing the top most label of the MPLS packet to reduce the overhead of the double lookup on the LER.

Have a look at the MPLS special Labels for more information about MPLS labels.

MPLS stands for Multi-protocol Label Switching. MPLS is a packet forwarding technology that is capable of carrying any L3 protocol and here comes the word multi-protocol. MPLS is capable of tunneling L3 packets inside the MPLS network using  MPLS labels. The MPLS label is pushed into the packet between the layer two header and the layer three header of the packet at the ingress router and is used to switch the packets across the network to its destination.

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Service Providers or Large enterprises commonly change routing policies from time to time, specially when adding new links or peering relationships with other entities.

When you change the inbound policy of your BGP speaker you need to reprocess the updates you received from that peer. BGP4 has no mechanism of requesting a re-advertisements from one of its peers. One solution to this problem is to store the received information local on the router and reprocess them without the need to refer to the peer who sent these updates. This option consumes a lot of resources on the router when you have multiple peering relationships.

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IS-IS uses the concept of distributed map or database, each router originates LSPs for his own links and relays its adjacent  LSPs to build up the distributed map, a local computation is performed on the database to extract IP reachability information to formalize the forwarding plane.

What is the problem?

IS-IS is an extended protocol by nature, basically it wasn’t  carrying IP information but now it does by some extensions, the extendability aspect of IS-IS came from the concept of  using TLVs (Type, Length, Value) which is theoretically allowing the IS-IS to carry any information, adding more and more TLVs to convey additional  information resulting in big messages, the exchanging of these big messages requires a discovery mechanism to discovering the network MTU,  to prevent messages drops ( Due to MTU mismatch).

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BGP is a critical component of the internet, bring BGP down and you bring the internet down or at least large portions of the internet. The problem is that BGP is highly vulnerable to many types of attacks for its implementation.

BGP runs over TCP on port 179 and inherits all types of TCP common attacks like replay, man-in-the-middle or DOS attacks. Also BGP is an application has its unique set of attacks against its implementation and messages.

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When it comes to choosing your PE-CE routing protocol, Which one do you think is best?

Choosing the PE-CE routing protocol for MPLS VPN  is an ongoing debate between back end network teams and those who have customer interface roles. They are always trying to satisfy the customer and we are always trying to keep the network simple, clean and stable.

To approach this issue we have to look from different perceptual views, as customers and service providers, as customer interface engineers and back end engineers. We need to see the big picture and work on a WIN-WIN solution.

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BGP is an intimate friend for all service provider engineers. Without BGP there is no internet, there is no MPLS VPN and there are no many other things now and in the days to come. I believe its healthy to visit your friends from time to time and know how you live

Any BGP speaker receives routing updates from other peers, processes the information for local use and then advertise selected routes to different peers based on predefined policies. In order for BGP to be able to perform its functions it stores this information is a special type of database called the BGP  Routing Information Base.

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