BGP Messages

BGP uses a variety of messages for establishing the connection, exchanging routing information, checking if the remote BGP neighbor is still there and/or notifying the remote side if any errors occur.

To do all of this, BGP uses 4 messages:

  • Open Message
  • Update Message
  • Keepalive Message
  • Notification Message

All of these BGP messages use a fixed-size header, it includes a type field that indicates what type of message it is.

To explain these BGP messages I will show you some Wireshark captures. I will use the following topology for this:

BGP R1 R2 AS1 AS2 Topology

Open Message

Once two BGP routers have completed a TCP 3-way handshake they will attempt to establish a BGP session, this is done using open messages. In the open message you will find some information about the BGP router, these have to be negotiated and accepted by both routers before we can exchange any routing information. Here are some of the items you will find in the open message:

  • Version: this includes the BGP version that the router is using. The current version of BGP is version 4 which is described in RFC 4271. Two BGP routers will try to negotiate a compatible version, when there is a mismatch then there will be no BGP session.
  • My AS: this includes the AS number of the BGP router, the routers will have to agree on the AS number(s) and it also defines if they will be running iBGP or eBGP.
  • Hold Time: if BGP doesn’t receive any keepalive or update messages from the other side for the duration of the hold time then it will declare the other side ‘dead’ and it will tear down the BGP session. By default the hold time is set to 180 seconds on Cisco IOS routers, the keepalive message is sent every 60 seconds. BGP routers will use the lowest configured hold down timer.
  • BGP Identifier: this is the local BGP router ID which is elected just like OSPF does:
    • Use the router-ID that was configured manually with the bgp router-id command.
    • Use the highest IP address on a loopback interface.
    • Use the highest IP address on a physical interface.
  • Optional Parameters: here you will find some optional capabilities of the BGP router. This field has been added so that new features could be added to BGP without having to create a new version.Things you might find here are:

Here’s an example of a wireshark capture of an open message between R1 and R2:

Wireshark Capture BGP Open Message

Above you can see the open message from R1 to R2. You can see the things that we discussed, the BGP version, AS number, hold time, BGP ID and the optional parameters (MP-BGP and route refresh). The marker field on top is used to indicate if we use MD5 authentication or not. When it’s filled with 1’s then we are not using authentication.

Update Message

Once two routers have become BGP neighbors, they can start exchanging routing information. This is done with the update message. In the update message you will find information about the prefixes that are advertised.In “BGP language” a prefix is referred to as NLRI (Network Layer Reachability Information). Here are some of the things you will find in an update message:

  • Withdrawn Route Length: this field shows the length of the Withdrawn Routes field in bytes. When it is set to 0, there are no routes withdrawn and the Withdrawn Routes field will not show up.
  • Withdrawn Routes: this field shows all the prefixes that should be removed from the BGP table.
  • Total Path Attribute Length: here you will find the total length of the Path Attributes field.
  • Path Attributes: the BGP attributes for the prefix are stored here, for example: origin, as_path, next_hop, med, local preference, etc. These path attributes are stored in TLV-format (Type, Length, Value).

Each of the BGP attributes also has an attribute flag that tells the BGP router how to treat the attribute. Here are the different bit flags:

  • Optional: when the attribute is well-known this bit is set to 0, when its optional it is set to 1.
  • Transitive: when an optional attribute is non-transitive this bit is set to 0, when it is transitive it is set to 1.
  • Partial: when an optional attribute is complete this bit is set to 0, when it’s partial it is set to 1.
  • Extended Length: when the attribute length is 1 octet it is set to 0, for 2 octets it is set to 1. This extended length flag may only be used if the length of the attribute value is greater than 255 octets.

Let’s take a look at an update message from R1:

R1(config)#router bgp 1
R1(config-router)#network 1.1.1.1 mask 255.255.255.255

Here’s the capture:

Wireshark Capture BGP Update Route Message

Above you can see a update message from R1. No routes are withdrawn and there are a couple of BGP attributes. You can see the ORIGIN, AS_PATH and MULTI_EXIT_DISC (MED). I also highlighted some of the flags. The AS_PATH attribute is transitive while MULTI_EXIT_DISC is optional. At the bottom you can find the NLRI information with our prefix.

Let’s remove the network command for the loopback interface on R1 so that we can see a withdrawn in the update message:

R1(config)#interface loopback 0
R1(config-if)#shutdown

Here’s the capture:

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Forum Replies

  1. Hi Rene

    Thank you for great article. I had a question though.
    Why didn’t R1 enter connect state and instead shifted to Active straight from idle? On the other hand i see R2 did enter connect state?

    thanks
    Kandhla

  2. Hei Rene,
    Thanks for the lesson, in my working experience, i am stuck in a situation for 2 sites. i have the ebgp peering up, but i am receiving 0 prefix from my isp. The isp said when they show ip bgp nei adverticed route that are hundreds of routes advertised, they can receive our route. Any good debug command to troule should this? We have plain bgp config, the same config (different peering ip though) is working with other sites.

    One interesting thing is that when i ping the isp with mtu 1500 it does not work, but it works with 1496. After reading cisco doc

    ... Continue reading in our forum

  3. Hi Jie,

    It’s possible that the MTU is causing your issue. Your ISP might be sending BGP updates of 1500 bytes which are dropped on your end. Here’s the default TCP segment size that is used for BGP:

    R1#show ip bgp neighbors 192.168.12.2 | include segment
    Maximum output segment queue size: 50
    Datagrams (max data segment is 1460 bytes):
    

    The segment size is 1460 bytes, add a TCP header (20 bytes) and IP header (20 bytes) and you have a 1500 byte packet. In other words, you should be able to send/receive up to 1500 bytes which is not possible at the moment.

    There’

    ... Continue reading in our forum

  4. OMG RENE! your FSM Diagram is so easy to understand.

    I bought this book called Routing TCP/IP, Volume II: CCIE Professional Development 2
    Great book but they have very complex descriptions on some things let me show you their chart lol… your chart below nocked it out of the ball park. don’t get me wrong that’s a great book but its hard as hex to understand and works better when paired with something like your website which can break it down in simple terms. I just had to laugh over the two diagrams though as yours was so easy to understand lol…

    //cdn-f

    ... Continue reading in our forum

  5. Hello Pinki

    It is possible to apply load balancing in BGP. BGP will not perform Equal Cost Multi-Path (ECMP) routing by default but it can be configured to do so. Specifically, you can use BGP multipath load sharing. You can see how to configure eBGP and iBGP to use more than one path in the following lesson:

    ... Continue reading in our forum

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