What does the BGP protocol consider when determining the best path?

Question

What does BGP protocol consider when determining the best path? Why is it considered the slower than interior routing protocols like RIP, OSPF, etc?

Answer 1

What does BGP protocol consider when determining the best path?

Below is a crude cheat sheet diagram of the best path selection attributes BGP evaluates.

Why is it considered the slower than interior routing protocols like RIP, OSPF, etc?

It’s a bit unfair to say BGP is slow since it is a fully-connected protocol. It’s trying to plot paths over what is, at the abstract level, a hypercube. The number of logical paths is therefore very, very large.

BGP is also a rather complex protocol. BGP4+ has to handle all manner of objects. Worse, because it’s an evolving protocol, it can’t compactify in any way. Everything that was supported has to still be supported and still behave in the same way, which means that there are going to be situations where the same thing can be expressed in different ways. BGP4+ has to know about all of that.

Third, people abused IPv4 chronically, and still do. Any given IP address can be anywhere in the network, CIDR is still a relatively recent idea. That means you can’t take any shortcuts.

OSPFv2 and v3, by comparison, are trying to deal with much simpler problems. For a start, they’re specialized and therefore v3 could shuck everything that didn’t apply. They’re also not working with hypercubes. Because of this, it doesn’t even matter that people abuse network addressing, as far less data has to be digested to cope with that.

It's almost as if comparing trucks (BGP) with race cars (RIP & OSPF).

BGP is built not for the speed of convergence, but for carrying a vast amount of information that other protocols cannot. The amount of policy-based route tuning you can do with BGP is enormous.

In general deployments, OSPF is used to pave the path between two subnets and on top of that fast converging network maintained by OSPF, we have BGP that carries some very detailed information about the subnets being connected. (MPLS VPNs)

Another would be connecting two different management entities (network domains). An enterprise connecting with an ISP, for example. An enterprise may use OSPF in their network to go with the fast convergence and then use BGP to send specific information of their subnets to connect with other networks elsewhere.

Answer 2

BGP has a rich set of attributes, and it uses those for its [complex] path selection algorithm. RFC 4271, A Border Gateway Protocol 4 (BGP-4) has the official algorithm, but some vendors have enhanced the algorithm. For example, Cisco has added the Weight attribute and commands to alter the original algorithm. See BGP Best Path Selection Algorithm:

BGP assigns the first valid path as the current best path. BGP then compares the best path with the next path in the list, until BGP reaches the end of the list of valid paths. This list provides the rules that are used to determine the best path:

1. Prefer the path with the highest WEIGHT.
   • Note: WEIGHT is a Cisco-specific parameter. It is local to the router on which it is configured.
2. Prefer the path with the highest LOCAL_PREF.
   • Note: A path without LOCAL_PREF is considered to have had the value set with the bgp default local-preference command, or to have a value of 100 by default.

3. Prefer the path that was locally originated via a network or aggregate BGP subcommand or through redistribution from an IGP.

   Local paths that are sourced by the network or redistribute commands are preferred over local aggregates that are sourced by the aggregate-address command.

   • Note: Be aware of this item:
     - If AIGP is configured AND the bgp bestpath aigp ignore command is not configured, the decision process considers the AIGP metric. See Configure the AIGP Metric Attribute for BGP for further details.

4. Prefer the path with the shortest AS_PATH.

   • Note: Be aware of these items:

     - This step is skipped if you have configured the bgp bestpath as-path ignore command.

     - An AS_SET counts as 1, no matter how many ASs are in the set.

     - The AS_CONFED_SEQUENCE and AS_CONFED_SET are not included in the AS_PATH length.

5. Prefer the path with the lowest origin type.

   • Note: IGP is lower than Exterior Gateway Protocol (EGP), and EGP is lower than INCOMPLETE.

6. Prefer the path with the lowest multi-exit discriminator (MED).

   • Note: Be aware of these items:

     - This comparison only occurs if the first (the neighboring) AS is the same in the two paths. Any confederation sub-ASs are ignored. In other words, MEDs are compared only if the first AS in the AS_SEQUENCE is the same for multiple paths. Any preceding AS_CONFED_SEQUENCE is ignored.

     - If bgp always-compare-med is enabled, MEDs are compared for all paths. You must disable this option over the entire AS. Otherwise, routing loops can occur.

     - If bgp bestpath med-confed is enabled, MEDs are compared for all paths that consist only of AS_CONFED_SEQUENCE. These paths originated within the local confederation.

     - THE MED of paths that are received from a neighbor with a MED of 4,294,967,295 is changed before insertion into the BGP table. The MED changes to to 4,294,967,294.

     - THE MED of paths that are received from a neighbor with a MED of 4,294,967,295 are considered valid and are inserted into BGP table with effect to Codes fixed for Cisco bug ID CSCef34800.

     - Paths received with no MED are assigned a MED of 0, unless you have enabled bgp bestpath med missing-as-worst . If you have enabled bgp bestpath med missing-as-worst , the paths are assigned a MED of 4,294,967,294. If you have enabled bgp bestpath med missing-as-worst , the paths are assigned a MED of 4,294,967,295 with effect to Codes fixed for Cisco bug ID CSCef34800.

     - The bgp deterministic-med command can also influence this step. Refer to How BGP Routers Use the Multi-Exit Discriminator for Best Path Selection for a demonstration.

7. Prefer eBGP over iBGP paths. If bestpath is selected, go to Step 9 (multipath).

   • Note: Paths that contain AS_CONFED_SEQUENCE and AS_CONFED_SET are local to the confederation. Therefore, these paths are treated as internal paths. There is no distinction between Confederation External and Confederation Internal.
8. Prefer the path with the lowest IGP metric to the BGP next hop.
   Continue, even if bestpath is already selected.
9. Determine if multiple paths require installation in the routing table for BGP Multipath. Continue, if bestpath is not yet selected.

10. When both paths are external, prefer the path that was received first (the oldest one). This step minimizes route-flap because a newer path does not displace an older one, even if the newer path would be the preferred route based on the next decision criteria (Steps 11, 12, and 13). Skip this step if any of these items is true:

    -You have enabled the bgp best path compare-routerid command.

    • *Note: Cisco IOS Software Releases 12.0.11S, 12.0.11SC, 12.0.11S3, 12.1.3, 12.1.3AA, 12.1.3.T, and 12.1.3.E introduced this command.

    -The router ID is the same for multiple paths because the routes were received from the same router.

    -There is no current best path.
    The current best path can be lost when, for example, the neighbor that offers the path goes down.

11. Prefer the route that comes from the BGP router with the lowest router ID. The router ID is the highest IP address on the router, with preference given to loopback addresses. Also, you can use the bgp router-id command to manually set the router ID.

    • Note: If a path contains route reflector (RR) attributes, the originator ID is substituted for the router ID in the path selection process.
12. If the originator or router ID is the same for multiple paths, prefer the path with the minimum cluster list length. This is only present in BGP RR environments. It allows clients to peer with RRs or clients in other clusters. In this scenario, the client must be aware of the RR-specific BGP attribute.

13. Prefer the path that comes from the lowest neighbor address.

    This address is the IP address that is used in the BGP neighbor configuration. The address corresponds to the remote peer that is used in the TCP connection with the local router.