{"id":5145,"date":"2026-03-27T17:21:46","date_gmt":"2026-03-27T17:21:46","guid":{"rendered":"https:\/\/geekmungus.co.uk\/?p=5145"},"modified":"2026-03-27T17:21:46","modified_gmt":"2026-03-27T17:21:46","slug":"bgp-test-lab-part-20-routing-information-base-rib-bgp-debug-logging-and-other-things","status":"publish","type":"post","link":"https:\/\/geekmungus.co.uk\/?p=5145","title":{"rendered":"BGP Test Lab – Part 20 – Routing Information Base (RIB), BGP Debug Logging and other Things"},"content":{"rendered":"\n

The Routing Information Base (RIB) is one of the key workings of the BGP protocol on a router. The RIB is not just a routing table, it’s a dynamic repository of all the information that a router has learned from BGP Neighbours, along with a host of associated other attributes, such as the route’s AS Path, MED, local preference and so on. The RIB as a whole makes a comprehensive set of data that forms the foundation for the BGP decision process, that allows a router to evaluate and select the best path for traffic.<\/p>\n\n\n\n

The article will be quick piecemeal, where i’ll throw in a number of different topics that, up until now we’ve not covered at all or only briefly mentioned.<\/p>\n\n\n\n

Structure of the RIB<\/h1>\n\n\n\n

The RIB is split into three main parts which are summarised below, but depending on the vendor and the different names for things, it can get a bit confusing.<\/p>\n\n\n\n

So to be clear, the BGP RIB has three parts, the Adj-RIB-In (RIB-In), the Local RIB (Loc-RIB) and the Adj-RIB-Out (RIB-Out). <\/p>\n\n\n\n

Adj-RIB-In (RIB-In)<\/h2>\n\n\n\n

The Adj-RIB-In collects all the routes (prefixes) from BGP Neighbours (peers). It’s just all the routes in raw form, before any modifications or policy filtering. So potentially within this you’ll find routes that your filters (policy) will discard too.<\/p>\n\n\n\n

Local RIB (Loc-RIB)<\/h2>\n\n\n\n

When the router receives the routes (from the Adj-RIB-In), it runs them through the various filters and policies after which the result is stored in the Local RIB. It is the Local RIB that contains only the routes that are active and “best” and compliant with any of the policies will make it in here and be actually used to govern the flow of traffic through the router onto its further destinations – when put into the Routing Table (Global RIB, FIB Forwarding Information Base).<\/p>\n\n\n\n

Adj-RIB-Out (RIB-Out)<\/h2>\n\n\n\n

Opposite to the Adj-RIB-In is the Adj-RIB-Out, this repository contains all the routes (prefixes) that a router is going to advertise to its BGP Neighbours. However, BGP ensures that only the “best” and validated routes (prefix) are only shared with its neighbours after they have been subject to any policy filtering (and\/or manipulation).<\/p>\n\n\n\n

Routing Table (Global RIB, FIB – Forwarding Information Base)<\/h2>\n\n\n\n

Note that network engineers often use the terms “Routing Table” and “RIB” interchangeably, but they are different. The RIB is the database of all routing information “warts and all” so to speak, while the Routing Table (or Global RIB or FIB – Forwarding Information Base) is the list of active routes that are being actively used by the Router to forward traffic.<\/p>\n\n\n\n

OK, that might be a bit confusing, so think of it like this. The BGP RIB has three parts, the incoming routes (RIB-In), the best selected routes that are compliant with policies etc. (Loc-RIB) and the outgoing routes (RIB-Out). The “best selected routes” from the (Loc-RIB) are the ones that go into the “Routing Table” (a.k.a Global RIB, FIB or CEF in Cisco-land) and are used in actually determining the forwarding of packets through the router.<\/p>\n\n\n\n

Administrative Distance (AD) also known as Route Priority<\/h1>\n\n\n\n

The Administrative Distance (AD) is a locally used value that indicates the trustworthiness of different routing sources (which come from different protocols), it is not carried in BGP, it is not visible to neighbours and is only used when installing routes into the Routing Table (Global RIB\/FIB) which will then be used in actually determining the forwarding of packets through the router.<\/p>\n\n\n\n

And sometimes it doesn’t apply, because in terms of BGP (or any other protocol): BGP does NOT compete using AD inside BGP. The AD only matters when competing against other routing protocols if two or more identical routes (network prefixes) are received via different protocols and the router needs a way to “tie-break” (referee) these to decide which is actually used by the router in forwarding packets.<\/p>\n\n\n\n

A vendor neutral list is of the Administrative Distance is shown below:<\/p>\n\n\n\n

Protocol<\/th>AD<\/th><\/tr><\/thead>
Connected<\/td>0<\/td><\/tr>
Static<\/td>1<\/td><\/tr>
eBGP<\/td>20<\/td><\/tr>
EIGRP (internal)<\/td>90<\/td><\/tr>
OSPF<\/td>110<\/td><\/tr>
RIP<\/td>120<\/td><\/tr>
iBGP<\/td>200<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Where is it Applied?<\/h3>\n\n\n\n

So if you remember the flow: Adj-RIB-In \u2192 Loc-RIB \u2192 Routing Table (Global RIB)<\/strong><\/p>\n\n\n\n

Then the Administrative Distance is evaluated at this stage: Loc-RIB \u2192 Routing Table (Global RIB)<\/strong>, and is done for multiple protocols (if they exist) to act as the “tie-breaker” of if to install a route into the Routing Table (Global RIB) if the router learns the same route (network prefix) via more than one network protocol at once.<\/p>\n\n\n\n

Metric<\/h3>\n\n\n\n

Before looking at some examples, we’ll quickly review what a Metric is, because we have three similar sounding things: Administrative Distance (Route Preference in EXOS), Route Metric, BGP attributes (like MED, LocalPref).<\/p>\n\n\n\n

The Metric <\/strong>is a value that is either set (or derived by the routing protocol a.k.a. the Origin) and used where we have two (or more) routes from the same protocol with the same destination (network\/route prefix), for example two routes learned via eBGP, it is at this point the Metric <\/strong>would be used. And of course although not important here, within BGP itself, attributes are used to select the “Best Path”.<\/p>\n\n\n\n

Although for static routes, here’s an example for the 10.10.10.128\/25 network, you can see it has two routes for the same network prefix, but different metrics 100 and 200, so within the same Origin (or routing protocol) the route with the lower metric (100) would win and be used.<\/p>\n\n\n\n

Switch # show iproute\nOri  Destination        Gateway         Mtr  Flags         VLAN       Duration\n#s   10.10.10.128\/25    10.10.10.129    100  UG---S-um--f- Default    0d:0h:2m:41s\n s   10.10.10.128\/25    10.10.10.130    200  UG---S-um---- Default    0d:0h:2m:40s<\/code><\/pre>\n\n\n\n
So in our case the\u00a0Administrative Distance (AD) a.k.a. Preference<\/strong>\u00a0is only used where you have two (or more) routes from different protocols with the same destination (network\/route prefix). For example, two routes for 10.10.10.128\/25, where one is being learned via OSPF and the other via\u00a0eBGP, different protocols; so same route, different protocols, AD is the tiebreaker.\u00a0<\/p>\n\n\n\n
Decision Layer<\/th>What Decides<\/th><\/tr><\/thead>
Between protocols<\/td>Administrative Distance<\/td><\/tr>
Within one protocol<\/td>Metric<\/td><\/tr>
Within BGP only<\/td>BGP decision process (attributes)<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
Where can I see the AD for a route?<\/h3>\n\n\n\n
Everything I have found on this seems to indicate you can’t see the AD alongside a route in the routing table, like another column, it doesn’t appear to be possible to see this value in the output.<\/p>\n\n\n\n
What you can see is the “Origin”, which is where the route has come from (been learned from), with this you are then able to determine the AD (from the table above – or your router’s configuration), to then understand why the route that is appearing in this output below (which is the Routing Table – Global RIB) is, or is not the one you were expecting to see!<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
Example 1 – eBGP vs OSPF<\/h3>\n\n\n\n
Let’s say on Router K, we’ve learned the route: 10.5.1.0\/24 via eBGP from Router L. But we have also learned this same route via OSPF from Router L.<\/p>\n\n\n\n
If we examine the Administrative Distance (AD) of each of these they are:<\/p>\n\n\n\n